(vi) Reaction with acetyl chloride or acetic anhydrides
NH2CONH2 + CH3COCl → NH2CONHCOCH3 + HCl
Acetyl chloride Acetyl urea (Ureide)
NH 2CONH 2 + (CH 3CO)2 O → NH 2CONHCOCH3 + CH 3COOH
Acetyl urea Acetic acid
(vii) Reaction with hydrazine
NH2CONH2 + H2 N.NH2 100oC → NH2CONH.NH2 + NH3
Urea Hydrazine Semicarbazide
(viii) Reaction with ethanol : H2 NCO NH2 + H OC2H5 heat → H2 NCOOC2H5 + NH3
Ethanol Urethane
NH 2 NHCl
(ix) Reaction with chlorine water : O = C + 2Cl2 → O = C + 2HCl
NH 2 NHCl
Urea Dichloro urea
(x) Dehydration : NH2CONH2 + SOCl2 → H2 N − C ≡ N + SO2 + 2HCl + H2O
(xi) Reaction with fuming sulphuric acid
NH2CONH2 + H2SO4 + SO3 → 2NH2SO3H + CO2
sulphamic acid
Oleum
(xii) Formation of cyclic ureides
NH H O O
|| ||
C2H5O C NH − C
CH2 PCl3 → O = C
O=C + CH2 + 2C2H5OH
NH − C
NH H C2 H 5 O C ||
O
Urea || Barbituric acid
O (Malonyl urea)
Diethyl malonate
NH H C2H5O CO PCl3 → O = C NH − C = O
O=C + + 2C2H5OH
NH − C = O
NH H C2H5O CO
Parabanic acid
Urea Diethyl oxalate (Oxalyl urea)
(xiii) Reaction with formaldehyde
O=C NH − H C2 H 5 O − CO CH → O = C NH − CO CH
+ NH − CH 3
NH − H HO − C 4 −Methyl urecil
CH3
CH2 = O + NH2CONH2 HCl → CH2(OH)NHCONH2 CH2=O →
Formaldehyde Monomethylol urea
CH 2 (OH)NHCONH(OH)CH 2 heat → Resin
Dimethylol urea
(Urea-Formaldehyde)
(4) Uses
(i) Mainly as a nitrogen fertilizer. It has 46.4% nitrogen.
(ii) In the manufacture of formaldehyde-urea plastic and semicarbazide.
(iii) As animal feed.
(iv) For making barbiturates and other drugs.
(v) As a stabilizer for nitrocellulose explosives.
(5) General Tests
(i) When heated with sodium hydroxide, ammonia is evolved.
(ii) When heated gently, it forms biuret which gives violet colouration with sodium hydroxide and a drop of
copper sulphate solution.
(iii) Its aqueous solution with concentrated nitric acid gives a white precipitate.
(iv) On adding sodium nitrite solution and dil. HCl (i.e., HNO2 ) to urea solution, nitrogen gas is evolved and
gives effervescence due to carbon dioxide.
Nitrogen containing compounds
The important nitrogen containing organic compounds are alkyl nitrites (RONO), nitro-alkanes (RNO2), aromatic
nitro compounds (ArNO2), alkyl cyanides (RCN), alkyl iso cyanides (RNC), amines (– NH2), aryl diazonium salts
(ArN2Cl), amides (– CONH2) and oximes (>C = N OH).
Alkyl nitrites and nitro alkanes.
Nitrous acid exists in two tautomeric forms.
H−O−N =O ⇌H − N O
Nitrite form O
Nitro form
Corresponding to these two forms, nitrous acid gives two types of derivatives, i.e., alkyl nitrites and nitro
alkanes.
R−O− N =O; R− N O
Alkyl nitrite O
Nitro alkane
It is important to note that nitro alkanes are better regarded as nitro derivatives of alkanes, while alkyl nitrites
are regarded as alkyl esters of nitrous acid.
(1) Alkyl nitrites : The most important alkyl nitrite is ethyl nitrite.
Ethyl nitrite (C2H5ONO)
(i) General methods of preparation : It is prepared
(a) By adding concentrated HCl or H2SO4 to aqueous solution of sodium nitrite and ethyl alcohol at very low
temperature (0°C).
NaNO2 + HCl → NaCl + HNO2
C2 H5OH + HNO2 → C2 H5ONO + H 2O
Ethyl nitrite
(b) From Ethyl iodide
C2H5I + KONO → C2 H5ONO + KI
Ethyl nitrite
Ethyl iodide Pot. nitrite
(c) By the action N 2O3 on ethyl alcohol.
2C2 H5OH + N 2O3 → 2C2 H5ONO + H 2O
(ii) Physical properties
(a) At ordinary temperature it is a gas which can be liquified on cooling to a colourless liquid (b.p.17°C) having
characteristic smell of apples.
(b) It is insoluble in water but soluble in alcohol and ether.
(iii) Chemical properties
(a) Hydrolysis : It is hydrolysed by aqueous alkalies or acids into ethyl alcohol.
C2 H5ONO + H 2O NaOH → C2 H5OH + HNO2
(b) Reduction : C2 H5ONO + 6H Sn → C2 H 5 OH + NH 3 + H2O
HCl
Small amount of hydroxylamine is also formed.
C2 H5ONO + 4H → C2 H5OH + NH 2OH
(iv) Uses
(a) Ethyl nitrite dialates the blood vessels and thus accelerates pulse rate and lowers blood pressure, so it is
used as a medicine for the treatment of asthma and heart diseases (angina pectoris).
(b) Its 4% alcoholic solution (known as sweet spirit of nitre) is used in medicine as a diuretic.
(c) Since it is easily hydrolysed to form nitrous acids, it is used as a source of nitrous acid in organic synthesis.
