Cationic Ligand (Positive)
Formula Name Formula Name
Nitronium NO + Nitrosonium
NO +
2 Hydrazinium Name
Dimethylglyoximeto
H 2 NNH +
3 (dmg)
Glycinate ion (gly)
Polydentate ligands (with two or more donor site)
Bidentate (Two donor sites)
Formula Name Formula
H 2 NCH 2CH 2 NH 2 Ethylenediamine (en)
Me − C = NO −
|
Me − C = NOH
OO Oxalato (ox) NH 2 − CH 2 − COO−
− || ||
O− C− C− O−
Formula Name
Diethylenetriaminediamine (dien)
Tridentate H2 N••(CH2)2 − N•• H − (CH2)2 N•• H2
Tetradentate Triethylenetetramine (trien)
Hexadentate H2 N••(CH2)2 − N•• H − (CH2)2 N•• H(CH2)2 N•• H2
Ethylenediamine tetra acetic acid
−: OOCH2C N•• − CH2 − CH2 − N•• CH2COO− : (EDTA)4–
−: OOCH2C CH2COO− :
Chelating Ligand : When polydentate ligands bind to the central metal ion they form a ring called chelate
and the ligand is referred as chelating ligand.
Ambidentate ligands : A ligand which possesses two donor atom but in forming complex it utilizes only
one atom depending upon the condition and type of complex.
NO2 (nitro) , ONO (nitrito), CN (cyano), NC (isocyano), SCN (thiocyanide), NCS (isothiocyanide)
π- acid ligand : Ligands which are capable of accepting an appreciable amount of π- e− density from the
metal atom into emptying π or π * orbital or their own called π − acceptor or π − acid ligands eg. CO.
(4) Co-ordination Sphere : Ligand with Central Ligand 3+
central metal ion is kept in square bracket [ ] retains metal ion
its identity in the same form is called co-ordination Co NH3 Coordination
sphere (non-ionisable) Number
6
(5) Co - ordination Number : Number of
monodentate ligands attached to central atom/ ion Coordination sphere
are called coordination number of the central metal
atom/ion.
(6) Ionisation Sphere : The part present out side of the square bracket is called ionization sphere (ionisable).
IUPAC Nomenclature of complex compounds.
In order to name complex compounds certain rules have been framed by IUPAC. These are as follows :
(1) The positive part of a coordination compound is named first and is followed by the name of negative part.
(2) The ligands are named first followed by the central metal. The prefixes di-, tri-, tetra-, etc., are used to
indicate the number of each kind of ligand present. The prefixes bis (two ligands), tris (three ligands), etc., are used
when the ligands includes a number e.g., dipyridyl, bis (ethylenediamine).
(3) In polynuclear complexes, the bridging group is indicated in the formula of the complex by separating it
from the rest of the complex by hyphens. In polynuclear complexes (a complex with two or more metal atoms),
bridging ligand (which links two metal atoms) is denoted by the prefix µ before its name.
(4) Naming of ligands : The different types of ligands i.e. neutral, negative or positive are named differently
in a complex compound.
When a complex species has negative charge, the name of the central metal ends in – ate. For some elements,
the ion name is based on the Latin name of the metal (for example, argentate for silver). Some such latin names
used (with the suffix – ate) are given below :
Fe Ferrate Cu Cuperate
Ag Argentate Au Aurate
Sn Stannate Pb Plumbate
(5) Point of attachment in case unidentate ligands with more than co-ordinating atoms
(ambidentate ligands) : The point of attachment in case of unidentate ligands with more than one co-ordinating
atoms is either indicated by using different names for the ligands (e.g, thiocyanato and isothiocyanato) or by placing
the symbol of the donor atom attached, the name of the ligand separated by a hypen.
(NH 4 )3[Cr(SCN)6 ] (NH 4 )2[Pt(NCS)6 ]
Ammonium hexathioxyanato -S-chromate (III) Ammonium hexathiocyanato -N-platinate (IV)
or or
Ammonium hexathiocyanatochromate (III) Ammonium hexaisothiocyanatoplatinate (IV)
(6) Name of the bridging groups : If a complex contains two or more central metal atoms or ions, it is
termed as polynuclear. In certain polynuclear complexes. ligands may link the two metal atoms or ions. Such
ligands which link the two metal atoms or ions in polynuclear complexes are termed as bridge ligands. These bridge
ligands are separated from the rest of the complex by hyphens and denoted by the prefix µ . If there are two or
more bridging groups of the same kind, this is indicated by di- µ−, tri −µ−, etc.
[(NH 3 )]5 Co − NH 2 − Co(NH 3 )5 ](NO3 )5 ; [(NH 3 )5 Co − OH − Co(NH 3 )5 ]Cl5
µ-amidobis [pentaamminecobalt (III)] nitrate µ - hydroxobis [pentaamminecobalt (III) ] chloride
[(CO)3 Fe(CO)3 Fe(CO)3] [Be4 O(CH 3COO)6 ]
tri- µ carbonylbis [tricarbonyliron (0)] hexa -µ -acetato(O,O')-µ4oxotetraberyllium(II)
(7) If any lattice component such as water or solvent of crystallisation are present, these follow the name and
are preceded by the number of these groups (molecules of solvent of crystallisation) in Arabic numerals.
For example,
[Cu(H2O)4 ]SO4.H2O [Cr(H2O)4 Cl2]Cl.2H2O
Tetraaquacopper(II)sulphate1− water tetraaquadichlorochromium (III( chloride 2- water
(8) Following punctuation rules should also be followed while writing the name of the complex compounds.
(i) The name of the complete compound should not start a capital letter, e.g., [Cu(NH3)4 ]SO4
Tetraamminecopper (II) sulphate (Correct)
Tetraamminecopper (II) sulphate (Incorrect)
(ii) The full name of the complex ion should be written as one word without any gap.
(iii) There should be a gap between the cation and anion in case of ionic complexes.
(iv) The full name of non-ionic complexes should be written as one word without any gap.
Isomerism in co-ordination compounds.
Compounds having the same molecular formula but different structures or spatial arrangements are called
isomers and the phenomenon is referred as isomerism.
Isomerism
Structural isomerism Stereoisomerism
Ionisation Co-ordination Linkage Hydrate Geometrical isomerism Optical isomerism
isomerism isomerism isomerism isomerism
(1) Structural isomerism : Here the isomers have different arrangement of ligands around the central metal
atom. It is of the following types :
(i) Ionisation isomerism : The co-ordination compound having the same composition or molecular
formula but gives different ions in solution are called ionization isomers.
There is exchange of anions between the co-ordination sphere and ionization sphere.
Example : [Co Br (NH 3 )5 ] SO4 [Co SO4 (NH 3 )5 ] Br
Pentaaminebromo cobalt (III) Sulphate Pentaaminesulphato cobalt (III) bromide
SO42− present in ionisation sphere Br − present in ionization sphere
Gives white precipitate with BaCl2 Gives light yellow precipitate with AgNO3
(ii) Co-ordination isomerism : In this case compound is made up of cation and anion and the isomerism
arises due to interchange of ligands between complex cation and complex anion.
Example : [Co(NH 3 )6 ] [Cr (CN)6 ] [Cr (NH 3 )6 ][Co(CN)6 ]
hexaamine cobalt (III) hexacyano chromate (III) hexaamine chromium (III) hexacyanocobalt (III)
complex cation contains → NH3 ligand (with cobalt) complex anion contains → NH3 ligand (with
chromium)
complex anion contains → CN − ligand (with chromium) complex anion contains → CN− ligand (with
cobalt)
(iii) Linkage isomerism : In this case isomers differ in the mode of attachment of ligand to central metal ion
and the phenomenon is called linkage isomerism.
Example : [Co ONO (NH 3 )5 ]Cl2
Pentaamminenitritocobalt (III) [Co NO2 (NH3)5]Cl2
Pentaaminenitrocobalt (III) chloride
: O − NO− oxygen atom donates lone pair of electrons (nitrito) NO − nitrogen atom donates lone pair of
2
electrons (nitro)
(iv) Hydrate isomerism : Hydrate isomers have the same composition but differ in the number of water
molecules present as ligands and the phenomenon is called hydrate isomerism.
Examples :(i) [Cr (H 2O)6 ]Cl3 hexaaquachromium (III) chloride (violet)
(ii) [Cr (H 2O)5 Cl ]Cl2. H 2O pentaaquachlorochromium (III) chloride monohydrate (blue green)
(iii) [Cr (H 2O)4 Cl ]Cl2. 2H 2O tetraaquadichlorochromium (III) chloride dihydrate (green)
(2) Stereo isomerism or space isomerism : Here the isomers differ only in the
spatial arrangement of atoms of groups about the central metal atom. It is of two types :
(i) Geometrical or Cis-trans isomerism : This isomerism arises due to the
difference in geometrical arrangement of the ligands around the central atom. When
identical ligands occupy positions opposite to each other called cis-isomer. When identical
ligands occupy positions opposite to each other called trans –isomer. It is very common in disubstituted complexes
with co-ordination number of 4 and 6.
Complexes of co-ordination number 4
Tetrahedral geometry : In this case all the four ligands are symmetrically arranged with respect to one
another as such geometrical isomerism is not possible.
Square planar geometry : The four ligands occupy position at the four corners and the metal atom or ion is
at the center and lie in the same plane.
Type : I [Ma2b2 ] , M = Pt, a = Cl, b = NH3
Example : [Pt Cl (NH 3 )(Py)2 ]
Note : Square planar complexes of types cis –isomer (pale yellow) trans –isomer (dark- yellow)
Ma4 , Ma3b, Mab3 do not exhibit geometrical
isomerism as all the spatial arrangements of the
ligands relative to each other are equivalent.
Complexes of co-ordination number 6
Octahedral geometry : Here the metal atom or ion lies at the center and 1 to 6 position are occupied by
the ligands.
Cis–Positions : 1–2, 2–3, 3–4, 4–5
Trans – position : 1–4, 2–5, 3–6
Type –I Ma4b2 , M = Co, a = NH 3 , and b = Cl
Example : [CoCl 2 ( NH 3 )4 ]+ ion
Type –II [Ma3b3 ] , M = Rh, a = Cl, and b = Py Cis–isomer (Blue violet) Trans-isomer (Green)
Example : [Rh Cl3 (Py)3
Type –III [M(aa)2(en)2]++ , M = Co, a − a = CH 2 NH 2 (bidentate), b = Cl (monodentate)
|
CH 2 NH 2
(ii) Optical isomerism
(a) Optical isomers are mirror images of each other and have chiral centers.
(b) Mirror images are not super imposable and do and have the plane of symmetry.
(c) Optical isomers have similar physical and chemical properties but differ in rotating the plane of plane
polarized light.
(d) Isomer which rotates the plane polarized light to the right is called dextro rotatory (d-form) and the isomer
which rotates the plane polarized light to the left is called laevorotatory (l–form)
Example : (a) [Ma2b2c2]n−+ ; [Pt(Py)2(NH3)2 Cl2]2+
Pt 2+ py 2+
Cl py Cl
py
Pt Pt
Cl NH3
H3N Cl
NH3
Cis-d- isomer Mirror NH3
Cis-l- isomer
(b) [M a b c d e f]; Pt (py) NH 3 NO2Cl Br]
Br Br
py NO2
O2N py
Pt
Cl NH3 H3N Pt
I Mirror Cl
d- isomer I
l- isomer
(c) [M(AA)3 ]n± ;[CO(en)3 ]3+
3+ en
en en
Co Co en en Co en en
en
en 'Meso' or optically inactive form
d- form l- isomer
Mirror
(d) [M(AA)2 a2 ]n± ;[Co(en)2 Cl2 ]+ (e) [M(AA)2 ab]n± ; [Co(en)2 NH3Cl]2+
en + en + en 2+ en 2+
Co Cl Cl Cl Mirror Co Cl
Cl Co Co en NH3
en
Cl en H3N en
Cis-d- isomer Mirror Cis-d- isomer Cis-l- form
Cis-l- isomer
Bonding in co-ordination compounds.
Werner was able to explain the bonding in complex.
Primary valency (Pv) : This is non- directional and ionizable. In fact it is the positive charge on the metal ion.
Secondary valency (Sv) : This is directional and non- ionizable. It is equal to the number of ligand atoms
co-ordinated to the metal (co-ordination number).
Example : [Co (NH 3 )6 ]Cl3 or Co(NH 3 )6 ]3+ 3Cl −
Pv → 3Cl − [3] Sv → 6NH 3 (6)
[Co(NH 3 )5 Cl]Cl 2 or [Co(NH 3 )5 Cl]2+ 2Cl −
Pv → 2Cl − (2) Sv → 5NH 3 + 1Cl − (6)
[Co(NH 3 )4 Cl 2 ] Cl or [Co(NH 3 )4 Cl 2 ]+ Cl −
Pv → Cl − (1) Sv → 4 NH 3 + 2Cl − (6)
Nature of the complex can be understood by treating the above complexes with excess of AgNO3 .
CoCl3 . 6NH 3 → 3AgCl, [Co(NH 3 )6 Cl3 (three chloride ion)
CoCl3 . 5NH 3 → 2AgCl, [Co(NH 3 )5 Cl2 (two chloride ion)
CoCl3 . 4 NH 3 → 1AgCl, [Co(NH 3 )4 Cl2 (one chloride ion)
CoCl3 . 3NH 3 → no AgCl, [Co(NH 3 )3 Cl3 (no chloride ion)
The nature of bonding between central metal atom and ligands in the coordination sphere has been explained
by the three well-known theories. These are :
(1) Valence Bond theory of coordination compounds
(i) The suitable number of atomic orbitals of central metal ion (s,p,d) hybridise to provide empty hybrid
orbitals.
(ii) These hybrid orbitals accept lone pair of electrons from the ligands and are directed towards the ligand
positions according to the geometry of the complex.
(iii) When inner d-orbitals i.e. (n-1) d orbitals are used in hybridization, the complex is called – inner orbital
or spin or hyperligated complex.
(iv) A substance which do not contain any unpaired electron is not attracted by 2 magnet. It is said to be
diamagnetic. On the other hand, a substance which contains one or more unpaired electrons in the electrons in the
d-orbitals, is attracted by a magnetic field [exception O2 and NO]. It is said to be paramagnetic.
Paramagnetism can be calculated by the expression, µ s = n(n + 2), where µ = magnetic moment.
s= spin only value and n= number of unpaired electrons.
Hence, if n = 1, µ s = 1(1 + 2) = 1.73 B.M. , if n = 3, µ s = 3(3 + 2) = 3.87 B.M. and so on
On the basis of value of magnetic moment, we can predict the number of unpaired electrons present in the
complex. If we know the number of unpaired electrons in the metal complex, then it is possible to predict the
geometry of the complex species.