Note : Isoamyl nitrite is used as an antispasmodic in angina pectoris and as a restorative in cardiac
failure.
(2) Nitro alkanes or Nitroparaffins : Nitro alkanes are regarded as nitro derivatives of hydrocarbons.
(i) Classification : They are classified as primary, secondary and tertiary depending on the nature of carbon
atom to which nitro groups is linked.
RCH 2 NO2 ; R CHNO2 ; R C − NO2
R R
Primary nitro alkane R Tertiary nitro alkane
Secondary nitro alkane
(ii) General methods of preparation
(a) By heating an alkyl halide with aqueous alcoholic solution of silver nitrite
C2 H5 Br + AgNO2 → C2 H5 NO2 + AgBr
Some quantity of alkyl nitrite is also formed in this reaction. It can be removed by fractional distillation since
alkyl nitrites have much lower boiling points as compared to nitro alkanes.
(b) By the direct nitration of paraffins (Vapour phase nitration)
CH 3CH 3 + HONO2 (fuming) 400°C → CH 3CH 2 NO2 + H 2O
With higher alkanes, a mixture of different nitro alkanes is formed which can be separated by fractional
distillation.
(c) By the action of sodium nitrite on α-halo carboxylic acids
CH2ClOOH NaNO2 → CH2 NO2COOH heat → CH3 NO2 + CO2
− NaCl α – Nitro acetic acid Nitro methane
α – Chloro acetic acid
(d) By the hydrolysis of α–nitro alkene with water or acid or alkali (Recent method)
CH3 CH3
| |
CH 3 − HOH H+orOH– → CH 3 − = O + CH3 NO2
CO= HC2HNO2 + C Nitro methane
Acetone
2-Methyl, 1-nitro propene
(e) Tertiary nitro alkanes are obtained by the oxidation of t-alkyl amines with KMnO4.
R3CNH 2 KMnO4 → R3CNO2 + H 2O
(iii) Physical properties
(a) Nitro alkanes are colourless, pleasant smelling liquids.
(b) These are sparingly soluble in water but readily soluble in organic solvents.
(c) Their boiling points are much higher than isomeric alkyl nitrites due to polar nature.
(d) Again due to polar nature, nitro alkanes are excellent solvents for polar and ionic compounds.
Note : 1° and 2° - Nitro alkanes are known to exist as tautomeric mixture of nitro-form and aci-form.
CH3 − N = O CH2 = N − OH
O↓ O↓
(nitro-form) (aci-form)
(iv) Chemical properties
(a) Reduction : Nitro alkanes are reduced to corresponding primary amines with Sn and HCl or Fe and HCl or
catalytic hydrogenation using nickel as catalyst.
RNO2 + 6H → RNH 2 + 2H 2O
However, when reduced with a neutral reducing agent (Zinc dust + NH4Cl), nitro alkanes form substituted
hydroxylamines.
R – NO2 + 4 H Zn+NH4Cl → R − NHOH + H 2O
(b) Hydrolysis : Primary nitro alkanes on hydrolysis form hydroxylamine and carboxylic acid.
RCH 2 NO2 + H 2O HCl or 80%H2SO4 → RCOOH + NH 2OH
secondary nitro alkanes on hydrolysis form ketones.
2R2CHNO2 HCl → 2R2CO + N 2O + H 2O
Ketone
(c) Action of nitrous acid : Nitrous acid reacts with primary, secondary and tertiary nitro alkanes differently.
R − CH 2 + O = NOH −H2O → R −C = NOH NaOH → R −C = NONa
Nitrous acid | |
| NO2 NO2
NO2
Primary Nitrolic acid Red coloured sodium salt
R2 C H + HON = O −H2O → R2 C− NO Etheror → Blue colour
NaOH
| |
NO2
NO2
Seconary
Pseudo nitrol
Tertiary nitro alkanes do not react with nitrous acid.
(d) Thermal decomposition : R.CH 2 .CH 2 NO2 >300°C → R.CH = CH 2 + HNO2
moderately
On rapid heating nitro alkanes decompose with great violence.
CH 3 NO2 heat, Rapidly → 1 N2 + CO2 + 3 H2
2 2
(e) Halogenation : Primary and secondary nitro alkanes are readily halogentated in the α-position by
treatment with chlorine or bromine.
CH3 CH3
| |
CH 3 − NO2 Cl2 → CCl 3 NO2 ; Cl2+ NaOH → CH 3 NO2
NaOH CH 3 – C H – NO2 – C–
Chloropicrin or nitro chloroform (insecticide) 2−Nitropropane |
Cl
(f) Condensation with aldehyde : CH 3CHO + CH 3 NO2 → CH 3CH(OH)CH 2 NO2
β -Hydroxy nitropropane
(nitro alcohol)
(g) Reaction with grignard reagent : The aci-form of nitroalkane reacts with Grignard reagent forming alkane.
RCH = N OH + CH3MgI → CH4 + RCH = N OMgI
O O
Methane
Note : The nitrogen of –NO2 carrying a positive charge exerts a powerful – I effect and thus activates the
hydrogen atom of the α-carbon. Thus the important reactions of nitroalkanes are those which involve
α-hydrogen atom of primary and secondary nitroalkanes (tertiary nitroalkanes have no α-
hydrogen atom and hence do not undergo such type of reactions).
Acidic character :The α-hydrogen atom of primary and secondary nitroalkanes are weakly acidic
and thus can be abstracted by strong alkalies like aq. NaOH. Therefore, 1° and 2° nitroalkanes
dissolve in aq. NaOH to form salts. For examples.
CH3 − N+ O NaOH → Na + C− H 2 – N+ O ↔ H2C = N+ O N− a
O– I O O−
Thus 1° and 2° nitroalkanes are acidic mainly due to following two reasons,
(a) Strong electron withdrawing effect of the – NO2 group.
(b) Resonance stabilisation of the carbanion (I) formed after the removal of proton.
The aci-form of nitroalkanes is relatively more acidic because it produces relatively more conjugate base.
(v) Uses : Nitro alkanes are used,
(a) As solvents for polar substances such as cellulose acetate, synthetic rubber etc.
(b) As an explosive.
(c) For the preparation of amines, hydroxylamines, chloropicrin etc.
Distinction between Ethyl nitrite and Nitro ethane
Test Ethyl nitrite (C2H5ONO) Nitro ethane (C2H5NO2)
(Alkyl nitrite, RONO) (Nitro alkane, RNO2)
Boiling point
Low, 17°C Much higher, 115°C
Reduction Gives alcohol + hydroxyl amine or NH3. Gives corresponding primary amine.
with metal C2H5ONO + 4H → C2H5OH + NH2OH
and acid RONO + 6H → ROH + NH3 + H2O C2H5 NO2 + 6H → C2H5 NH2 + 2H2O
(Sn/HCl) or Readily hydrolysed to give corresponding RNO2 + 6H → RNH2 + 2H2O
alcohol and sodium nitrite
with LiAlH4. (decomposition).
C2H5ONO + NaOH → C2H5OH + NaNO2
Action of RONO + NaOH → ROH + NaNO2 Not decomposed, i.e., alcohols are not produced.
But it may form soluble sodium salt, because in
NaOH presence of alkali the nitro form changes into aci
form, which dissolves in alkalies to form sodium
(alkalies).
salt.
CH3 − CH = N OH NaOH → CH3 − CH = N ONa
O O
Action of No action with nitrous acid. Primary nitro alkanes forms nitrolic acid, which
dissolve in alkali to give red solution.
HNO2
(NaNO2+ Secondary nitro alkane yields pseudo-nitrol,
HCl) which dissolves in alkali to give blue solution.
Aromatic Nitro Compounds.
Aromatic nitro compounds are the derivatives of aromatic hydrocarbons in which one or more hydrogen atom
(s) of the benzene nucleus has been replaced by nitro (– NO2) group.
(1) Preparation
(i) Nitration (Direct method) : The number of – NO2 groups introduced in benzene nucleus depends upon
the nature and concentration of the nitrating agent, temperature of nitration and nature of the compound to be
nitrated.
(a) The nature of the nitrating agent : For example, NO2
NO2
Fuming HNO3 conc. HNO3
100°C
100°C
O2N
NO2 Benzene NO2
(b) Temperature Soyfn-nTirtirnaitrtoiobnen:zeFnoe r example,
NO2 m-Dinitrobenzene
NO2
m-Dinitro benzeNneO2 HNO3 + H2SO4 HNO3 + H2SO4
60°C 60°C
Benzene Nitrobenzene
(c) Nature of the compound to be nitrated : Presence of electron-releasing group like –OH, –NH2, –CH3, –OR,
etc., in the nucleus facilitates nitration. Thus aromatic compounds bearing these groups (i.e. phenol, aniline, toluene,
etc.) can be nitrated readily as compared to benzene. Thus benzene is not affected by dilute HNO3 while phenol,
aniline and toluene forms the corresponding ortho- and para-nitro compounds.
NO2
conc. HNO3 dil. HNO3 No reaction
H2SO4
OH OH OH OH
NO2 NO2
O2N HNO3 dil. HNO3 +
H2SO4
NO2 Phenol o-Nitrophenol NO2
2, 4, 6-Trinitrophenol p-Nitrophenol
On the other hand, nitration of aromatic compounds having electron withdrawing groups like – NO2, – SO3 H
requires powerful nitrating agent (like fuming HNO3 + H2SO4) and a high temperature.
(ii) Indirect method : The aromatic nitro compounds which can not be prepared by direct method may be
prepared from the corresponding amino compound.
NH2 N2BF4 NO2
NaNO2 NaNO2
HBF4 Cu, heat
NO2 NO2 NO2
p-Nitroaniline p-Dinitroaniline
(2) Physical properties
(i) Aromatic nitro compounds are insoluble in water but soluble in organic solvents.
(ii) They are either pale yellow liquids or solids having distinct smells. For example, nitro benzene (oil of
Mirabane) is a pale yellow liquid having a smell of bitter almonds.
(3) Chemical properties
(i) Resonance in nitrobenzene imparts a partial double bond character to the bond between carbon of benzene
nucleus and nitrogen of the – NO2 group with the result the – NO2 group is firmly bonded to the ring and therefore
cannot be replaced other groups, i.e., it is very inert. Oδ – Oδ –
O– O O– O– O– O– O– O–
N+ N+ N+ N+ N+
+ + δ+ δ+
+ δ+
Resonance hybrid
Resonating structures of nitrobenzene
of nitrobenzene
(ii) Displacement of the – NO2 group : Although – NO2 group of nitrobenzene cannot be replaced by other
groups, but if a second – NO2 group is present on the benzene ring of nitrobenzene in the o- or p- position, it can be
replaced by a nucleophile. For example, NO2 Nu
+ aq. KOH, NH3 or C2H5OK
NO2 NO2
p-Dinitrobenzene
(Where, Nu = OH, NH2 or OC2H5)
(iii) Reduction : Aromatic nitro compounds can be reduced to a variety of product as shown below in the
case of nitrobenzene.
C6 H5 NO2 → C6 H5 NO → C6 H5 NHOH → C6 H5 NH 2
Nitrobenzene Nitrosobenzene Phenylhydroxylamine Aniline
The nature of the final product depends mainly on the nature (acidic, basic or neutral) of the reduction medium
and the nature of the reducing agent.