(v) There are two types of ligands namely strong field and weak field ligands. A strong field ligand is capable
of forcing the electrons of the metal atom/ion to pair up (if required). Pairing is done only to the extent which is
required to cause the hybridization possible for that Co-ordination number. A weak field ligand is incapable of
making the electrons of the metal atom/ ion to pair up.
Strong field ligands : CN − , CO, en, NH 3 , H 2O, NO− , Py .
Weak field ligands : I − , Br − , Cl − , F − , NO3− , OH − , C2O42− , NH 3 , H 2O .
Geometry (shape) and magnetic nature of some of the complexes (Application of valence bond theory)
(2) Ligand field theory : According to this theory when the ligands come closer to metal atom or ion, a field
is created. This field tends to split the degenerate d-orbitals of the metal atom into different energy levels. The
nature and number and number of lignads determine the extent of splitting on the basis of which the magnetic and
spectroscopic properties of the complex can be explained.
Stability of co-ordination in solution and Spectrochemical series.
Stronger is the metal-ligand bond, less is the dissociation in the solution and hence greater is the stability of a
coordination compounds.
Instability constant for the complex ion [Cu(NH3)4 ]2+ i.e.
[Cu(NH3)4 ]2+ ⇌ Cu2+ + 4 NH3 , is given by the expression; Ki = [Cu2+ ][NH3]4 .
[Cu(NH3)4 ]2+
Stability constant of the above complex i.e.
Cu2+ + 4 NH 3 ⇌ [Cu(NH 3 )2+ is given as under ; K = [Cu(NH 3 )42+ ] 4 = 1
[Cu2+ ][NH 3 ] K,
Greater is the stability constant, stronger is the metal – ligand bond
Factors affecting the stability of complex ion
(1) Nature of central metal ion : The higher the charge density on the central metal ion the greater is the
stability of the complex
For example, the stability constant of [Fe(CN)6 ]3− is much greater than the stability constant of [Fe(CN)6]4–.
Fe 2+ + 6CN − [Fe (C N)6 ]4− ; K = 1.8 × 106
Fe 3+ + 6CN − [Fe (C N)6 ]3− ; K = 1.2 × 10 31
Effective atomic number (EAN) or Sidgwick theory : In order to the stability of the complexes sidgwick
proposed effective atomic number. EAN generally coincides with the atomic number of next noble gas in some
cases. EAN is calculated by the following relation :
EAN = Atomic no. of the metal –e − lost in ion formation +No. of e − gained from the donor atom of the
ligands.
EAN = Atomic number – Oxidation number + co-ordination no. × 2
Complex Metal oxidation At. No. Coordination Effective atomic number
state of metal number
K 4[Fe (CN )6 ] (26 – 2) + (6 × 2) = 36 [Kr]
[Cu (NH 3 )4 ]SO4 + 2 26 6 (29 – 2) + (4 × 2) = 35
[Co (CH 3 )6 ]Cl3 4 (27 – 3) + (6 × 2) = 36 [Kr]
Ni (CO )4 + 2 29 6 (28 – 0) + (4 × 2) = 36 [Kr]
K 2[Ni (CN )4 ] 4 (28 – 2) + (4 × 2) = 34
+ 3 27 4
0 28
+ 2 28
K 2[PtCl6 ] +4 78 6 (78 – 4) + (6 × 2) = 86 [Rn]
K 3[Cr (C 2O4 )3 ] +3 24 6 (24 – 3) + (6 × 2) = 33
K 3[Fe (CN )6 ] +3 26 6 (26 – 3) + (6 × 2) = 35
K 2[HgI 4 ] +2 80 4 (80 – 2) + (4 × 2) = 86 [Rn]
[Ag NH 3 )2 ]Cl +1 47 2 (47 – 1) + (2 × 2) = 50
K 2[PdCl 4 ] +2 46 4 (46 – 2) + (4 × 2) = 52
(2) Nature of ligand : Greater the base strength is the ease with which it can donate its lone pair of electrons
and therefore, greater is the stability of the complex formed by it.
For example : [Cu(NH 3 )4 ]2+ ; K = 4.5 × 1011 ; [Cu(CN)4 ]2− ; K = 2.0 × 10 27
(3) Presence of chelate ring : Chelating ligands form more stable complex as compared to monodentate
ligands. For example : Ni 2+ + 6NH 3 [Ni(NH 3 )6 ]2+ ; K = 6 × 108 ; Ni 2+ + 3en [Ni(en)3 ]2+ ; K = 4 × 108
Spectro chemical series : Ligands can be arranged in increasing order of their strength (ability to cause crystal
field splitting) and the series so obtained is called as spectro chemical series.
I− < Br − < Cl − <F− < OH − < OX 2− < H 2O < Py < NH 3 < en < NO − < CN − < CO .
2
Ligands arranged left to NH 3 are generally regarded as weaker ligands which can not cause forcible pairing
of electrons within 3d level and thus form outer orbital octahedral complexes.
On the other hand NH 3 and all ligands lying right to it are stronger ligands which form inner orbital
octahedral complexes after forcible pairing of electrons within 3d level.
Preparation and Application of coordination compounds.
(1) Preparation : Coordination compounds are generally prepared by the application of the following
methods,
(i) Ligand substitution reaction : A reaction involving the replacement of the ligands attached to the
central metal ion in the complex by other ligands is called a ligand substitution reaction.
[Cu(H 2O)4 ]2+ (aq) + 3NH 3(aq)) → [Cu(NH 3 )4 ]2+ (aq)
Light blue Deep blue
+4 H2O(l)
(ii) Direct mixing of reagent : PtCl2 + 2H 2 N − CH 2 − CH 2 − NH 2 → [Pt(en)2 Cl2 ]
(en) (Dichloro bis
(ethylene diam mine platinum (II)
(iii) Redox reactions : In these reactions, either oxidation or reduction is involved
2CO(NO3 )2 + 8 NH 3 + 2NH 4 NO3 + H 2O2 2[CO(NH 3 )NO2 ](NO3 )2 + 2H 2O
Penta ammine nitrocobalt (II) nitrate.
Application
(1) Estimation of hardness in water, as Ca ++ and Mg 2+ ions form complexes with EDTA.
(2) Animal and plant world e.g. chlorophyll is a complex of Mg 2+ and haemoglobin is a complex of Fe 2+
vitamin B12 is a complex of Co 2+ .
(3) Electroplating of metals involves the use of complex salt as electrolytes e.g. K [Ag (CN )2 ] in silver plating.
(4) Extraction of metals e.g. Ag and Au are extracted from ores by dissolving in NaCN to form complexes.
(5) Estimation and detection of metal ions e.g. Ni 2+ ion is estimated using dimethyl glyoxime.
(6) Medicines e.g. cis-platin i.e. cis [PtCl2 (NH 3 )2 ] is used in treatment of cancer.
Organometallic compounds.
These are the compounds in which a metal atom or a metalloid (Ge, Sb) or a non-metal atom like B, Si, P,
etc, (less electronegative than C) is directly linked to a carbon atom of a hydrocarbon radical or molecule.
Organometallic compounds contain at least one..
(1) Metal – Carbon bond, (2) Metalloid – Carbon bond, (3) Non metal – Carbon bond.
Example :
Compounds : C2 H 5 MgBr , (C2H5)2 Zn , C6 H 5Ti(OC3 H7 )3 , (CH 3 )4 Si
Organometallic bond : Mg − C , Zn − C , Ti − C , Si − C
Note : B(OCH3 )3 , (C3 H7O)4 Ti cannot be regarded as organometallics as there is not metal carbon
bond.
Classification of organometallic compounds : Organometallics
have been classified as :
(1) σ–bonded organometallic compounds : Compounds such as
RMgX, R2 Zn, R3 Pb, R3 Al, R4 Sn etc, contains M − C σ − bond and are
called σ − bonded organometallic compound.
(2) π-bonded organometallic compounds : The transition metals Fe Cr
binds to unsaturated hydrocarbons and their derivatives using their d-
orbitals. Here metal atom is bonded to ligands in such a way that donations
of electrons and back acceptance by the ligand is feassible. These are called
π − orbitals of the ligand. These are called π complexes.
Examples : (i) π − cyclopentadienyl – iron complex
Ferrocene [Fe (η5 − CH5)2] , Bis (cyclopentadienyl) iron (II)
It is a π bonded sandwitch compound. The number of carbon atoms
bonded to the metal ion is indicated by superscript on eta (η x ) i.e. η 5 in this complex.
(ii) Dibenzene chromium ( π − complex) Ferrocene Dibenzene
It is also a π − bonded sandwitch compound. Its formula is [Cr(η 6 − C6 H6 )2 ] Fe(η5 − C5 H5 )2 chromium
(iii) Alkene complex ( π − complex) Cr(η6 − C6H6)
Zeise’s salt K Pt Cl3 (η 2 − C2 H 4 )] ; Potassium trichloroethylene platinate (IV).
It is a π bonded complex. µ 2 indicates that two carbons of ehylene are bonded to metals.
(3) Complexes containing both σ– and π– bonding characteristics : Metal carbonyls, compounds
formed between metal and carbon monoxide belong to this class. Metal carbonyls have been included in
organometallics.
(a) Mononuclear carbonyls : Contain one metallic atom per molecule. e.g Ni (CO)4, Fe(CO)5, Cr(CO)6
(b) Polynuclear carbonyls : Contain two or more metallic atoms per molecule. e.g.,
Mn2 (CO)10 , Fe(CO)9 , Fe(CO)12
Applications of organometallics
(1) Grignard reagent (RMgX) has been extensively used for synthesis of various organic compounds.
(2) Wilkinson's catalyst [(PH 3 P)3 RhCl] i.e. tris (triphenylphosphine) chlororhodium (I) is used as a
homogeneous catalyst for the hydrogenation of alkenes.
(3) Zeigler Natta catalyst (composed of a transition metal salt, generally TiCl4 and trialkyl aluminium) are
used as heterogeneous catalysts in the polymerisation of alkenes.
Some Important points.
(1) Flexidentate character : Polydentate ligands are said to have flexidentate character if they do not use
all its donor atoms to get coordinated to the metal ion e.g. EDTA generally act as a hecadentate ligand but it can
also act as a pentadentate and tetradentate ligand.
(2) Badecker reaction : This reaction involves the following chemical change.
Na2[Fe(CN)5 NO] + Na2SO3 → Na4 [Fe(CN)5 (NO. SO3 )]
(3) Everitt's salt : It is K2[Fe(CN)6 ] obtained by reduction of prussian blue.
(4) Masking : Masking is the process in which a substance without physical separation of it is so transformed
that is does not enter into a particular reaction e.g., masking of Cu2+ by CN − ion.
(5) Macrocyclic effect : This term refers to the greater thermodynamic stability of a complex with a cyclic
polydentate ligand when compared to the complex formed with a non-cyclic ligand. e.g., Zn(II) complex with
ligand;
(6) Prussian blue and Turnbull's blue is pot. ferric ferrocyanide. However colour of Turnbull's blue is less
intense than prussian blue. Decrease in colour is due to the presence in it of a white compound of the formula
K2{Fe[Fe(CN)6 ]} named as potassium ferrous ferrocyanide.
Halogen derivatives
Compounds derived from hydrocarbons by the replacement of one or more hydrogen atoms by the corresponding
number of halogen atoms are termed as halogen derivatives. The halogen derivatives of the hydrocarbons are broadly
classified into three classes:
• Halogen derivatives of saturated hydrocarbons (Alkanes)- Halo-alkanes.
• Halogen derivatives of unsaturated hydrocarbons (Alkenes and alkynes)-Halo-alkene or alkyne.
• Halogen derivatives of aromatic hydrocarbons (Arenes)-Halo-arenes.
General characteristics of Halo-Alkanes.
(1) Organic compounds in which halogen atom (F,Cl, Br, I) is directly linked with saturated carbon atom are
known as halo-alkanes. General formula is Cn H 2n+2−m Xm ( X = F, Cl, Br, I ) and m = no.of halogen atom;
n = no.of carbon atoms.
(2) Depending on the number of halogen atoms present in the halogen derivative, these are termed as mono-,
di-, tri-, tetra-, and polyhalogen derivatives.
MCetHhan4e −H → CH 3 − X −H → CH 2− X2 −H → CH − X3 −H → C −X 4
+X Mono +X + X Tri +X
Di Tetra
(i) Monohalogen derivatives are termed as alkyl halides.
Example : CH3Cl C2H5Br C3H7I
Methyl chloride Ethyl bromide Propyl iodide
Monohalogen derivatives or alkyl halides are classified as primary (1°), secondary (2°) or tertiary (3°)
depending upon whether the halogen atom is attached to primary, secondary or tertiary carbon atoms.
H Primary carbon R' Secondary carbon R' Tertiary carbon
| | |
R− C− X
R − C− X R − C− X |
| | R"
H Tertiary alkyl halide
H Secondary alkyl halide
Primary alkyl halide
(ii) The dihalogen derivatives are mainly of three types
(a) Gem-dihalides : In these derivatives both the halogen atoms are attached to the same carbon atom. These
are also called alkylidene halides.
CH3CH Cl ; CH3 − CBr2 − CH3
Cl Isopropylidene bromide
Ethylidene chloride
(b) Vic-dihalides : In these derivatives, the halogen atoms are attached to adjacent (Vicinal) carbon atoms.
These are also termed as alkylene halides.
CH2Cl.CH2Cl ; CH 3CHCl.CH 2Cl
Ethylene chloride Propylene chloride
(c) α-ω halides (Terminal dihalides) : In these derivatives, the halogen atoms are attached to terminal carbon
atoms. These are also called polymethylene halides.
CH 2 BrCH 2CH 2 Br ; Cl − CH 2 − CH 2 − CH 2 − CH 2 − Cl
Trimethylene bromide Tetra-methylene chloride
(iii) The tri-halogen derivatives are termed as halo-forms
Example : CHCl3 ; CHBr3 ; CHI 3
Chloroform
Bromoform Iodoform
(iv) In tetra-halogen derivatives all the four halogen atoms are attached to the same carbon atom in
derivatives of methane.
Example : CCl4 ; CBr4
Carbon tetrachloride Carbon tetrabromide
In other derivatives, the four halogen atoms are attached to different carbon atoms, e.g., CHCl2
|
CHCl2
Acetylene tetrachloride or
1,1,2,2- tetrachloroethane
(3) The common and IUPAC names of some halogen derivatives are listed here.
Formula of halogen derivatives Common name IUPAC name
Methyl chloride Chloromethane
CH3Cl Ethyl bromide
Isopropyl bromide Bromoethane
CH3CH2Br n-Butyl chloride 2-Bromopropane
Isobutyl bromide 1-Chlorobutane
CH3CHBrCH3 1-Bromo -2- methylpropane
Tertiary butyl bromide
CH3CH2CH2CH2Cl 2-Bromo –2-methylpropane
Ethylidene chloride
CH3 CH − CH2Br 1,1-Dichloroethane
CH3 Ethylene chloride
Chloroform 1,2,-Dichloroethane
CH3 Iodoform Trichloromethane
| Tri-iodomethane
CH3 C− CH3 Carbon tetrachloride
− Tetrachloromethane
|
Br
CH3CH Cl
Cl
CH2Cl.CH2Cl
CHCl3
CHI 3
CCl4
Usually, the simple and lower members are called by common names and higher members are given IUPAC names.
(4) Higher members of alkyl halides show following types of isomerism,
CH3
|
(i) Chain isomerism : CH3 − CH 2 − CH 2 − CH 2 − X ←→ CH 3 − CH − CH 2 − X
1-Halo butane 1-Halo -2-methylpropane
(ii) Position isomerism : CH3 − CH2 − CH2 − X ←→ CH3 − C H − CH3
1-Halopropane
|
X
2 − Halopropane
(iii) Optical isomerism : This is due to the presence of asymmetric carbon atom in secondary butyl halide.
H
|
CH3 C− CH2CH3
−
|
Br
(2- Bromobutane)
The total number of isomers in alkyl halides are: Propyl (C3H7 − X) has two isomers, Butyl (C4 H9 − X) has
four isomers, and Pentyl (C5H11 − X) has eight isomers.
(5) Halo-alkanes contain sp3 hybridised carbon atom bonded to halogen atom or atoms.
General methods of preparation of Alkyl Halides.
(1) From alkanes
(i) By halogenation : C2H6 (Excess) + Cl2 hv → C2 H5Cl + HCl
Ethane Ethyl chloride (Major product)
CH3CH2CH3 Cl 2 CH3CH2CH2Cl + CH3CHCH3
Propane |
UV light → 1−Chloropropane (45%)
Cl
2- Chloropropane (55%)
This reaction proceed through free radical mechanism.
Note : Order of reactivity of X2 for a given alkane is, F2 > Cl2 > Br2 > I2 .
The reactivity of the alkanes follows the order : 3°alkane > 2°alkane > 1°alkane.
(ii) With sulphuryl chloride : R − H + SO2Cl2 hv + HCl
Organicperoxide(R'CO2)2 → R − Cl + SO2
Note : In presence of light and trace of an organic peroxide the reaction is fast.
(2) From alkenes (Hydrohalogenation)
CH3 − CH = CH − CH3 + HBr → CH3CH2 − CH − CH3 → Electrophillic addition.
But − 2−ene |
Br
2- Bromobutane
Note : Addition of HBr to alkene in the presence of organic peroxide take place due to peroxide effect or
Kharasch's effect.
This addition take place by two mechanism,
Peroxide initiates free radical mechanism.
Markownikoff’s addition by electrophillic mechanism.
From alkyne we cannot obtain mono alkyl halide.
The order of reactivity of halogen acids is, HI > HBr > HCl .
(3) From alcohols
(i) By the action of halogen acids
Groove’s process R − OH+ H − X Anhy. ZnCl2 → RX + H2O
Alcohol 300°C Haloalkane
Note : The reactivity order of HX in the above reaction is : HI > HBr > HCl > HF .
Reactivity order of alcohols 3° > 2° > 1° > MeOH .
2° and 3° alcohols undergo SN1 ; where as 1° and MeOH undergo SN 2 mechanism.
Concentrated HCl + anhy. ZnCl2 is known as lucas reagent.
(ii) Using PCl5 and PCl3 : CH3CH2OH + PCl5 → CH3CH2Cl+ POCl3 + HCl
Phosphorus Chloroethane Phosphorus
pentachloride Oxychloride
3CH3CH2OH + PCl3 → 3CH3CH2Cl+ H3PO3
Chloroethane Phosphorus acid
Note : Bromine and iodine derivatives cannot be obtain from the above reaction, because PBr5 or PI5
are unstable.
This method gives good yield of primary alkyl halides but poor yields of secondary and tertiary alkyl
halides.
(iii) By the action of thionyl chloride
(Darzan's process) CH 3CH 2OH + SOCl2 Pyridine → CH 3CH 2Cl + SO2 + HCl
Note : Reaction takes place through SN 2 mechanism.
(4) From silver salt of carboxylic acids (Hunsdiecker reaction, Decarboxylation)
(Free radical mechanism) R− C− O − Ag + Br − Br CCl4 → R − Br + CO2 ↑ + AgBr ↓
Decarboxylation
||
O
Note : The reactivity of alkyl group is 1° > 2° > 3°
Not suitable for chlorination because yield is poor.
In this reaction iodine forms ester instead of alkyl halide and the reaction is called Birnbourn-
Simonini reaction, 2R − COOAg + I 2 → RCOOR′ + 2CO2 + 2AgI .
(5) From alkyl halide (Halide exchange method) : R − X + NaI Acetone → R − I + NaX(X = Cl, Br)
Reflux
Note : Alkyl fluorides can not be prepared by this method. They can be obtained from corresponding
chlorides by the action of Hg2F2 or antimony trifluoride.
2CH 3Cl + Hg 2 F2 → 2CH 3 F + Hg 2Cl2
Methyl fluoride
(6) Other method R−X
(i) ROH KI,H3PO4 →
(ii) ROH X2 +(PhO)3P →
Rydon method
(iii) Dihalide Zn−Cu→
HCl
(iv) RMgX X2 →
(v) ROR PCl5 →
Properties of Alkyl Halides.
(1) Physical properties
(i) CH3F, CH3Cl, CH3Br and C2H5Cl are gases at room temperature. The alkyl halides upto C18 are
colourless liquids while higher members are colourless solids.
(ii) Alkyl halides are insoluble in water but soluble in organic solvents.
(iii) They burn on copper wire with green edged flame (Beilstein test for halogens).
(iv) Alkyl bromides and iodides are heavier than water. Alkyl chlorides and fluorides are lighter than water.
(v) Alkyl iodides become violet or brown in colour on exposure as they decompose in light.
2RI Light → R − R + I2
(vi) For a given alkyl group, the boiling points of alkyl halides are in the order RI > RBr > RCl > RF and for a
given halogen the boiling points of alkyl halides increase with the increase of the size of the alkyl group.
(vii) Alkyl halides are in general toxic compounds and bring unconsciousness when inhaled in large amounts.
(2) Chemical properties : The alkyl halides are highly reactive, the order of reactivity is,
Iodide > Bromide > Chloride (Nature of the halogen atom)
Tertiary > Secondary > Primary (Type of the halogen atom)
Amongst the primary alkyl halide, the order of reactivity is : CH3 X > C2H5 X > C3H7 X , etc.
The high reactivity of alkyl halides can be explained in terms of the nature of C − X bond which is highly
polarised covalent bond due to large difference in the electronegativities of carbon and halogen atoms. The halogen
is far more electronegative than carbon and tends to pull the electrons away from carbon, i.e., halogen acquires a
small negative charge and carbon a small positive charge.
− δC+ − δX−
This polarity gives rise to two types of reactions,
(i) Nucleophilic substitution reactions (ii) Elimination reactions
(i) Nucleophilic substitution reactions : The Cδ + site is susceptible to attack by nucleophiles (An electron
rich species). : Nu− + R − X → Nu − R + X − :
R− X − X− → R+ Nu− → R − Nu ( SN1 reaction)
Slow Fast
Nu− + R − X Slow → Nu.....R.....X Fast → Nu − R + X − ( SN2 reaction)
Transition state
Examples of SN reactions,
(a) Hydrolysis : Alkyl halides are hydrolysed to corresponding alcohols by moist silver oxide (AgOH) or by
boiling with aqueous alkali solution (NaOH or KOH). The attacking nucleophile is OH − .
RX + AgOH → ROH+ AgX ; RX + KOH(aq) → ROH + KX
Alkyl halide Alcohol
Note : With the help of this reaction an alkene can be converted into alcohol. Alkene is first reacted with
HBr to form alkyl bromide and then hydrolysis is done.
CH 2 = CH 2 HBr → CH3CH 2 Br AgOH → CH3CH 2OH
Ethylene Ethyl bromide Ethyl alcohol
(b) Reaction with alkoxides or dry silver oxide : Ethers are formed by heating alkyl halides with sodium or
potassium alkoxides or dry silver oxide. The attacking nucleophile is OR− (Williamson's synthesis).
RX + NaOR' Heat → ROR' + NaX ; 2RX + Ag 2O → R − O − R+ 2 AgX
Unsym. ether Sym. ether
(c) Reaction with sodium or potassium hydrogen sulphide : Alkyl halides form thioalcohols with aqueous
alcoholic sodium hydrogen sulphide or potassium hydrogen sulphide. The nucleophile is SH − .
RX + NaSH → RSH + NaX
Sodium hydrogen Thioalcohol
or Alkanethiol
sulphide or Alkyl mercaptan
(d) Reaction with alcoholic potassium cyanide and silver cyanide : Alkyl cyanides are formed as the main
product when alkyl halides are heated with alcoholic potassium cyanide. The nucleophile is CN − .
RX + KCN Alcohol → RCN + KX
Alkyl cyanide or
Alkane nitrile
(e) Reaction with potassium nitrite or silver nitrite : On heating an alkyl halide with potassium nitrite in an
aqueous ethanolic solution, alkyl nitrite is obtained as the main product though some nitro alkane is also formed.
The nucleophile is NO2− .
RX + K − O − N = O → R − O − N = O+ KX
Alkyl nitrite
However, when alkyl halide is heated with silver nitrite in an aqueous ethanolic solution, nitro-alkane is the
main product. Some alkyl nitrite is also obtained.
RX + AgNO2 → R − N O AgX
O+
Nitro-alkane
(f) Reaction with ammonia : On heating with aqueous or alcoholic solution of ammonia in a sealed tube at
100°C, alkyl halides yield a mixture of amines and quaternary ammonium salt. The nucleophile is NH − in the first
2
reaction.
C2H5Br + H − NH2 → C2H5 NH2 + HBr ; C2H5 NH2 + BrC2H5 → C2H5 NHC2H5 + HBr
Ethylamine(p.) Diethylamine(sec.)
+−
(C2H5)2 NH + BrC2H5 → (C2H5)3 N + HBr ; (C2 H5 )3 N + BrC2 H5 → (C2 H5 )4 NBr
Triethylamine(tert.) Tetraethyl ammonium
bromide(Quaternary)
(g) Reaction with silver salts of fatty acids : On heating with silver salts of fatty acids in alcoholic solution, alkyl
halides yield esters. The nucleophile is R' COO− .
R' COOAg + XR → R' COOR + AgX
Ester
(h) Reaction with sodium acetylide : Alkyl halides react with sodium acetylide to form higher alkynes. The
nucleophile is CH ≡ C− .
RX + NaC ≡ CH → R − C ≡ CH+ NaX
Sodium acetylide Alkyne
(i) Reaction with sodium or potassium sulphide : Alkyl halides react with sodium or potassium sulphide in
alcoholic solution to form thioethers.
2RX + Na2S → R − S − R+ 2NaX
Thioether
Thioethers can also be obtained by heating alkyl halides with alcoholic solution of sodium mecaptide
(NaSR') , i.e., metallic derivative of a thioalcohol.
RX − NaSR' → R − S − R'+ NaX
C2 H5 Br + NaSCH 3 → C2 H5 − S − CH 3 + NaBr
Ethyl methyl thioether
(j) Reaction with halides : Alkyl chlorides react with sodium bromide or sodium iodide to form alkyl bromide
or alkyl iodide. Similarly, alkyl bromides react with sodium iodide in acetone or methanol to form alkyl iodides.
RCl + NaBr → RBr NaI → RI
Alkyl chloride Alkyl bromide Alkyl iodide
(ii) Elimination reactions : The positive charge on carbon is propagated to the neighbouring carbon atoms
by inductive effect. When approached by a strongest base (B), it tends to lose a proton usually from the β-carbon
atom. Such reactions are termed elimination reactions. They are also E1 and E2 reactions.
HH B..− H
HH
|| || |
E1 reaction : R − C − C − H Slow → R − C − C − H Fast → R − C C− H + B− H
|| −X− |+ | =
HX HH
B− : H H BH H H
| | | | |
E2 Reaction : R − − H Slow → R − H Fast → R − C X−
C− C C− C− | = C− H + B − H +
|| ||
HX HX H
Transiton state
As the above reactions involve leaving of X −, the reactivity of alkyl halides (Same alkyl group, different
halogens) should be limited with C − X bond strength.
Type of bond C−I C − Br C − Cl
Bond strength (kcal/mol) 45.5 54 66.5
Bond strength increases
The breaking of the bond becomes more and more difficult and thus, the reactivity decrease.
The order of reactivity (Tertiary > Secondary > Primary) is due to +I effect of the alkyl groups which
increases the polarity of C − X bond.
RR
RC X, CH X, R CH2 X
R R
The primary alkyl halides undergo reactions either by SN2 or E2 mechanisms which involve the formation of
transition state. The bulky groups cause steric hinderance in the formation of transition state. Therefore, higher
homologues are less reactive than lower homologues. CH3 X > C2H5 X > C3H7 X , etc.
Example of elimination reaction
(a) Dehydrohalogenation : When alkyl halides are boiled with alcoholic potassium hydroxide, alkenes are
formed.
Cn H 2n +1 X + KOH → CnH2n + KX + H2O
Alkene
(Alcoholic)
In this reactions, ether is a by-product as potassium ethoxide is always present in small quantity.
C2H5Br + KOC2H5 → C2H5 − O − C2H5 + KBr
(b) Action of heat : Alkyl halides when heated above 300°C, tend to lose a molecule of hydrogen halide
forming alkenes.
RCH2CH2 X 300°C → RCH = CH2 + HX ; C2H5Br 300°C → CH2 = CH2 + HBr
Alkene Ethene
The decomposition follows the following order,
Iodide > Bromide > Chloride (When same alkyl group is present) and
Tertiary > Secondary > Primary (When same halogen is present).
(iii) Miscellaneous reactions
(a) Reduction : Alkanes are formed when alkyl halides are reduced with nascent hydrogen obtained by
Zn / HCl or sodium and alcohol or Zn/Cu couple or LiAlH4 .
RX + 2H → R − H + HX
Reaction is used for the preparation of pure alkanes
(b) Wurtz reaction : An ether solution of an alkyl halide (Preferably bromide or iodide) gives an alkane when
heated with metallic sodium.