(a) Reduction in acidic medium NO2 NO2
+ 6H Sn + HCl + 2H2O
Nitrobenzene Aniline
Reduction of dinitrobenzene with ammonium sulphide reduces only one – NO2 group (selective reduction)
NO2 NO2
m-Dinitro benzeNneO2 (NH4)2S m-NitroanilineNH2
or Na2S
(b) Reduction in neutral medium : C6 H 5 NO2 + 2H Zn dust+ NH4Cl → C6 H 5 NO → C6 H 5 NHOH
Nitrobenzene (− H 2O)
Nitrosobenzene Phenylhydroxylamine
(intermediate)
(c) Reduction in alkaline medium : C6 H5 NO2 2[H] → C6 H5 NO (−H2O) → C6 H5 – NO
Nitrobenzene 4[H] →
Nitrosobenzene ||
C6H5 −N
C6 H5 NO2 C6 H5 NHOH Azoxybenzene
Nitrobenzene Phenylhydroxylamine
Azoxybenzene on further reduction yields azobenzene and hydrazobenzene.
C6 H5 – NO 2[H] → C6 H 5 – N 2[H] → C6 H 5 – NH
|| || |
C6H5 − N C6H5 − N C6H5 − NH
Azoxybenzene Azobenzene Hydrazobenzene
(d) Electrolytic reduction :
• Weakly acidic medium of electrolytic reduction gives aniline.
• Strongly acidic medium gives phenylhydroxylamine which rearranges to p-aminophenol.
NH2
NO2 NHOH
electrolytic rearrangement
reduction in presence
of conc. H2SO4
Nitrobenzene Phenylhydroxylamine OH
p-Aminophenol
• Alkaline medium of electrolytic reduction gives all the mono- and di-nuclear reduction products
mentioned above in point (c).
(iv) Electrophilic substitution : Since – NO2 group is deactivating and m-directing, electrophilic substitution
(halogenation, nitration and sulphonation) in simple aromatic nitro compounds (e.g. nitrobenzene) is very difficult as
compared to that in benzene. Hence vigorous reaction conditions are used for such reaction and the new group
enters the m-position. NO2 NO2 NO2
NO2
(a) + Cl2 AlCl3 (b) conc. HNO3 m-DinitrobenzenNeO2
conc. H2SO4
Nitrobenzene Cl Nitrobenzene
m-Chloronitrobenzene
(c) NO2 NO2
+ H2SO4 (fuming) 100°C
Nitrobenzene SO3H
m-Nitrobenzene sulphonic acid
Although nitrobenzene, itself undergoes electrophilic substitution under drastic conditions, nitrobenzene having
activating groups like alkyl, – OR, – NH2 etc. undergoes these reactions relatively more readily.
CH3 HNO3 CH3 HNO3 O2N CH3
NO2 H2SO4 NO2 H2SO4 NO2
o-Nitrotoluene NO2 2, 4, 6-TriniNtroOto2luene (TNT)
2, 4-Dinitrotoluene
Sym-trinitrobenzene (TNB) is preferentially prepared from easily obtainable TNT rather than the
direct nitration of benzene which even under drastic conditions of nitration gives poor yields.
CH3 COOH
O2N NO2 Na2Cr2O7 O2N NO2 Sodalime O2N NO2
H2SO4 (–CO2)
NO2 NO2 NO2
(TNT) 2, 4, 6-Trinitro benzoic acid 2, 4, 6-Trinitrophenol (TNB)
(v) Nucleophilic Substitution : Benzene is inert to nucleophiles, but the presence of – NO2 group in the
benzene ring activates the latter in o- and p-positions to nucleophiles. NO2
NO2 NO2
OH
KOH +
fuse
Nitro benzene o-Nitrophenol OH
p-Nitrophenol
(vi) Effect of the – NO2 group on other nuclear substituents
(a) Effect on nuclear halogen : The nuclear halogen is ordinarily inert, but if it carries one or more electron-
withdrawing groups (like – NO2) in o- or p-position, the halogen atom becomes active for nucleophilic subtitutions
and hence can be easily replaced by nucleophiles (KOH, NH 3 , NaOC2 H 5 ).Nu
Cl
NO2 NO2
+ KOH, NH3 or C2H5ONa
NO2 NO2
2, 4-Dinitrochlorobenzene (Where, Nu = OH, NH2, OC2H5)
(b) Effect on phenolic –OH group : The acidity of the phenolic hydroxyl group is markedly increased by the
presence of – NO2 group in o- and p-position.
The decreasing order of the acidity of nitrophenols follows following order
OH OH OH OH
O2N NO2 NO2 NO2
NO2 NO2 o- and p-Nitrophenols Phenol
2, 4, 6-Trinitro phenal 2, 4-Dinitrophenol
Increased acidity of o- and p-nitrophenols is because of the fact that the presence of electron-withdrawing
– NO2 group in o-and p-position (s) to phenolic –OH group stabilises the phenoxide ions (recall that acidic nature of
phenols is explained by resonance stabilisation of the phenoxide ion) to a greater extent.
–O – N+ = O –O – N+ – O–
O– O– O
Phenoxide ion Extra stabilisation of p-nitrophenate ion due to –NO2 group
(no –NO2 group)
Due to increased acidity of nitrophenols, the latter react with phosphorus pentachloride to give good yields of
the corresponding chloro derivative, while phenol itself when treated with PCl5 gives poor yield of chlorobenzene.
OH Cl
NO2 NO2
+ PCl5
(4) Uses NO2 NO2
2, 4-Dinitrophenol 2, 4-Dinitrochlorobenzene
(i) On account of their high polarity, aromatic nitro compounds are used as solvents.
(ii) Nitro compounds like TNT, picric acid, TNB etc. are widely used as explosives.
(iii) These are used for the synthesis of aromatic amino compounds.
(iv) Nitro benzene is used in the preparation of shoe polish and scenting of cheap soaps.
Cyanides and Isocyanides
Hydrogen cyanide is known to exist as a tautomeric mixture.
H–C≡N⇌ H−N C
Hence, it forms two types of alkyl derivatives which are known as alkyl cyanides and alkyl isocyanides.
R–C≡N R–N C
Alkyl Cyanide Alkyl isocyanide
Nomenclature : According to IUPAC system, cyanides are named as "alkane nitriles". In naming the
hydrocarbon part, carbon of the – CN group is also counted.
Formula As cyanide IUPAC name
CH3CN Methyl cyanide(Acetonitrile) Ethane nitrile
C2H5CN Ethyl cyanide(Propiononitrile) Propane nitrile
C3H7CN Propyl cyanide Butane nitrile
C4H9CN Butyl cyanide Pentane nitrile
Iso cyanides are named as "Alkyl carbylamine" or "Carbyl amino alkane".
Formula As isocyanide(Comman name) IUPAC name
CH3NC Methyl isocyanide (Methyl isonitrile) Methyl carbylamine (Carbylamino
C2H5NC Ethyl isocyanide (Ethyl isonitrile) methane)
Ethyl carbylamine (Carbylamino ethane)
C3H7NC Propyl isocyanide (Propyl isonitrile) Propyl carbylamine (Carbylamino
propane)
(1) Alkyl Cyanides
(i) Methods of preparation
(a) From alkyl halides : The disadvantage of this method is that a mixture of nitrile and isonitrile is formed.
RX + KCN(orNaCN) → RCN + RNC
Alkyl Nitrile Isonitrile
halide (Major product) (Minor product)
(b) From acid amides : RCONH 2 P2O5 → RCN ; CH 3 CONH 2 P2O5 → CH 3 CN + H2O
−H2O Acetamide Methyl cyanide
Industrially, alkyl cyanides are prepared by passing a mixture of carboxylic acid and ammonia over alumina at
500°C.
RCOOH + NH 3 → RCOONH 4 Al2O3 → RCONH 2 Al2O3 → RCN
Acid Ammonium salt – H 2O – H2O Alkyl cyanide
Amide
(c) From Grignard reagent
RMgX+ ClCN → RCN + Mg X ; CH 3 MgBr + ClCN → CH 3CN + Mg Br
Grignard Alkyl Cl Cl
reagent cyanide Methyl magnesium Cyanogen Methylcyanide
bromide chloride
(d) From primary amines : Primary amines are dehydrogenated at high temperature to form alkyl cyanides.
This is also a commercial method.
RCH 2 NH 2 Cu orNi → RCN + 2H2 ; CH3CH 2 NH 2 CuorNi → CH 3CN + 2H2
Primary amine 500°C Ethylamine 500°C
Methyl cyanide
H
|
(e) From oximes : R− P2O5 → R − CN + H2O
C = NOH −H2O
Alkyl cyanide
Aldoxime
(ii) Physical properties
(a) Alkyl cyanides are neutral substance with pleasant odour, similar to bitter almonds.
(b) Lower members containing upto 15 carbon atoms are liquids, while higher members are solids.
(c) They are soluble in water. The solulbility decreases with the increase in number of carbon atoms in the
molecule.
(d) They are soluble in organic solvents.
(e) They are poisonous but less poisonous than HCN
(iii) Chemical properties
(a) Hydrolysis
RCN H2O → RCONH 2 H2O → RCOOH+ NH 3
Alkyl H+ H + Acid
cyanide Amide
CH 3CN H2O → CH 3CONH 2 H2O → CH 3COOH + NH 3
Methyl H+ Acetamide H+ Acetic acid
cyanide
(b) Reduction : When reduced with hydrogen in presence of Pt or Ni, or LiAlH4 (Lithium aluminium hydride)
or sodium and alcohol, alkyl cyanides yield primary amines.
RCN 4H → RCH 2 NH 2
Primary
Alkyl cyanide amine
However, when a solution of alkyl cyanides in ether is reduced with stannous chloride and hydrochloric acid
and then steam distilled, an aldehyde is formed (Stephen's reaction).
R − C ≡ N SnCl2 HCl → RCH = NH.HCl H2O → RCHO + NH 4 Cl
[2H] Imine hydrochloride Aldehyde
(c) Reaction with Grignard reagent : With grignard's reagent, an alkyl cyanide forms a ketone which further
reacts to form a tertiary alcohol.
R′ R′ OH
| | X
R−C ≡ N + R' MgX → R− = NMgX 2H2O → R − = O+ NH3 + Mg
C C
Ketone
R′ R′ R′ OH
| | | X
R – = O + R′′MgX → R − − OMgX H2O → R − = OH + Mg
C C C
||
R′′ R′′
Tertiary alcohol
(d) Alcoholysis : RCN + R′OH+ HCl → − CN|+| −HO2 R′ Cl − H2O → RCOOR′ + NH 4 Cl
Alkyl Alcohol Ester
R
cyanide
imido ester
(iv) Uses : Alkyl cyanides are important intermediates in the laboratory synthesis of a large number of
compounds like acids, amides, esters, amines etc.
(2) Alkyl Isocyanides
(i) Methods of preparation
(a) From alkyl halides : R − X + AgCN → RNC + RCN ; CH 3Cl + AgCN → CH 3 NC + CH 3CN
Alkyl halide Isocyanide Cyanide
(Isonitrile) (Nitrile) Methyl chloride Methyl isocyanide
(Main product)
Main product Minor product
(b) From primary amines (Carbylamine reaction) : RNH 2 + CHCl3 + 3KOH → RNC + 3KCl + 3H 2O
Isocyanide
Primary amine Chloroform
O
||
(c) From N-alkyl formamides : POCl3 → R −N C+ H2O
R − NH − C− H Pyridine
N −alkyl formamide Isocyanide
(ii) Physical properties
(a) Alkyl isocyanides are colourless, unpleasant smelling liquids.