2RX + 2Na → R − R + 2NaX
(c) Reaction with magnesium : Alkyl halides form Grignard reagent when treated with dry magnesium powder
in dry ether.
Rx + Mg Dryether → R − Mg − X
(Powder ) Grignard reagent
Grignard reagents are used for making a very large number of organic compounds.
(d) Reaction with other metals : Organometallic compounds are formed.
When heated with zinc powder in ether, alkyl halides form dialkyl zinc compounds. These are called
Frankland reagents.
2C2H5Br + 2Zn EHtehaetr →(C2H5)2 Zn + ZnBr2
When heated with lead-sodium alloy, ethyl bromide gives tetra ethyl lead which is used an antiknock
compound in petrol.
4C2 H5 Br + 4Pb(Na) →(C2 H5 )4 Pb + 4 NaBr + 3Pb
Alkyl halides form dialkyl mercury compounds when treated with sodium amalgam.
2C2H5 Br + Na − Hg →(C2H5 )2 Hg+ NaBr
Diethyl mercury
Reaction with lithium : Alkyl halides react with lithium in dry ether to form alkyl lithiums.
RX + 2Li Ether → R − Li + LiX ; C2H5Br + 2Li → C2H5 − Li + LiBr
Ethyl bromide
Alkyl lithiums are similar in properties with Grignard reagents. These are reactive reagents also.
(e) Friedel-Craft's reaction : Alkyl halides react with benzene in presence of anhydrous aluminium halides to
form a homologue of benzene.
C6H6 + RCl AlCl3 → C6H5R + HCl ; C6H6 + C2H5Br AlBr3 → C6H5C2H5 + HBr
Benzene Alkyl benzene
(f) Substitution (Halogenation) : Alkyl halides undergo further halogenation in presence of sunlight, heat
energy or peroxide.
C2 H5 Br Br2 → C2 H 4 Br2 Br2 → C2 H 3 Br3 .....
hv hv
Preparations and properties of Dihalides.
(1) Methods of preparation of dihalides
(i) Methods of preparation of gemdihalide
X
|
(a) From alkyne (Hydrohalogenation) : R−C ≡C−H + HX → R − C C− H +HX → R − C− CH
| = 3
| |
XH
X
(b) From carbonyl compound : RCHO + PCl5 → RCHCl2 + POCl3
[Terminal dihalide]
Note : If ketone is taken internal dihalide formed.
(ii) Methods of preparation of vicinal dihalide
(a) From alkene [By halogenation] : R − CH = CH2 + Cl2 → R − C H − C H 2
| |
Cl Cl
(b) From vicinal glycol : R − CH − OH + 2PCl5 → R − CH − Cl + 2HCl + 2POCl3
| |
CH2−OH CH2−Cl
(2) Properties of dihalides
(i) Physical properties
(a) Dihalide are colourless with pleasant smell liquid. Insoluble in water, soluble in organic solvent.
(b) M.P and B.P ∝ -molecular mass.
(c) Reactivity of vicinal dihalides > Gem dihalide.
(ii) Chemical properties of dihalide
(a) Reaction with aqueous KOH : RCHX2 + 2KOH(aq.) −KX → RCH(OH)2 −H2O → RCHO
HX
| |
(b) Reaction with alcoholic KOH : RCH 2 − CHX 2 Alc.KOH → R − = H NaNH2 → R − C ≡ CH
−(KX + H2O) C C− −(NaX + NH3 )
(c) Reaction with Zn dust
Gem halide (di) form higer symmetrical alkene.
Vicinal dihalide form respective alkene.
(d) Reaction with KCN : R − CHX2 + 2KCN −2KX → RCH(CN)2 H3O⊕ → RCH(COOH)2
Hydrolysis
(e) Other substitution reaction
CH2 −X NH3 / 373K → CH2 − NH2
| |
CH2 − X CH2 − NH2
Ethylene diamine
CH 2 − X 2CH3COONa → CH 2 −OCOCH3 2NaX.
| |
CH2 − X CH2 −OCOCH3 +
Tri-halides (Chloroform and iodoform).
Chloroform or trichloromethane, CHCl3
It is an important trihalogen derivative of methane. It was discovered by Liebig in 1831 and its name
chloroform was proposed by Dumas as it gave formic acid on hydrolysis. In the past, it was extensively used as
anaesthetic for surgery but now it is rarely used as it causes liver damage.
(1) Preparation
(i) Chloroform is prepared both in the laboratory and on large scale by distilling ethyl alcohol or acetone with
bleaching powder and water. The yield is about 40%. The available chlorine of bleaching powder serves both as
oxidising as well as chlorinating agent.
CaOCl2 + H2O → Ca(OH)2 + Cl2
Bleaching powder
(a) From alcohol
Alcohol is first oxidised to acetaldehyde by chlorine.
[Cl2 + H2O → 2HCl + O]; CH3CH2OH+ O → CH3CHO+ H2O
Ethyl alcohol Acetaldehyde
Acetaldehyde then reacts with chlorine to form chloral (Trichloro acetaldehyde).
CH3CHO+ 3Cl2 → CCl3CHO+ 3HCl
Acetaldehyde Chloral
[So Cl2 acts both as an oxidising and chlorinating agent]
Chloral, thus, formed, is hydrolysed by calcium hydroxide.
CCl CHO OHC CCl3
3 Hydrolysis → 2CHCl3 + (HCOO)2 Ca
H +
− O − Ca − O − H Chloroform Calcium formate
(b) From acetone : Acetone first reacts with chlorine to form trichloro acetone.
CH3 − CO − CH3 + 3Cl2 → CCl3COCH3 + 3HCl
Trichloro acetone
Trichloro acetone is then hydrolysed by calcium hydroxide.
CCl3 COCH3 H3C.CO CCl3 Hydrolysis → 2CHCl3 + (CH3COO)2 Ca
H + H Chloroform Calcium acetate
−O−Ca −O−
(ii) From carbon tetrachloride : Now-a-days, chloroform is obtained on a large scale by the reduction of
carbon tetrachloride with iron fillings and water. This method is used in countries like U.S.A.
CCl4 + 2H Fe /H2O → CHCl3 + HCl
This chloroform is not pure and used mainly as a solvent.
(iii) Pure Chloroform is obtained by distilling chloral hydrate with concentrated sodium hydroxide solution.
CCl3CH(OH)2 + NaOH → CHCl3 + HCOONa + H2O
Chloral hydrate
Note : Chloral hydrate is a stable compound inspite of the fact that two − OH Cl H O
CCll − C − C − HO
groups are linked to the same carbon atom. This is due to the fact that intramolecular H
hydrogen bonding exists in the molecule between chlorine and hydrogen atom of − OH
group.
(2) Physical properties
(i) It is a sweet smelling colourless liquid.
(ii) It is heavy liquid. Its density is 1.485. It boils at 61°C.
(iii) It is practically insoluble in water but dissolves in organic solvents such as alcohol, ether, etc.
(iv) It is non-inflammable but its vapours may burn with green flame.
(v) It brings temporary unconsciousness when vapours are inhaled for sufficient time.
(3) Chemical properties
(i) Oxidation : When exposed to sunlight and air, it slowly decomposes into phosgene and hydrogen
chloride.
Cl C Cl + [O] Light and air → Cl C Cl → HCl + Cl C=O
Cl H Cl OH Cl
Chloroform Phosgene
or CHCl3 + 1 O2 Light and air → COCl2 + HCl
2
Phosgene is extremely poisonous gas. To use chloroform as an anaesthetic agent, it is necessary to prevent
the above reaction. The following two precautions are taken when chloroform is stored.
(a) It is stored in dark blue or brown coloured bottles, which are filled upto the brim.
(b) 1% ethyl alcohol is added. This retards the oxidation and converts the phosgene formed into harmless
ethyl carbonate.
COCl2 + 2C2H5OH →(C2H5O)2 CO+ 2HCl
Ethyl carbonate
(ii) Reduction : When reduced with zinc and hydrochloric acid in presence of ethyl alcohol, it forms
methylene chloride.
CHCl3 + 2H Zn /HCl → CH 2Cl2 + HCl
(alc.)
When reduced with zinc dust and water, methane is the main product.
CHCl3 + 6H Zn /H2O → CH4 + 3HCl
(iii) Chlorination : Chloroform reacts with chlorine in presence of diffused sunlight or UV light to form
carbon tetrachloride.
CHCl3 + Cl2 UV light → CCl4 + HCl
Carbon tetrachloride
(iv) Hydrolysis : Chloroform is hydrolysed when treated with hot aqueous solution of sodium hydroxide or
potassium hydroxide. The final product is sodium or potassium salt of formic acid.
Cl + Na OH(aq.) OH O O
OH OH ONa
H −C Cl + Na OH(aq.) −NaCl → HC OH −H2O → H −C NaOH → H − C
− H 2O
Cl + Na OH(aq.)
Formic acid Sodium formate
Unstable
(Orthoformic acid)
[So, CHCl3 + 4KOH(aq.) Hydrolysis → HCOOK + 3KCl + 2H 2O ]
(v) Nitration : The hydrogen of the chloroform is replaced by nitro group when it is treated with concentrated
nitric acid. The product formed is chloropicrin or trichloronitro methane or nitro chloroform. It is a liquid, poisonous
and used as an insecticide and a war gas.
CHCl3 + HONO2 → CNO2Cl3 + H 2O
Nitric acid Chloropicrin (Tear gas)
(vi) Heating with silver powder : Acetylene is formed when chloroform is heated at high temperature with
silver powder.
H − C − Cl3 + 6 Ag + Cl3 − C − H → CH ≡ CH+ 6 AgCl
Acetylene
(vii) Condensation with acetone : Chloroform condenses with acetone on heating in presence of caustic
alkalies. The product formed is a colourless crystalline solid called chloretone and is used as hypnotic in medicine.
Cl3CH + O = C CH3 (NaOH) → HO C CH3
CH3 Cl3C CH3
Chloretone
(1,1,1- Trichloro- 2-methyl- 2- propanol)
(viii) Reaction with sodium ethoxide : When heated with sodium ethoxide, ethyl orthoformate is formed.
Cl + Na OC2 H5 OC2 H5
H −C Cl + Na OC2 H5 −3NaCl → H − C OC2 H5
Cl + Na OC2 H5 OC2 H5
Ethyl orthoformate
Halogen derivatives
(ix) Reimer-Tiemann reaction : Chloroform reacts with phenol when heated in presence of sodium
hydroxide or potassium hydroxide. The product formed is salicylaldehyde.
C6 H5OH + CHCl3 + 3NaOH 65°C → C6 H4 OH + 3 NaCl + 2H 2O
CHO
Hydroxy benzaldehyde
(x) Carbylamine reaction (Isocyanide test) : This reaction is actually the test of primary amines.
Chloroform, when heated with primary amine in presence of alcoholic potassium hydroxide forms a derivative
called isocyanide which has a very offensive smell.
RNH 2 + CHCl3 + 3KOH(alc.) ∆ → RNC + 3KCl + 3H2O
Carbylaminoalkane
(Alkyl isonitrile)
This reaction is also used for the test of chloroform.
(4) Uses
(i) It is used as a solvent for fats, waxes, rubber, resins, iodine, etc.
(ii) It is used for the preparation of chloretone (a drug) and chloropicrin (Insecticide).
(iii) It is used in laboratory for the test of primary amines, iodides and bromides.
(iv) It can be used as anaesthetic but due to harmful effects it is not used these days for this purpose.
(v) It may be used to prevent putrefaction of organic materials, i.e., in the preservation of anatomical species.
(5) Tests of chloroform
(i) It gives isocyanide test (Carbylamine test).
(ii) It forms silver mirror with Tollen's reagent.
(iii) Pure Chloroform does not give white precipitate with silver nitrate.
Iodoform or tri-iodomethane, CHI3
Iodoform resembles chloroform in the methods of preparation and properties.
(1) Preparation
(i) Laboratory preparation : Iodoform is prepared in the laboratory by heating ethanol or acetone with
iodine and alkali.
Ethanol : CH3CH2OH + I2 → CH3CHO+ 2HI
Acetaldehyde
CH3CHO + 3I2 → CI3CHO+ 3HI
Iodal
CI3CHO + KOH → CHI3 + HCOOK
Tri −iodoacetaldehyde Iodoform Pot. formate
Acetone : CH3COCH3 + 3I2 → CI3COCH3 + 3HI
Tri − iodoacetone
CI3COCH3 + KOH → CHI3 + CH3COOK
Iodoform Pot. acetate
Sodium carbonate can be used in place of KOH or NaOH. These reactions are called iodoform reactions.
(ii) Industrial preparation : Iodoform is prepared on large scale by electrolysis of a solution containing
ethanol, sodium carbonate and potassium iodide. The iodine set free, combine with ethanol in presence of alkali to
form iodoform. The electrolysis carried out in presence of CO2 and the temperature is maintained at 60-70°C.
KI ⇌ K+ + I− Anode
Cathode
K+ + e− → K 2I − → I 2 + 2e −
K + H2O → KOH + 1 H2
2
KOH is neutralised by CO2 : C2H5OH + 4I2 + 3Na2CO3 → CHI3 + HCOONa + 5NaI + 3CO2 + 2H2O
(2) Properties
(i) It is a yellow crystalline solid.
(ii) It has a pungent characteristic odour.
(iii) It is insoluble in water but soluble in organic solvents such as alcohol, ether, etc.
(iv) It has melting point 119°C. It is steam volatile.
(3) Reactions of iodoform KOH HCOOK
Hydrolysis Potassium formate
Reduction CH2I2
Red P/HI
Methylene iodide
CHI3 Heating CH ≡ CH
Ag powder
Acetylene
Carbylamine reaction
C6H5NH2+KOH (alc.) C6H5 NC
Phenol isocyanide
Heating alone Iodine vapours, 4CHI3+3O2 → 4CO + 6I2 + 2H2O
With AgNO3 (Less stable than CHCl3)
Yellow precipitate of AgI
(This reaction is not given by chloroform)
(4) Uses : Iodoform is extensively used as an antiseptic for dressing of wounds; but the antiseptic action is
due to the liberation of free iodine and not due to iodoform itself. When it comes in contact with organic matter,
iodine is liberated which is responsible for antiseptic properties.
(5) Tests of iodoform
(i) With AgNO3 : CHI3 gives a yellow precipitate of AgI .
(ii) Carbylamine reaction : CHI3 on heating with primary amine and alcoholic KOH solution, gives an
offensive smell of isocyanide (Carbylamine).
(iii) Iodoform reaction : With I2 and NaOH or I2 and Na2CO3 , the iodoform test is mainly given by ethyl
OO
|| ||
alcohol (CH3CH2OH), acetaldehyde (CH3 − C− H), α-methyl ketone or 2-one (− C− CH3), secondary alcohols or
2-ol (−CHOH ⋅ CH3) and secondary alkyl halide at C2(−CHCICH3) . Also lactic acid ( CH3 − CHOH − COOH) ,
OO
|| ||
Pyruvic acid (CH3 − C− COOH) and methyl phenyl ketone (C6H5 − C− CH3) give this test.
Tetra-halides (Carbon tetrachloride or tetrachloromethane, CCl4).
It is the most important tetrahalogen derivative of methane.
(1) Manufacture
(i) From methane : Chlorination of methane with excess of chlorine at 400°C yields impure carbon
tetrachloride.
CH4 + 4Cl2 400°C → CCl4 + 4 HCl
Methane used in this process is obtained from natural gas.
(ii) From carbon disulphide : Chlorine reacts with carbon disulphide in presence of catalysts like iron,
iodine, aluminium chloride or antimony pentachloride.
CS2 + 3Cl2 → CCl4 + S2Cl2
Sulphur
monochloride
S2Cl2 further reacts with CS2 to form more of carbon tetrachloride.
CS2 + 2S2Cl2 → CCl4 + 6S
Carbon tetrachloride is obtained by fractional distillation. It is washed with sodium hydroxide and then
distilled to get a pure sample.
(iii) From propane : Propane is reacted with chlorine at about 400°C and at a pressure of 70-100
atmosphere.
C3 H8 + 9Cl2 Heat → CCl4 + C2Cl6 + 8 HCl
Pressure
Carbon tetrachloride Hexachloroethane
(Solid)
(Liquid)
(2) Physical properties
(i) It is a colourless liquid having characteristic smell.
(ii) It is non-inflammable and poisonous. It has boiling point 77°C.
(iii) It is insoluble in water but soluble in organic solvents.
(iv) It is an excellent solvent for oils, fats, waxes and greases.
(3) Chemical properties : Carbon tetrachloride is less reactive and inert to most organic reagents. However,
the following reactions are observed.
(i) Reaction with steam (Oxidation) : Carbon tetrachloride vapours react with steam above 500°C to form
phosgene, a poisonous gas.
CCl4 + H2O 500°C → COCl2 + 2HCl
Phosgene (Carbonyl chloride)
(ii) Reduction : It is reduced by moist iron filling into chloroform.
CCl4 + 2H Fe /H2O → CHCl3 + HCl
(iii) Hydrolysis : On heating with aqueous potassium hydroxide it forms carbon dioxide which combines with
potassium hydroxide to give KCl and potassium carbonate (Inorganic salts).
CCl4 + 4 KOH −4KCl →[C(OH)4 ] −2H2O → CO2 2KOH → K2CO3 + H2O
Unstable
(iv) Reaction with phenol (Reimer-tiemann reaction) : It combines with phenol in presence of sodium
hydroxide to form salicylic acid.
C6 H5OH + CCl4 +4 NaOH → C6 H4 OH + 4 NaCl + 2H2O
COOH
Salicylic acid
(4) Uses
(i) It is used as a fire extinguisher under the name pyrene. The dense vapours form a protective layer on the
burning objects and prevent the oxygen or air to come in contact with the burning objects.
(ii) It is used as a solvent for fats, oils, waxes and greases, resins, iodine etc.
(iii) It finds use in medicine as helmenthicide for elimination of hook worms.
Unsaturated halides (Halo-alkene).
Vinyl chloride or chloroethene, CH2=CHCl
(1) Synthesis : Vinyl chloride can be synthesised by a number of methods described below:
(i) From ethylene chloride : It is easily prepared in the laboratory by the action of dilute alcoholic solution
of potassium hydroxide on ethylene chloride.
CH 2 Cl + Alc. KOH CHCl + KCl + H 2O
| ||
CH2Cl CH2
Ethylene chloride Vinyl chloride
Vinyl chloride can also be obtained from ethylene chloride by thermal decomposition at 600-650°C.
CH 2 Cl CHCl + HCl
| ||
CH2Cl
CH2
(ii) From ethylene : Free radical chlorination of ethylene at 500°C yields vinyl chloride.
CH2 = CH2 + Cl2 500°C → CH2 = CHCl
Vinyl chloride
(iii) From acetylene : Vinyl chloride is obtained by controlled addition of HCl on acetylene. Acetylene is
passed through dilute hydrochloric acid at about 70°C in presence of HgCl2 as a catalyst to form vinyl chloride.
This method is also used for its manufacture.
CH ≡ CH + HCl HgCl2 → CH2 = CHCl
70°C Vinyl chloride
(2) Properties : It is a colourless gas at room temperature. Its boiling point is –13°C. The halogen atom in
vinyl chloride is not reactive as in other alkyl halides. However, C = C bond of vinyl chloride gives the usual
addition reactions.
The non-reactivity of chlorine atom is due to resonance stabilization. The lone pair on chlorine can participate
in delocalization (Resonance) to give two structures.
CH 2 = CH − .. ←→ C− H2 − CH = C+ l
Cl
(i) (ii)
The following two effects are observed due to resonance stabilization.
(i) Carbon-chlorine bond in vinyl chloride has some double bond character and is, therefore, stronger than a
pure single bond.
(ii) Carbon atom is sp2 hybridized and C − Cl bond length is shorter (1.69Å) and stronger than in alkyl
halides (1.80Å) due to sp3 hybridization of the carbon atom.
Addition reactions Br2 CH2Br – CHBrCl
CH2 = CHCl CCl4
HBr 1,2-Dibromo-1-Chloroethane
Polymerisation CH3 – CHBrCl
Peroxide
NaOH 1-Bromo-1-Chloroethane
Cl Cl
− | | −
CH 2 − H − CH2 − H
C C
Polyvinyl chloride (PVC)
n
No reaction
(3) Uses : The main use of vinyl chloride is in the manufacture of polyvinyl chloride (PVC) plastic which is
employed these days for making synthetic leather goods, rain coats, pipes, floor tiles, gramophone records,
packaging materials, etc.
Allyl iodide or 3-iodopropene-1, ICH2CH = CH2
(1) Synthesis : It is obtained,
(i) By heating allyl chloride with sodium iodide in acetone. Allyl chloride required in the reactions is prepared
either by chlorination of propene at 500°C or by action of PCl3 on allyl alcohol.
CH3CH = CH2 + Cl2 500°C → C H2 − CH = CH2
Propene
|
Cl
Allyl chloride
Or 3 C H 2 − CH = CH 2 + PCl 3 Heat → 3 C H 2 − CH = CH 2 + H 3 PO3
| |
OH Cl
Allyl alcohol
C H 2 − CH = CH 2 + NaI Acetone → C H 2 − CH = CH 2 + NaCl
Heat
| |
Cl I
Allyl chloride Allyl iodide
This is halogen- exchange reaction and is called Finkelstein reaction.
(ii) By heating glycerol with HI.
C H2OH C H 2 I C H 2 I
| + 3HI −3H2O → | Heat → |
C HOH C HI −I2 CH
||
| |
CH2OH CH 2
Glycerol CH 2 I
1,2,3-Tri-iodopropane Allyl iodide
(2) Properties : It is a colourless liquid. It boils at 103.1°C.The halogen atom in allyl iodide is quite reactive.
The p-orbital of the halogen atom does not interact with π-molecular orbital of the double bond because these are
separated by a saturated sp3 -hybridized carbon atom. Thus, the halogen atom in allyl halides can be easily
replaced and the reactions of allyl halides are similar to the reaction of alkyl halides.
In terms of valence bond approach, the reactivity of halogen atom is due to ionisation to yield a carbonium
ion which can stabilize by resonance as shown below,
CH2 = CH − CH2I →[CH2 = CH − C+ H2 ←→ C+ H2 − CH = CH2] + I −
Substitution reactions : Nucleophilic substitution reactions occur,
CH2 = CHCH2I NaOH CH2 = CH – CH2OH
KCN Allyl alcohol
NH3 CH2 = CH – CH2CN
CH3ONa Allyl cyanide
CH2 = CH – CH2NH2
Allyl amine
CH2 = CH – CH2OCH3
Allyl methyl ether
AgNO2 CH2 = CH – CH2NO2
3-Nitropropene-1
Addition reactions : Electrophilic addition reactions take place in accordance to Markownikoff's rule.
CH2 = CH − CH2I + Br2 → CH2Br ⋅ CHBr ⋅ CH2I ; CH2 = CH − CH2I + HBr → CH3CHBrCH2I
1,2 − Dibromo - 3 - iodopropane 2 − Bromo -1- iodopropane
Allyl iodide is widely used in organic synthesis.
Halo-arenes.
In these compounds the halogen is linked directly to the carbon of the benzene nucleus.
(1) Nomenclature : Common name is aryl halide IUPAC name is halo-arene.
Example : Cl Br
;
Benzylchloride Benzylbromide
Chlorobenzene Bromobenzene
(2) Structure
Sidewise overlapping possible
CC
C C Cl
CC Sidewise overlapping possible
Chlorobenzene
(3) Methods of preparation
(i) By direct halogination of benzene ring
+ X2 Lewis acid → X + HX Lewis acid = FeX 3 , AlX3 ,Tl(OAC)3 ; X 2 = Cl2 , Br2
(ii) From diazonium salts CuCl C6H5Cl
CuBr C6H5Br
C6H5I
KI C6H5F
H⊕BF4
C6 H5 NH 2 NaNO2, HCl → C6 H 5 N ⊕ −
5°C 2
Cl
(iii) Hunsdiecker reaction : C6H5COO− Ag + Br2 → C6H5Br + CO2 + AgBr
(iv) From Aryl thalium compound : ArH + Tl(OOCCF3)3 −CF3CO2H → ArTl(OOCF3)2 KI → ArI
∆
Aryl thallium trifluoroacetate
(4) Physical properties
(i) Physical state : Haloarenes are colourless liquid or crystalline solid.
(ii) Solubility : They are insoluble in water, but dissolve readily in organic solvents. Insolubility is due to
inability to break hydrogen bonding in water. Para isomer is less soluble than ortho isomer.
(iii) Halo-arenes are heavier than water.
(iv) B.P. of halo-arenes follow the tend. Iodo arene > Bromo arene > Chloro arene.
(5) Chemical properties
Inert nature of chlorobenzene : Aryl halides are unreactive as compared to alkyl halides as the halogen
atom in these compounds is firmly attached and cannot be replaced by nucleophiles. Such as OH − , NH − , CN − etc.
2
..
: Cl: Cl⊕ Cl⊕ Cl⊕
Thus delocalization of electrons by resonance in aryl halides, brings extra stability and double bond character
between C − X bond. This makes the bond stronger and shorter than pure single bond. However under vigorous
conditions the following nucleophilic substitution reactions are observed,
(i) Nucleophilic displacement : C6 H5Cl NaOH,350°C → C6 H 5OH
500 atm.
(ii) Electrophilic aromatic substitution
Cl Cl Cl
+ HNO3 H2SO4 NO2 +
Cl Cl Cl Cl NO2 Cl Cl
+ Br ; CH3 +
Br2 /Fe CH3Cl
AlCl3 CH3
Br
(iii) Wurtz – fittig reaction : C6 H5 Br + CH3Br Na → C6 H5CH3 + 2NaBr
Ether
(iv) Formation of grignard reagent : C6 H5 Br Mg → C6 H5 MgBr
Ether
(v) Ullmann reaction
2 I Cu + CuI2
Some more important halogen derivatives.
(1) Freons : The chloro fluoro derivatives of methane and ethane are called freons. Some of the derivatives
are: CHF2Cl (monochlorodifluoromethane), CF2Cl2 (dichlorodifluoromethane), HCF2CHCl2 (1,1-dichloro-2,2-
difluoroethane). These derivatives are non-inflammable, colourless, non-toxic, low boiling liquids. These are stable
upto 550°C. The most important and useful derivative is CF2Cl2 which is commonly known as freon and freon-12.
Freon or freon-12 (CF2Cl2) is prepared by treating carbon tetrachloride with antimony trifluoride in the
presence of antimony pentachloride as a catalyst. 3CCl4 + 2SbF3 SbCl5 → 3CCl 2 F2 + 2SbCl3
Catalyst
Or it can be obtained by reacting carbon tetrachloride with hydrofluoric acid in presence of antimony
pentafluoride.
CCl4 + 2HF SbF5 → CCl2F2 + 2HCl
Under ordinary conditions freon is a gas. Its boiling point is –29.8°C. It can easily be liquified. It is chemically
inert. It is used in air-conditioning and in domestic refrigerators for cooling purposes (As refrigerant). It causes
depletion of ozone layer.
(2) Teflon : It is plastic like substance produced by the polymerisation of tetrafluoroethylene (CF2 = CF2) .
Tetrafluoroethylene is formed when chloroform is treated with antimony trifluoride and hydrofluoric acid.
CHCl3 SbF3 → CHF2Cl 800°C → CF2 = CF2
HF − HCl (b.pt.-76°C)
On polymerisation tetrafluoroethylene forms a plastic-like material which is called teflon.
nCF2 = CF2 →(−CF2 − CF2 −)n
Tetrafluoroethylene Teflon
Teflon is chemically inert substance. It is not affected by strong acids and even by boiling aqua-regia. It is
stable at high temperatures. It is, thus, used for electrical insulation, preparation of gasket materials and non-sticking
frying pans.
(3) Acetylene tetrachloride (Westron), CHCl2∙CHCl2 : Acetylene tetrachloride is also known as sym.
tetrachloroethane. It is prepared by the action of chlorine on acetylene in presence of a catalyst such as ferric
chloride, aluminium chloride, iron, quartz or kieselguhr.
CH ≡ CH + 2Cl2 → CHCl2 ⋅ CHCl2
(1,1,2,2− Tetrachloroethane)
In absence of catalyst, the reaction between chlorine and acetylene is highly explosive producing carbon and
HCl. The reaction is less violent in presence of a catalyst.
It is a heavy, non-inflammable liquid. It boils at 146°C. It is highly toxic in nature. Its smell is similar to
chloroform. It is insoluble in water but soluble in organic solvents.
On further chlorination, it forms penta and hexachloroethane. On heating with lime (Calcium hydroxide), it is
converted to useful product westrosol (CCl2 = CHCl) .
2CHCl2 − CHCl2 + Ca(OH)2 → 2CHCl = CCl2 + CaCl2 + 2H 2O
Westron Westrosol
(Trichloroethene)
Both westron and westrosol are used as solvents for oils, fats, waxes, resins, varnishes and paints, etc.