(b) They are insoluble in water but freely soluble in organic solvents.
(c) Isonitriles are much more poisonous than isomeric cyanides.
(iii) Chemical properties
RN = C +
(a) Hydrolysis : 2H 2 O H+ → RNH 2 + HCOOH
Alkyl isocyanide
Primary amine Formic acid
(b) Reduction : R − N = C + 4H Ni → RNHCH 3
Alkyl isocyanide secondary amine
(c) Action of heat : When heated for sometime at 250°C, a small amount of isonitrile changes into isomeric
nitrile.
RNC heat → RCN
(d) Addition reaction : Alkyl isocyanide give addition reactions due to presence of unshared electron pair on
carbon atom.
R : N ::: C : or R − N+ ≡ C−
The following are some of the addition reactions shown by alkyl iscoyanides.
RNC + X2 → RNCX2 ; RNC + S → RNCS ; RNC + HgO → RNCO+ Hg
(Halogen) Alkyl iminocarbonyl Alkyl Alkyl
halide isothiocyanate isocyanate
(iv) Uses : Due to their unpleasant smell, alkyl isocyanides are used in detection of very minute leakage.
Carbylamine reaction is used as a test for the detection of primary amino group.
Note : Methyl isocyanate (MIC)gas was responsible for Bhopal gas tragedy in Dec. 1984.
Cyanides have more polar character than isocyanides. Hence cyanides have high b.p., and are more
soluble in water. However, both isomers are more polar than alkylhalides, hence their boiling points
are higher than the corresponding alkyl halides.
Being less polar, isocyanides are not attacked by OH– ions.
Comparison of Alkyl Cyanides and Alkyl Isocyanides
Test Ethyl cyanide Ethyl isocyanide
Smell Strong but pleasant Extremely unpleasant
More (≈ 4D) Less (≈ 3D)
Dipole moment 98°C(i.e. High) 78°C (i.e. low)
Soluble Only slightly soluble
B.P. Gives propionic acid (Acid, in general) Give ethyl amine (1° amine, in general)
Same as above No action
Solubility in water.
Gives ethylmethyl amine (2° amine, in
Hydrolysis with acids general)
Does not occur
Hydrolysis with
alkalies Ethyl cyanide is formed
Reduction Gives propylamine (1° amine, in general)
Stephen's reaction Gives propionaldehyde (Aldehyde, in
general)
Heating (250°C)
No effect
Amines.
Amines are regarded as derivatives of ammonia in which one, two or all three hydrogen atoms are replaced
by alkyl or aryl group. NH3
–H +R –2H + 2R –3H + 3R
RNH2 R2NH R3N
(Primary) (Secondary) (Tertiary)
Amines are classified as primary, secondary or tertiary depending on the number of alkyl groups attached to
nitrogen atom.
||
The characteristic groups in primary, secondary and tertiary amines are: – NH2 ; – NH ; −N
(amino)
(imino) |
(tert −nitrogen)
In addition to above amines, tetra-alkyl derivatives similar to ammonium salts also exist which are called
quaternary ammonium compounds.
NH 4 I ; R4 NI ; (CH 3 )4 NI or − R + X−
R R
Quaternary Tetramethyl |
N−
|
R
ammonium iodide ammonium iodide
Tetra-alkyl
ammonium salt
(1) Simple and mixed amines : Secondary and tertiary amines may be classified as simple or mixed
amines according as all the alkyl or aryl groups attached to the nitrogen atom are same or different. For example,
Simple amines : (CH 3 )2 NH ; (CH 3CH 2 )3 N ; (C6 H5 )2 NH
Dimethylamine Triethylamine Diphenylamine
Mixed amines : C2 H 5 − NH ; C6 H5 − NH; C3 H7 − N − C H 3
| | |
CH3 CH3 C2H5
Ethylmethylamine Methylaniline Ethylmethyl-n-
propylamine
The aliphatic amines have pyramidal shape with one electron pair. In amines, N undergoes sp3
hybridisation.
(2) Nomenclature : In common system, amines are named by naming the alkyl groups attached to nitrogen
atom followed by suffix-amine.
CH 3 NH 2 ; C2 H5 NH 2 ; CH 3CH 2CH 2 NH 2
Methylamine Ethylamine n−Propylamine
C| H3 ; CH3 NH ; C2 H 5 NH ; CH 3 NH ; CH 3 CH 3 CH 3 N
CH 3 − C H − NH 2 CH3 C2 H 5 C2 H 5 CH 3 N ; CH 3 N ; C2H5
CH 3
Isopropylamine C2 H 5 C3 H7
Dimethylamine Diethylamine Ethylmethylamine Trimethylamine Ethyldimethylamine Ethylmethylpropylamine
In IUPAC system, amino group is considered as substituent and amines are named as amino derivatives of
alkanes (Amino alkanes).
CH 3 NH 2 ; C2 H5 NH 2 ; C| H3
Aminoethane CH3 − C H − NH2
Aminomethane
2- Aminopropane
Secondary amines are named as alkyl aminoalkanes and tertiary as dialkyl amino alkanes with highest
rank to the amino alkane (primary amine).
CH3 NH ; C2 H 5 NH ; CH 3
CH3 CH3 CH3 N
N-Methyl amino N-Methyl amino CH 3
methane ethane
N,N -Dimethyl amino
methane
Alternatively, in IUPAC system, primary amines are named by replacing the final-e of the parent alkane by
-amine (Alkanamine). A number is added to indicate the position of – NH2 group.