(4) p-Dichlorobenzene : It is prepared by chlorination of benzene.
It is a white, volatile solid having melting point of 325 K, which readily sublimes. It resembles chlorobenzene
in their properties.
It is used as general insecticides, germicide, soil fumigant deodorant. It is used as a larvicide for cloth moth
and peach tee borer.
(5) DDT; 2, 2-bis (p-Chlorophenyl) –1,1,1-trichloroethane : It is synthesised by heating a mixture of
chloral (1mol) with chlorobenzene (2mol) in the presence of concentrated H2SO4.
HH Cl H Cl
| | + H2O
CCl3 – C = O + Conc. H2SO4 Cl3C – C Cl
H Cl D.D.T.
Chloral Chlorobenzene
(1mol) (2mol)
Properties and uses of D.D.T.
(i) D.D.T. is almost insoluble in water but it is moderately soluble in polar solvents.
(ii) D.D.T. is a powerful insecticide. It is widely used as an insecticide for killing mosquitoes and other insects.
Side Effects of D.D.T. : D.D.T. is not biodegradable. Its residues accumulate in environment and its long
term effects could be highly dangerous. It has been proved to be toxic to living beings. Therefore, its use has been
abandoned in many western countries. However, inspite of its dangerous side effects, D.D.T. is still being widely
used in India due to non-availability of other cheaper insecticides.
(6) BHC (Benzene hexachloride), C6H6Cl6 : It is prepared by chlorination of benzene in the presence of
sunlight.
Cl
Cl Cl
+ 3Cl2 Sunlight
Cl Cl
Cl
Benzene BHC
Uses : It is an important agricultural pesticide mainly used for exterminating white ants, leaf hopper, termite,
etc. It is also known by the common name gammaxene or lindane or 666.
Note : aaaeee conformation of C6H6Cl6 is most powerful insecticide.
(7) Perfluorocarbons (PFCs) : Perfluorocarbons (CnF2n+2) are obtained by controlled fluorination of
vapourized alkanes diluted with nitrogen gas in the presence of a catalyst.
C7 H16 + 16F2 Vapourphase, N2,573K → C7 F16 + 16 HF
CoF2 (Catalyst)
Perfluoroheptane
These are colourless, odourless, non-toxic, non-corrosive, non-flammable, non-polar, extremely stable and
unreactive gases, liquids and solids. These are stable to ultraviolet radiations and other ionising radiations and
therefore, they do not deplete the ozone layer like freons.
These are good electrical insulators. These have many important uses such as :
(i) These are used as lubricants, surface coatings and dielectrics.
(ii) These are used as heat transfer media in high voltage electrical equipment.
(iii) These are used for vapour phase soldering, gross leak detection of sealed microchips etc. in electronic
industry.
(iv) These are also used in health care and medicine such as skin care cosmetics, wound healing, liquid
ventilation, carbon monoxide poisoning and many medical diagnosis.
Organometallic compounds.
Organic compounds in which a metal atom is directly linked to carbon or organic compounds which contain
at least one carbon-metal bond are called organometallic compounds.
Example : Methyl lithium → CH3Li ; Dialkyl zinc → R2Zn ; Alkyl magnesium halide → R − Mg − X
(1) Methyl lithium : CH3I + 2Li Ether → CH3Li + LiI
−10°C
Methyl iodide Methyl lithium
Note : High reactivity of CH3Li over grignard reagent is due to greater polar character of C − Li bond in
comparison to C − Mg bond.
Chemical properties
(i) CH3 − Li + H ⋅ OH → CH4 + LiOH
(ii) CH3 − Li + CH2 − CH2 → CH3CH2CH2OLi H2O → CH3CH2CH2OH + LiOH
O
O
||
(iii) CH 3 − Li + CO2 → CH 3 Li H2O → CH 3COOH + LiOH
− C− O −
(iv) CH3 − Li + H−C = O → CH3CH2 −O− Li H2O → CH3CH2OH + LiOH
|
H
Note : Unlike grignard reagents, alkyl lithium can add to an alkenic double bond.
R − Li + CH2 = CH2 → R − CH2 − CH2 − Li
(2) Dialkyl zinc : First organometallic compound discovered by Frankland in 1849.
Heat → 2R Heat →
2RI + 2Zn CO2 − Zn − I CO2 R2 Zn + ZnI 2
Dialkyl zinc
Chemical properties
Preparation of quaternary hydrocarbon : (CH3)3 CCl + (CH3)2 Zn →(CH3)4 C+ CH3ZnCl
Neopentane
(3) Grignard reagent : Grignard reagent are prepared by the action of alkyl halide on dry burn magnesium
in presence of alcohol free dry ether.
Dry ether dissolves the grignard reagent through solvolysis.
C2H5 R C2H5
| | |
:O: Mg :O:
|
| |
X
C2H5 C2H5
Grignard reagents are never isolated in free sate on account of their explosive nature.
Note : For given alkyl radical the ease of formation of a grignard reagent is, Iodide > Bromide >
Chloride
Usually alkyl bromides are used.
For a given halogen, the ease of formation of grignard reagent is, CH3 X > C2H5 X > C3H7 X..........
Since tertiary alkyl iodides eliminate HI to form an alkene, tertiary alkyl chlorides are used in their
place.
Grignard reagent cannot be prepared from a compound which consists in addition to halogen, some
reactive group such as − OH because it will react rapidly with the grignard reagent.
The C − Mg bond in grignard reagent is some what covalent but highly polar.
| Mδ +g X or δR−− Mδ +g X
− Cδ−
|
The alkyl group acts as carbanion. The majority of reaction of grignard reagent fall into two groups:
(i) Double decomposition with compound containing active hydrogen atom or reactive halogen atom
RMgX + HOH → RH + Mg(OH)X ; RMgX + D2O → RD + Mg(OD)X
RMgX + R' OH → RH + Mg(OR')X ; RMgX + R' NH2 → RH + Mg(R' NH)X
RMgX + R' I → R − R'+MgIX ; RMgX + ClCH2OR' → RCH2OR'+MgClX
(ii) Addition reaction with compounds containing C = O ; − C ≡ N, C = S etc.
C = O + RMgX → H OH C− OH + Mg OH
C− O MgX → X
|
|
R
R
N H OH Mg OH
−C ≡ + RMgX → − C = N MgX → − C = O+ NH 3 + X
| O H2 |
R
R
Hydroxy compounds and ethers
Introduction of hydroxy compounds.
Hydroxy compounds are compounds in which the hydroxy group, – OH is directly linked with the aliphatic or
aromatic carbon. Hydroxy compounds can be classified into following three categories.
(1) Aliphatic hydroxy compounds (alcohols) : Alcohols are regarded as hydroxy derivatives of
hydrocarbons.
R − H −H → R − OH (R = alkyl group)
Hydrocarbon +OH Alcohol
They can be mono, di or tri-hydric alcohols depending upon whether they contain one, two or three hydroxy
groups.
Monohydric alcohols Dihydric alcohol Trihydric alcohol Polyhydric alcohol
CH3OH C| H2OH CH2OH
CH2OH CHOH
Methanol |
| |
C2H5OH (CHOH)4
CH2OH CH2OH
Ethanol |
Glycol Glycerol
CH2OH
sorbital or Mannitol
(2) Aromatic hydroxy compounds (Phenols) : Phenols are regarded as hydroxy derivatives of aromatic
hydrocarbons (arenes).
Ar − H −H → Ar − OH
Arene +OH Phenol
They can be mono, di or tri-hydric phenols depending upon whether they contain one, two or three hydroxy
groups. OH OH OH OH
Monohydric phenols : CH3 m-cresol CH3 CH3
OH
Phenol o-cresol p-cresol
OH OH
OH
Dihydric phenols : OH OH
OH Quinol
Catechol Resorcinol
OH OH
OH OH
Trihydric phenols :
HO Phloroglucinol OH
OH
OH
Pyrogallol
Hydroxy quinol
(3) Aromatic alcohols : Compounds in which the hydroxy group is present in the side chain are termed
aromatic alcohols.
CH2OH CH2CH2OH
Benzyl alcohol 2 phenyl ethanol
Monohydric alcohols.
Alcohols containing one hydroxyl group are known as monohydric alcohols. These alcohols may be saturated
or unsaturated depending on the nature of hydrocarbon groups. Saturated monohydric alcohols form a homologous
series of the general formula Cn H 2n+1OH . They are also represented as R – OH where R represents an alkyl group.
They may be regarded as derivatives of water, i.e., one hydrogen atom of the water molecule is replaced by an alkyl
group.
HOH −H → R − OH
Water +R Alcohol
(1) Classification : Monohydric alcohols are subdivided into three classes
(i) Primary alcohols : In these alcohols, the hydroxyl group is attached with primary (1o ) carbon atom. They
possess a characteristic group − CH 2OH and their general formula is RCH 2OH . R may be H in the first member
and alkyl group in the rest of the members.
Examples : HCH 2OH ; CH 3CH 2OH ; CH 3CH 2CH 2OH ; CH 3 CH − CH 2OH
Ethyl alcohol n-propyl alcohol CH 3
Methyl alcohol isobutyl alcohol
(ii) Secondary alcohols : In these monohydric alcohols, the hydroxyl group is attached with secondary (2°)
carbon atom. They possess a characteristic group CHOH and the general formula R CHOH (R and R′ may be
R′
same or different).
Example : CH3OH CHOH ; C2 H 5 CHOH
CH3OH CH3
Isopropyl alcohol sec. butyl alcohol
(iii) Tertiary alcohols : In these monohydric alcohols, the hydroxyl group is attached with tertiary (3°) carbon
R′′
atom. They contain a characteristic group COH and have the general formula R′ OH (R, R′ and R′′ may be
R
same or different).
CH3 CH3
||
Examples : CH 3 − C− OH ; C2 H 5 C− OH
| −
|
CH3 CH3
tert. Butyl alcohol tert. Amyl alcohol
(2) Nomenclature
(i) Common system : Named as alkyl alcohol.
Note : In higher members, it is always indicated whether the – OH group is attached to primary,
secondary and tertiary carbon. By prefixing n for primary sec. for secondary and tert. for tertiary.
CH3
|
Example : CH 3CH 2OH ; CH 3 − CH 2 CH CH 3 ; CH3 C− OH
Ethyl alcohol − − −
| |
OH CH3
sec. butyl alcohol
tert. butyl alcohol
(ii) Carbinol system : CH 3OH is called carbinol. All other members are considered its alkyl derivatives.
Example : CH3OH ; CH 3 CHOH ; CH3 CHCH2OH
Carbinol CH 3 CH3
Di methyl carbinol Isopropyl carbinol
(iii) IUPAC system : Named as alkanol.
Example : CH 3OH ; CH3CH2CH2OH ; CH3CH2CH2CH2OH
Methanol Propanol -1 Butanol -1
(3) Structure : Oxygen of – OH group is bonded to sp3 hybrid carbon by a sigma bond.
H σ bond O H H ..
H H C –––– O O:
C sp3 sp3 |
HH 108.5
C – O – H = 108.5 H–C H
C – O bond length = 142 pm
HH |
H
(4) Isomerism
C|H3
(i) Chain isomerism : CH3CH2CH2CH2OH ↔ CH3 − CH− CH2OH
Butanol -1 2 Methyl propanol-1
(ii) Positional isomerism : CH 3CH 2CH 2OH ↔ CH 3 CH CH 3
Propanol-1
|
OH
Propanol - 2
(iii) Functional isomerism : CH3CH2OH ↔ CH3OCH3
Ethanol Methoxy methane
(5) General methods of preparation of monohydric alcohols
(i) From alkyl halide : C2H5 Br + KOH → C2 H 5 OH + KBr ; C2H5 Br + AgOH → C2H5OH+ AgBr
Ethanol Moist silver oxide Ethanol
Bromoethane (Aqueous) Bromoethane
Note : 1° alkyl halide gives good yield of alcohols.
2° alkyl halide gives mixture of alcohol and alkene.
3° alkyl halide gives alkenes due to dehydrohalogenation.
C| H3 C| H3
CH 3 − C− CH3 + KOH CH3 − C = CH2 + KBr H2O
| → 2 - Methylpropene +
Br (Aqueous) (Major product)
Oxidation number of carbon in different organic compounds are given below in increasing order,
Alkane,alkene,alkyneO, axlikdyal thioanlidneu,malbcoerhoofl,ccaarrbboonnyinl cinocmrepaosuinngdso,radceirds and acidderivatives →
Alkanes and alkenes can be converted into alcohols by oxidation method while carbonyl
compounds, acid and their derivatives can be converted into alcohol by reduction.
Oxidation no. of carbon in RX and ROH is same hence RX converts into R–OH by displacement
reaction.
(ii) From alkenes
(a) Hydration
Direct process : C=C HOH → − | |
Alkene
C− C−
||
OH H
Alcohol
Indirect process : CH 2 = CH 2 + HOSO2OH → CH3CH 2OSO2OH H2O → CH 3CH 2 OH + H 2SO4
Ethyl hydrogen sulphate Boil
Ethene Sulphuric acid Ethanol
In case of unsymmetrical alkenes
CH 3CH = CH 2 + HOSO2OH Markownikoff''s → CH 3 − CH − CH 3 H2O → CH 3 − CH − CH 3
Propene rule Boil
| |
OSO2OH OH
Propan- 2- ol
CH3 CH3 CH3
| | |
CH 2 H+ H2SO4 → CH 3 H2O → C− CH
− C = + − C− − 3
|
⊕ OH
(b) Oxymercuration-demercuration Alcohol
C=C + H2O + Hg(OAc)2 Oxymercuration → − | − | NaBH4 → − C− C−
Mercuric acetate C C− ||
| H|gOAcDemercuration
OH H
OH
Alcohol
This reaction is very fast and produces the alcohol in high yield. The alcohol obtained corresponds to
Markownikoff’s addition of water to alkene.
(c) Hydroboration oxidation (HBO) : (Antimarkownikoff’s orientation)
C=C +H−B → − | − | H2O2,OH− → − | |
C C− C− C−
|| ||
HB H OH
Alcohol
Diborane is an electron defficient molecule. It acts as an electrophile reacting with alkenes to form alkyl boranes
R3 B .
R − CH = CH 2 + H − BH 2 → R − CH − C H 2 RCH=CH2 → (R CH 2 CH 2 )2 BH RCH2 =CH2 → (RCH 2CH 2 )3 B
Dialkyl borane Trialkyl borane
| |
B H2
H
Alkyl borane
Other examples : (B2H6) (CH2 – CH2)3B H2O → CH2 – CH2OH
Antimarkownikoff's rule
1-Alkanol
CH = CH2 Hg(OCOCH3)2 CH –––– CH2 H + → CH – CH3
||
|
OCOCH3 HgOCOCH3 Markownikoff's rule 2-Alkanol
OH
H3O+ ⊕ CH2 – CH3 C⊕H – CH3 H2O → CH2 – CH3
OH
Less stable
Note : Carbocation are not the intermediate in HBO hence no rearrangement take place.