CH3
|
CH 3 NH 2 ; CH 3CH 2 NH 2 ; CH 3 − C H − NH 2
Methanamine Ethanamine 2-Propanamine
When two or more amino groups are present, words di, tri- etc., are used with position numbers.
H 2 NCH 2 − CH 2 NH 2 ; H 2 NCH 2 CH 2 C HCH 2 CH 3
1,2-Ethane-di-amine
(1,2-di-amino ethane) |
N
1,3-Pentane-di-amine
Secondary or tertiary amines are named as N-substituted derivatives of primary amines. The largest
group attached to nitrogen is taken as the alkyl group of the primary amine.
NH3 C2H5 CH3
| | |
CH 3CH 2 NHCH 3 ; CH 3CH 2 − N − CH 2CH 2CH 3 ; C2 H 5 N− C H C H C H C H C H
N -Methylethanamine N-Ethyl- N-methylpropanamine − 2 − − 2 − 2 − 3
1 2 3 4 5
N , N -Diethyl - 2- methyl - pentanamine
(3) Isomerism : Amines are represented by a general formula, CnH2n+3N and exhibit following types of
isomerism,
(i) Functional isomerism : This is due to the presence of different functional groups.
Molecular formula C3H9N represents three functional isomers.
CH 3CH 2CH 2 NH 2 ; CH3 NH ; CH 3
n-Propylamine C2H5 CH 3 N
(Primary) 1° Ethylmethylamine CH 3
(Secondary)2°
Trimethylamine
(Tertiary)3°
(ii) Chain isomerism : This is due to the difference in the carbon skeleton of the alkyl group attached to the
amino group.
CH 3CH 2CH 2CH 2 NH 2 ; CH3
n-Butylamine
|
CH 3C HCH 2 NH 2 (C4 H11 N)
Isobutylamine
(iii) Position isomerism : This is due to the difference in the position of amino group in the carbon chain.
CH3
|
CH 3CH 2CH 2 NH 2 ; CH 3 − C H − NH 2 ; (C3 H9 N)
n-Propylamine Isopropylamine
(I-amino propane) (2-amino propane)
(iv) Metamerism : This is due to different alkyl groups attached to the same polyvalent functional group.
CH3 NH ; C2 H 5 NH ; (C4 H11 N)
C3 H7 C2 H 5
Methyl propylamine Diethylamine
(4) General methods of preparation
(i) Methods yielding mixture of amines (Primary, secondary and tertiary)
(a) Hofmann's method :The mixture of amines (1°, 2° and 3°) is formed by the alkylation of ammonia with
alkyl halides.
CH3I + NH3 → CH3 NH2 CH3I →(CH3)2 NH CH3I →( CH3)3 N CH3I →( CH3)4 NI
Methyliodide Methylamine Dimethylamine Trimethylamine Tetramethyl
(1°) (2°) (3°) ammonium iodide
The primary amine may be obtained in a good yield by using a large excess of ammonia. The process is also
termed as ammonolysis of alkyl halides. It is a nucleophilic substitution reaction.
(b) Ammonolysis of alcohols : CH 3OH + NH 3 Al2O3 → CH 3 NH 2 CH3OH →(CH 3 )2 NH CH3OH →(CH 3 )3 N
350°C
Primary amine may be obtained in a good yield by using a large excess of ammonia.
(ii) Methods yielding primary amines
(a) Reduction of nitro compounds
R − NO2 + 6[H ] Sn HCl or→ RNH 2 + 2H2O ; C2 H 5 − NO2 + 6[H] → C2 H 5 NH 2 + 2H 2O
Zn HCl or Ni or LiAlH4
(b) Reduction of nitriles (Mendius reaction)
R − C ≡ N + 4[H] → R − CH 2 NH 2 ; CH 3C ≡ N + 4[H] → CH 3 − CH 2 NH 2
Methyl cyanide Ethylamine
The start can be made from alcohol or alkyl halide.
R − OH SOCl2 → R − Cl KCN → R − CN LiAlH4or → RCH2 NH2
Alcohol Na + C2H5OH Primary amine
Alkyl chloride Alkyl nitrile
This sequence gives an amine containing one more carbon atom than alcohol.
(c) By reduction of amides with LiAlH4
RCONH2 LiAlH4 → RCH2 NH2 ; CH 3 CONH 2 LiAlH4 → CH 3 CH 2 NH 2
Acetamide Ethylamine
(d) By reduction of oximes : The start can be made from an aldehyde or ketone.
RCHO H2NOH → RCH = NOH LiAlH4 → RCH 2 NH 2
Oxime orH2 Ni Primary amine
Aldehyde
R C = O + H2 NOH → R C = NOH LiAlH4 → R CH − NH2
R R R
Ketone Oxime
(e) Hofmann's bromamide reaction or degradation (Laboratory method) : By this method the amide (–CONH2)
group is converted into primary amino (– NH2) group.
R − CO − NH 2 + Br2 + 4KOH → R − NH 2 + 2KBr + K 2CO3 + 2H 2O
Amide Pri-amine
This is the most convenient method for preparing primary amines.
This method gives an amine containing one carbon atom less than amide.
(f) Gabriel phthalimide synthesis : This method involves the following three steps.
• Phthalimide is reacted with KOH to form potassium phthalimide.
• The potassium salt is treated with an alkyl halide.
• The product N-alkyl phthalimide is put to hydrolysis with hydrochloric acid when primary amine is formed.
CO CO CO COOH
CO NK C2H5X COOH
NH KOH NC2H5 HOH C2H5NH2 +
Phthalimide –H2O HCl Phthalic acid
CO CO
Potassium phthalimide N-Ethyl phthalimide
When hydrolysis is difficult, the N-alkyl phthalimide can be treated with hydrazine to give the required amine.