(iii) By reduction of carbonyl compounds : Bouveault Blanc
RCHO + H2 Pd → RCH 2 OH ; RCOR′+ H2 NaBH4 → R − CH − R′
LiAlH4 Ketone or Ni / Pt
Aldehyde Primary alcohol |
OH
Secondary alcohol
CHH3O+ 3⊕ ⊕ CH3 H2O → CH3 CH3
HO OH
CH2 CH2
B2H6 HO – CH2
CH2OH
LiAlH4 also reduces epoxides into alcohol : CH2 − CH2 + LiAlH4 → CH3 − CH2OH
O
Hydride selectively attacks the less alkylated carbon of the epoxide.
CH3 CH3
| |
LiAlH4 → CH C− CH
CH3 − C− CH2 H− H+ 3 − 3
O |
OH
(iv) By reduction of carboxylic acids and their derivatives
R − COOH (i) LiAlH4 → RCH 2OH ; RCOOH → RCOOR′ H2 → RCH 2OH + R′OH
(ii) H2O Ester Catalyst
Carboxylic acid primary alcohol Carboxylic acid
Esters are also reduced to alcohols
O
||
(Bouveault Blanc reaction) CH3 + 4[H] Na /C2H5OH → CH3CH2OH + CH3OH
− C− OCH3
Methyl acetate Ethanol Methanol
(Ester)
Note : Reduction with aluminium isopropoxide is known as Meerwein-Ponndorff verley reduction (MPV)
reduction.
Me 2 C = O + (CH3 )2 CHOH Al(OCHMe2) → Me 2 CHOH + CH 3 C=O
Isopropyl alcohol CH 3
OO
|| ||
(v) By hydrolysis of ester : R − C− O R′ + HO / Na(aq) → R − C− ONa + R′OH .
Sod. salt of acid Alcohol
(vi) From primary amines : CH3CH 2 NH 2 + HONO NaNO2/ HCl → CH3CH 2OH + N 2 + H 2O
Aminoethane Ethanol
Note : It is not a good method of preparation of alcohols because number of by product are formed like
alkyl chloride alkenes and ethers.
(vii) From Grignard reagent
(a) With oxygen : 2R − Mg − X + O2 ∆ → 2R − O − Mg − X 2HOH → 2ROH + 2Mg(X)OH
Al2O3
(b) With ethylene oxide
Rδ − − Mgδ + − X δ+ δ+ → RCH 2CH 2 − OMgX H2O → RCH 2CH 2OH + Mg(X)OH
+ CH 2 − CH 2
Oδ −
Other examples
With cyclic ester
OO OMgBr OH OH OMgBr HO OH
CH3 C CH3
+ CH3Mg – Br → COH3 H2O COH3 CH3MgBr H2O
CH3
CH3
Cl Br + Mg → Cl Mg Br CH2O → Cl CH2OMg Br
Cl CH2OH Hydrolysis
H H H
|
(c) With carbonyl compounds : δR− ← δ+ X + R′ − | H2O → R′ − | − R
Cδ +
Mg− → R′ − C− R C
|| | |
Oδ − OMgX OH
Note : If R′ = H, product will be 1°alcohol.
If R′ = R, product will be 2°alcohol.
If carbonyl compound is ketone, product will be 3° alcohol.
It is the best method for preparation of alcohol because we can prepare every type of alcohols.
(viii) The oxo process : It is also called carbonylation or hydroformylation reaction. A mixture of alkene
carbon monoxides and hydrogen. Under pressure and elevated temperature in the presence of catalyst forms
aldehyde.
Catalyst is cobalt carbonyl hydride [CoH(CO)4 ] product is a mixture of isomeric straight chain (major) and
branched chain (minor) aldehydes. Aldehydes are reduced catalytically to the corresponding alcohols.
2CH 3 − CH = CH 2 + 2CO + 2H 2 → CH 3 − CH 2 − CH 2 − CHO CH3 – CH – CHO
|
CH3 CH3 – CH – CH2OH
|
CH3
H2 CH3 – CH2 – CH2 – CH2 – OH
Zn – Cu
(6) Physical properties of monohydric alcohols
(i) Character : Alcohols are neutral substances. These have no effect on litmus paper. This is analytical test for
alcohols.
(ii) Physical state : The lower alcohols (upto C12) are colourless alcohol with characteristic smell and burning
taste. The higher members with more than 12-carbon atoms are colourless and odourless solids.
(iii) Polar character : Oxygen atom of the – OH group is more electronegative than both carbon and
hydrogen. Thus the electron density near oxygen atom is slightly higher. Hydrogen bonding shown below
H − O- - - -H − O- - - H − O- - - -H − O . This gives polar character to OH bond.
| || |
R RR R
(iv) Solubility : The lower alcohols are miscible in water.
H − O :δ − - - - - -δ + H − O : Solubility ∝ Size of 1 groups
|| alkyl
RH
Increase in carbon-chain increases organic part hence solubility in water decreases.
Isomeric 1°, 2°, 3° alcohols have solubility in order 1° > 2° > 3°.
(v) Boiling points : Due to intermolecular hydrogen bonding boiling points of alcohols are higher than
hydrocarbon and ethers.
B.P. ∝ No. of 1 ; B.P. follows the trends : 1° alcohols > 2° > 3° alcohol
branches
(vi) Density : Alcohols are lighter than water. Density ∝ Molecular masses.
(vii) Intoxicating effects : Methanal is poisonous and is not good for drinking purposes. It may cause
blindness and even death. Ethanal is used for drinking purposes.
(7) Chemical properties : Characteristic reaction of alcohol are the reaction of the – OH group. The reactions
of the hydroxyl group consists of either cleavage of C – O bond or the cleavage of O – H bond.
| → δO− ← Hδ +
− Cδ +
|
Polar bond
C – O bond is weaker in the case of tertiary alcohols due to +I effect of alkyl groups while – OH bond is
weaker in primary alcohols as electron density increase between O – H bond and hydrogen tends to separates as a
proton.
H weaker bond R CH − OH ; R
R
| R C−O−H
Secondary R weaker bond
R → C−O − H ;
| Tertiary
H
Primary
Thus primary alcohols give the most of reaction by cleavage of O – H bond while tertiary alcohols are most
reactive because of cleavage of C – O bond. Hence – O – H cleavage reactivity order : Primary > Secondary >
Tertiary and C – O – cleavage reactivity order : Tertiary > Secondary > Primary alcohol
(i) Reaction involving cleavage of with removal of ‘H’ as proton
Alcohols are stronger acids than terminal acetylene but are not acidic enough to react with aqueous NaOH or
KOH. Acidic nature is in the order HOH > ROH > CH ≡ CH > NH3 > RH .
Acidic nature of alcohol decrease with increase of alkyl groups on – OH bonded carbon due to +I (inductive)
effect of alkyl group.
H R R H
H > R → C↓ − O H > R → C↓ − O
|
| R↑
R → − C−O
| H
H
(a) Reaction with Na : (Active metals)
2RO − H + 2M → 2ROM + H 2 (M = Na, K, Mg, Al, etc.)
Evolution of H2 shows the presence of –OH and reaction show that alcohols are acidic in nature. Alcohols acts
as Bronsted acids because they donate a proton to a strong base (: B− ) .
.. ..
Example : R − O.. − H+ : B− → R − O.. :− + B−H
Base Conjugate acid
Alcohol Alkoxide
(acid) (conjugate base)
On reaction of alkoxide with water, starting alcohol is obtained.
.. ..
H O.. − H R O.. : R−O−H + OH −
− + → Conjugate base
Conjugate acid
Acid Base
This is the analytical test for alcohols. Conc. H2SO4 RCOOR′+ H2O
(b) Reaction with carboxylic acid [Esterification] : RCO− OH + H − OR′ Ester
acid Alcohol
When HCl gas is used as catalyst, the reaction is called fischer-speier esterification.
Presence of bulky group in alcohol or in acid decreases the rate of esterification. This is due to steric hinderence
of bulky group. Reactivity of alcohol in this reaction is 1o > 2o > 3o .
(c) Reaction with acid derivatives : (Analytical test of alcohol)
OO
|| ||
CH 3 − Cl + H− O − CH 2CH 3 Pyridine → CH3 − OCH2CH3 + HCl
C− C−
Ethanoyl chloride Ethanol Ethyl ethanoate
(Ethyl acetate)
OO O
|| || ||
Acylation : CH 3 − C− O − C− CH 3 + H − OCH 2CH 3 → CH 3 − C− OCH 2CH 3 + CH 3COOH
Acetic anhydride Ethyl ethanoate
(d) Reaction with grignard reagents : CH 3OH+ C2 H5 MgBr → C2 H6 + CH 3OMgBr
Methyl Ethyl magnesium Ethane
alcohol bromide
(e) Reaction with ketene : R − δO−− δH++ CH 2 = δC+ = δO− → CH 2 = C− O − R → CH 3 − C− O − R
||
|
H −O O
(Keto form)
(enol form)
(f) Reaction with isocyanic acid : R − δO−− δH++ H − N = δC+ → H − N = C − O − R → H − NH − C− O − R
|| | ||
Oδ − OH O
amino ester
(Urethane)
(g) Reaction with ethylene oxide : R−O−H + CH2 − CH2 → CH 2 − CH 2 ROH → CH2 − CH2
O | | −H2O ||
OH OH
OR OR
1, 2-dialkoxyethane
(h) Reaction with diazomethane : R − OH + CH 2 N 2 → R − O − CH3 + N 2
(Ether)
(i) Alkylation : ROH + R′2SO4 → ROR′ + R′HSO4
||
(ii) Reaction involving cleavage of − C − OH with removal or substitution of –OH group
||
(a) Reaction with hydrogen halides : Alcohols give alkyl halide. The reactivity of HX is in the order of
HI > HBr > HCl and the reactivity of ROH is in the order of allyl, benzyl > 3° > 2°> 1°. The reaction follows a
nucleophilic substitution mechanism.
Grove’s process : ROH + HX ZnCl2 → R − X + H 2O
anhydrous ∆
If alcohols react with HI and red phosphorus, alkane will be formed.
C2 H5OH + 2HI RedP → C2 H 6 + I2 + H2O
heat
Primary alcohols follow SN 2 mechanism . R − OH + + X− → δ−X --- R - - - OH δ+ → R− X + H2O
2 2
Protonated
1o alcohol
In secondary and tertiary alcohols, the SN1 mechanism operates
R − OH H+ R − OH2+ –H2O R+ X− → R − X
(b) Reaction with PCl5 : ROH + PX5 → RX + POX3 + HX ; X = Cl (Analytical test for alcohols)
(c) Reaction with PCl3 : 3ROH + PCl 3 → 3 RCl + H 3 PO3
Alcohol Alkyl
Phosphorus chloride Phosphorus
trichloride acid
(d) Reaction with thionyl chloride [SOCl2] : ROH + SOCl2 Pyridine → RCl + SO2 + HCl
(e) Reaction with ammonia : ROH + NH 3 Al2O3 → RNH 2 ROH → R2 NH ROH → R3 N
360o C Al2O3 Al2O3
Primary Secondary Tertiary
amine amine amine
(f) Reaction with HNO3 : R − OH + conc.HNO3 → R − O − N O H2O
O+
alkyl nitrite
Mechanism : HNO3 → H + + NO3−
R − O − H + H+ → R − O⊕ − H − H2O → R⊕ ; R⊕ + NO3 → R − O − N O
O
| alkyl nitrite
H
(g) Reaction with H2SO4 [Dehydration of alcohol] : The elimination of water from a compound is known as
dehydration. The order of case dehydration is Tertiary > Secondary > primary alcohol. The products of
dehydration of alcohols are depend upon the nature of dehydrating agents and temperature. ⊕
–(H+)
Conc. H2SO4
170 CH2 = CH2 CH3 OH CH3
Ethylene
CH 3 − CH 2 − OH H2SO4 → 110° CH3 | | | CH H+/H2SO4
H2SO4 C2H5HSO4 C
− C − C − − 3 Shifting +
Ethyl hydrogen sulphate |
| |
140°C C2H5O – C2H5 CH3 CH3 CH3
Diethyl ether
⊕
Alcohol leading to conjugated alkene are dehydrated to a greater extent than those of alcohols leading to
nonconjugated alkene. Thus dehydration is in order CH 2 = CH − CH − CH3 > CH3 − CH 2 − CH − CH 3
||
OH OH
CH3 CH3 CH3 CH3 / CH 3
| | || \ CH 3
CH 3 C CH CH H2SO4 → CH C− CH CH → CH 3 C CH − CH 3 −H+ → CH 3 −C = C
− − − 3 −H2O 3 − C| H3⊕ − 3 − − |
| | ⊕ CH3
CH3 OH
2-alkene
− CH 2 − CH = CH − CH 3
− CH 2 − CH − CH 2 − CH 3 H+ → − CH 2 − CH − CH 2 − CH 3
H 2 SO4
| ⊕
OH −H2O
− CH = CH − CH 2 − CH 3
More amount
(iii) General reaction of alcohols
(a) Reduction : R − OH + 2HI ∆ → R − H
(b) Oxidation : Difference between 1°, 2° and 3° alcohols.
1° → RCH 2OH → R−C =O→ R−C =O
| |
H OH
Aldehyde Carboxylic acid
2° → R − CH − R′ CrO3 → R − C− R′ O → RCOOH + CO2 + H2O
| || Drastic conditions
OH O
Secondary alcohol
CH3 CH3
| |
3° → CH3 C− OH 4[O] → O 4[O] → CH3COOH CO2 H2O
− CH3 − C = Acetic acid + +
| (Under strong Acetone (Under strong (Lesser number
CH3 condition) (Lesser number condition)
Tert. butyl alcohol of carbon atoms) of carbon atoms)
(Tertiary)
Note : 3° alcohols are resistant to oxidation, but on taking stronger oxidising agent they form ketone.
(c) Catalytic oxidation/dehydrogentaion
HH
| |
1° CH 3 O H Cu,573K → CH 3 − = O+ H2
C− C
| Ethanal
(Acetaldehyde)
H
Ethanol
(Pri. alcohol)
2° CH 3 C| H3 Cu,573K → CH 3 C| H3
− C−OH − C = O + H2
| Propanone
(Acetone)
H
2-Propanol
(Sec. alcohol)
CH3 CH3
| |
3° CH 3 Cu,573K → CH 3 − = CH 2 + H2O
− C− OH C
|
CH3 2-Methylpropene
(Alkene)
2- Methylpropan- 2- ol
(Tert. alcohol)
This is dehydration process and difference between 1°, 2° and 3°alcohols.
(d) Oxidation through Fenton’s reagent : [FeSO4 + H 2O2 ]
Mechanism
CH3 CH3
Fe +2 + H 2O2 → Fe +3 + OH − + O• H ; | O• H | C• H 2 +
CH 3 C CH H CH 3 H2O
− − 2 − + → − C−
| |
OH OH
CH3 CH3 CH3 CH3
| | | |
CH3 − C• H2 + CH 2 − C H3 → CH3 − CH2 − CH2 − CH3
C− C− C− C−
| | | |
OH OH OH OH
2, 5-dimethyl hexandiol - 2, 5
Important reagents used for oxidation of alcohols
≅ PCC [Pyridinium chloro chromate (C6 H5 N⊕ H Cl CrO3− ) ] to oxidise 1° alcohols to aldehydes and 2°
alcohols to ketones.
≅PDC [Pyridinium di chromate (C5 H5 . NH)22+ Cr2O72− ] to oxidise 1° alcohols to aldehyde and 2° alcohol to
ketones.
≅ H 2CrO4 (chromic acid) to oxidise 1° alcohol to carboxylic acid.
≅ CrO3 ⋅ H 2SO4 / Acetone to oxidise 2°alcohol to ketones.
≅Jones reagents (chromic acid in aqueous acetone solution) oxidise 1° alcohol to aldehyde and 2° alcohol to
ketone without affecting (C = C) double bond.
≅ MnO2 selectively oxidises the –OH group of allylic and benzylic 1° and 2° alcohols to give aldehyde and
ketone respectively.
≅ N 2O4 in CHCl3 (field’s) oxidises primary and secondary benzyl alcohol.
(e) Self condensation : Guerbet’s reaction
R
|
R − CH 2 − CH 2 − OH + H − CH − CH 2 − OH NaOC2H5, ∆ → R − CH 2 − CH 2 − OH
CH 2 − CH −
| higher alcohol
R
(f) Reaction with cerric ammonium nitrate : Cerric ammonium nitrate + ROH → Red colour solution of
Yellow colour
complex. This is analytical test for alcohols.
(g) Iodoform test : When a few drops of alcohol are warmed with iodine and KOH yellow precipitate of
iodoform with characteristic smell is obtained. Any alcohol consists CH3CHOH group give iodoform test.
Example : CH 3CH 2OH , CH 3 − CHOH − CH 3 , C6 H5 − CHOH − CH 3
Since reaction takes place with alkali solution as one of the reagents hence alkyl halide like CH3 − CH 2Cl and
CH 3 − CH− R will also give this test.
|
Cl
2NaOH + I 2 → NaI + NaOI + H 2O ; CH 3CH 2OH + 2NaOI → CH 3CHO + 2NaOH + 2HI
CH3CHO + 3NaOH → CI 3CHO + 3NaOH ; CI 3 − CHO + NaOH → CHI 3 + HCOONa
Iodofiorm
CI 3 − CHO + HCl → CHI 3 + HCOOH
Other example COONa
CHOH – CH3 COCH3
(O) KOH/I2 CHI3 +
OO O O
COCI3 HCl COOH
CHOHCH3 I2/NaOH COCH3 I2 + CHI3
H2O
(8) Individual members of monohydric alcohols
Methanol
(i) Preparation WOOD
(a) By destructive distillation Destructive distillation
Volatile Non-volatile residue
products (Charcoal)
Passed into condenser
Uncondensed gases Distillate
(Wood gas) allowed to settle
Lower coloured Upper layer
layer (Wood tar) (Pyroligneous acid)
Acetic acid 10%, methanol 2.5%
and acetone 0.5%
Passed through milk of
lime and distilled
Distillate Residue
(Methanol and acetone) (Calcium acetate)
Fractionally distilled Distilled with
H2SO4
Acetone (impure) Methanol (impure) CH3COOH
(b.pt. 56° C) (b.pt. 64.5° C)
Purified by Acetic acid
Purified by anhydrous
sodium bisulphite Calcium chloride
(b) From water gas : C + H 2O 1300oC → CO + H 2 ; CO + 2H 2 ZnO+Cr2O3 → CH 3OH
water gas 300o C
Compressed gases Methyl alcohol
(c) From natural gas : 2CH4 + O2 → 2CH3OH
(9:1) by volume
(ii) Physical properties
(a) It is a colourless liquid and boils at 64.5° C.
(b) It is miscible in water, good solvent for fats and oils.
(c) It is inflammable and burns with a faint luminous flame.
(d) It has pleasant smell and burning taste.
(iii) Uses
(a) For the manufacture of formaldehyde and formaline. CH 3OH K2Cr2O7 → CH 2O + H 2O
(b) A 20% mixture of methyl alcohol and gasoline is a good motor fuel.
(c) Use as an antifreeze for automobile radiators.
(d) To denaturate ethyl alcohol, mixture is called methylated spirit.
(e) In the preparation of dyes, medicines and perfumes.
Ethanol
(i) Preparation
(a) From Ethylene : C2 H 4 + H 2O H3PO4 → C2 H 5 OH Yield of ethyl alcohol is 95%.
300o C,70atm
(b) From acetylene : CH H2O H2SO4 (40%) → CH 2 → CH 3CHO
||| + ||
CH 1% HgSO4 ,60o C CHOH Acetaldehyde
Acetylene Vinyl alcohol
(Unstable)
CH 3 CHO + H2 Ni → C2 H 5 OH
110−140o C Ethyl alcohol
(c) Fermentation : This word is given by Lebeig. Fermentation process is exothermic. Ethanol is prepared by
molasses and invert sugar molasses is the waste product in sugar industry. It is a mixture of sugar (30%) and invert
sugar (32-40%) combine form of glucose and fructose called invert sugar. Temperature should be 25-30°C. low
concentration (8-10%) is favourable.
C12 H 22O11 + H2O yeast cell → C6 H12O5 + C6 H12O6 ; C6 H12O6 yeast cell → C2 H 5 OH + CO2 + H2O
glucose fructose
invertase enzyme zymase enzyme
Certain amounts of inorganic compound such as ammonium sulphate, phosphate should be added. Oxygen is
necessary for the growth of ferments. Boric acid, mercury salts etc. should not be present in the solution as these
retards the fermentation.
(d) From starch : 2(C6 H10O5 )n + nH 2O diastase → nC12 H 22O11 ; C12 H 22O11 + H 2O maltose → 2C6 H12O6
starch maltose (wort) maltose glucose
2C6 H12O6 zymase → C2 H6OH + CO2 + H2O + energy
By this method ethanol prepared is 10-12% called wash. Raw spirit Fractional → C2 H5OH + H2O
distillation 95.5%
4.5%
rectified spirit
Manufacturing of ethyl alcohol (absolute) from rectified spirit called Azeotrophic distillation.
• Absolute alcohol is prepared by distillating rectified spirit with benzene.
• The first fraction consists of constant boiling mixture of ethyl alcohol, water and benzene [18.5% + 7.4%
+ 74.1%] and distills over at 64.8°C.
• The second fraction consists of a boiling mixture of ethyl alcohol and benzene [32.4% + 67.6%] and
distills at 68.2o C .
• Pure alcohol distills at 78.3o C and known as absolute alcohol (100%).
(ii) Properties : Same as monohydric alcohols.
(iii) Uses
(a) In the manufacture of alcoholic beverages.
(b) As a preservative for biological specimens.
(c) As a low freezing and mobile liquid in scientific apparatus such as thermometers and spirit levels.
(d) In hospitals as an antiseptic.
(e) As a petrol substitute (power alcohol).
(9) Interconversion of monohydric alcohols
(i) Primary alcohol into secondary alcohols
C3 H7 OH SOCl2 → C3 H7 Cl alc KOH → CH 3 CH = CH 2 HBr → CH 3 CH CH 3 aq. KOH → CH 3 CH CH 3
Propan-1-ol Propene
(1° alcohol) | |
Br OH
Propan- 2- ol
(2° alcohol)
(ii) Secondary alcohol into tertiary alcohol
OH O OMgBr OH
| || | |
[O] → CH 3 CH3MgBr → CH 3 C CH H + ,H2O → CH 3 CH 3
CH 3 − CH − CH 3 − C− CH 3 − − 3 − C−
Propan- 2- ol K2Cr2O7 / H + |
(Iso-propyl alcohol) |
(2° alcohol) CH3 CH3
2- Methtylpropan- 2- ol
(3°) (tert. butyl alcohol)
(iii) Primary alcohol into tertiary alcohol
CH3 CH3 CH3 CH3
| | | |
H2SO4, Heat → CH CH 2 HBr→ CH C− CH aq. KOH → CH 3 C− CH
CH3 CH CH2 OH Dehydration 3 −C = 3 − 3 − 3
2-Methylpropan-1-ol (1°) Markownikoff's | |
(Iso butyl alcohol) rule Br OH
2-Methylpropan-2-ol (3°)
(tert. butyl alcohol)
(iv) Lower alcohol into higher alcohol (ascent of series)
CH 3 OH HI → CH3 I KCN → CH3CN 4(H) → CH 3 CH 2 NH 2 HONO → CH3CH 2OH
Methanol Reduction Ethanol
(1 carbon atom) (2 carbon atoms)
(v) Higher alcohol into lower alcohol [Descent series]
C2 H 5 OH K2Cr2O7, H+ → CH 3 COOH NaOH → CH3COONa NaOH +CaO → CH 4 Cl2 → CH3Cl aq. KOH → CH3OH
Ethanol [O] Heat Methanol
(2 carbon atoms) (one carbon atom)
(10) Alcoholic beverages : Liquors used for drinking purposes containing ethyl alcohol as the principal
constituent are called alcoholic beverages. Besides alcohol, these contain large amounts of water, colouring and
flavouring materials. Alcoholic beverages are of two types :
(i) Undistilled beverages : These are prepared from fruit juices and grains. Those prepared from grapes and
other fruit juices are known as wines. Wines contain 18-20% of ethyl alcohol and are used as such after
fermentation, i.e., with distillation. The natural wines when made stronger by the addition of rectified alcohol are
known as fortified wines.
Name Undistilled Percentage of alcohol Source
Beer 3-6 Barley
Cider 2-6 Apples
Wine (Champagne) 8-10 Grapes
Claret 7-13 Grape juice
Port and Sherry (Fortified) 14-24 Grape juice
(ii) Distilled beverages : These are prepared by the fermentation of molasses, barley, maize, etc. The
fermented liquor is then distilled. These contain a higher percentage of ethyl alcohol which may be as high as 50%.
Name Distilled Percentage of alcohol Source
Whisky 35-40 Malt
Rum 45-55 Molasses
Gin 40-45
40-45 Maize
Brandy 40-50 Grape juice
Cognac Grape juice
(11) Alcoholometry : The process of determining the percentage of alcohol in a given sample is known as
alcoholometry. An alcohol water mixture having specific gravity 0.91976 at 15°C and containing 57.1% of ethyl
alcohol by volume or 49.3% by mass is called proof-spirit. A sample having higher percentage of ethyl alcohol in
comparison to proof-spirit is referred to as over-proof (O.P.) and the one having lower alcohol content than proof-
spirit is known as under-proof (U.P.). Thus 15 U.P. means that 100 ml of the sample contains as much alcohol as 85
ml of proof spirit. Similarly, 15 O.P. means that 100 ml of the sample contains as much of alcohol as 115 ml of
proof spirit.
(12) Power alcohol : Alcohol used for the generation of power is called power alcohol. Generally it is a
mixture of 80% petrol and 20% absolute alcohol with cosolvent benzene. It is cheaply obtained from waste petrol.
(13) Methylated spirit : Ethyl alcohol containing 5 to 10% of methyl alcohol is known as methylated spirit or
denatured spirit. Denaturing can also be done by adding 0.5% pyridine, petroleum naptha, rubber distillate
(caoutchoucine) or CuSO4 .
(14) Toxicity of alcohols : The toxicity of alcohols is mainly due to their biological to oxidation takes place
in living organism. Methyl alcohol is highly toxic and its consumption causes, blindness and death. Ethyl alcohol is
non toxic but produces physiological effect disturbing brain activity on drinking. The commercial alcohol is made
unfit for drinking by mixing in it copper sulphate (which gives its colour) and pyridine (which makes it foul smelling
liquid). It is known as denaturation of alcohol.
(15) Distinguish between primary, secondary and tertiary monohydric alcohols
(i) Lucas test : A mixture of anhydrous ZnCl 2 + conc. HCl is called as Lucas reagent.
Primary R − CH2 − OH conc. HC /ZnCl2 anhy. → R − CH2 − Cl → ppt. appears after heating
Secondary
Tertiary −H2O
R2CH − OH conc. HCl /ZnCl2anhy. → R2 − CH − Cl → ppt. appears with in 5 minutes
− H 2O
R3C − OH ZnCl2 / HCl → R3C − Cl → ppt. appears immediately
(ii) Victor mayer test : Also known as RBW test. RBW → Red, Blue, White test.
Primary C2H5OH P +I2 → C2H5I AgNO2 → C2H5 NO2 HONO → CH3 − C− NO2 NaOH → CH3 − C− NO2
|| ||
NOH NONa
Nitrolic acid Sod. salt of nitrolic acid (Red colour)
Secondary (CH3 )2 CHOH P +I2 →(CH3)2 CHI HONO →(CH3)2 − C NO2 NaOH → No reaction (Blue colour)
|
NO
Tertiary (CH3)3 COH P+I2 →(CH3)3 Cl AgNO2 →(CH3)3 CNO2 HONO → No reaction (colourless)
Difference between methanol and ethanol
Methanol Ethanol
(i) When CH3OH is heated on Cu coil it gives formalin (i) It does not give formalin like smell.
like smell.
(ii) When CH3OH is heated with salicylic acid in H2SO4 (ii) No such odour is given.
(conc.) then methyl salicylate is formed which has odour
like winter green oil.
(iii) It does not give haloform or iodoform test. (iii) It gives haloform test
Dihydric alcohols.
These are compound containing two hydroxyl groups. These are dihydroxy derivatives of alkanes. Their
general formula is Cn H 2n+2O2 . The simplest and most important dihydric alcohol is ethylene glycol. They are
classified as α, β, γ….. glycols, according to the relative position of two hydroxyl groups. α is 1, 2 glycol, β is 1, 3
glycol.
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