CO NH2 heat CO –NH
NH + | | + RNH2
CO NH2
CO –NH
Hydrazine
Nitrogen containing compound
(g) By decarboxylation of α-amino acids
R C HC OOH Ba(OH)2 → RCH 2 NH 2 ; CH 2 − COOH Ba(OH)2 → CH 3 NH 2
heat heat
| | Methyl amine
NH2 NH2
α -amino acetic acid
(Glycine)
(h) By means of a Grignard regent and chloramine : RMgX + ClNH2 → RNH 2 + MgXCl
(i) By hydrolysis of Isocyanides or Isocyanates
H OH 2H 2O (HCl) → R − NH 2 + HCOOH ; CH 3 − NC+ 2HOH H+ → CH 3 NH 2 HCOOH
R− N C+ Alkyl amine Aceto isonitile
H ≡ OH − +
Alkyl isocyanide
H OH + K 2CO3 ; R − NCO + 2KOH → R − NH 2 + K 2CO3
CH 3 −N =C = O + 2KOH → CH 3 − NH 2
H OH Alkyl isocyanate
Methyl isocyanate
(j) By Schmidt reaction : R − COOH + N3H Conc.H2SO4 → R − NH 2 + N2 + CO2
Acid Hydrazoic Alkyl
acid amine
In this reaction the acyl azide (R – CON3) and alkyl isocyanate (R – NCO) are formed as an intermediate.
R – COOH + N3 H → RCON3 + H2O ; RCON 3 → R − N = C = O+ N2
Acyl azide Acyl azide Alkyl isocyanate
R − N = C = O + H2O → R − NH2 + CO2
Alkyl amine
The overall reaction which proceeds by the elimination of nitrogen from acyl azide followed by acidic or
alkaline hydrolysis to yield primary amine containing one carbonless, is called Curtius Degradation.
The method uses acid chloride to prepare primary amine through acyl azide.
O OO
|| || ||
R − OH SOCl2 → R− NaN3 → R
C− C− Cl − C− N3
Acyl
chloride Acyl azide
O
||
R− −N2 → R − N = C = O 2NaOH → R − NH 2 + Na 2 CO 3
C− N3 heat
The mechanism of curtius rearrangement is very similar to Hofmann degradation.
R N=N=N R – – N + N R N• •
N• • ≡
•• ••
C C –N2 C Intramolecular R–N=C=O
alkyl shift
|| || ||
O OO
Schmidt reaction converts R – COOH to R–NH2, which is a modification of curtius degradation. In this
reaction a carboxylic acid is warmed with sodium azide (Na+N3–) and conc. H2SO4. The carboxylic acid is directly
converted to the primary amine without the necessity of isolating alkyl azide.
O
||
R− NaN3 +H2SO4 (conc.) → RNH 2 + N2 + CO2
C− OH heat
(NaN 3 + H 2SO4 → N 3 H + NaHSO4 )
(k) By Ritter reaction : It is a good method for preparing primary amines having α-tertiary alkyl group.
(CH 3 )3 C − OH+ H 2 SO4 + HCN → (CH 3 )3 C − NH 2
Tert-butyl alcohol Tert −butylamine
(1°amine)
R3 C − OH H+ → H2O + R3 C + HCN → R3C N+ ≡ CH H2O →
Tert - carboniumion
CHO − R 3 CNH OH− → R3C − NH 2 + HCOO −
Pri-amine
O
||
(l) Reductive amination of aldehydes and ketones : NH 3 + H2 Ni,150°C → R − CH 2 − NH 2 + H2O
R − C− H+ 300atm Primary
Aldehyde amine
H H
| |
R − =O + H 2 HN (−H2O) →[R − NH ] H2 → RCH 2 − NH 2
C C= Ni
Imine
R − O + NH3 + H2 Ni,150°C → R − C| H3 NH 2
300atm CH −
||
C− CH3
Ketone
This reaction probably takes place through the formation of an imine (Schiff's base).
The primary amine can also be converted into sec. or tert. amines by the following steps
R − CHO + R′NH2 H2 Ni → RCH2NHR′ ; RNH 2 + 2H 2C = O + 2HCOOH → RN(CH 3 )2 + 2H 2O + 2CO2
Sec. amine Tert.-amine
(m) By reduction of azide with NaBH4 : R−X + NaN 3 → RN 3 NaBH4 → RNH 2
H2O
Alkyl halide Sodium Alkyl 1°amine
(1°or2°) azide azide
(n) By Leuckart reaction : Aldehydes or ketones react with ammonium formate or with formamide to give
formyl derivative of primary amine.
O
||
> C = O + 2HCOONH4 →> CHNH – C– H + 2H2O + CO2 + NH3
Amm.formate
O
||
> C = O + 2HCONH 2 →> CHNH − C− H + CO2 + NH 3
Formamide
These formyl derivatives are readily hydrolysed by acid to yield primary amine.
R O R
R || R
CHNH − H + HOH H+ → CHNH2 + H2O + CO2
C−
This is called Leuckart reaction, i.e.,
R C = O + HCOONH4 180−200°C → R CHNH2 + H2O + CO2
R′ Amm. formate ∆ R′
Ketone Primary amine
Note : On commercial scale, ethylamine is obtained by heating a mixture of ethylene and ammonia at
450°C under 20 atmospheric pressure in presence of cobalt catalyst.
CH 2 = CH 2 + NH 3 Cobalt catalyst → CH 3 CH 2 NH 2
Ethylene 450°C,20atm
(iii) Methods yielding secondary amines
(a) Reaction of primary amines with alkyl halides
R − NH2 + R − X ∆ → R2NH + HX → R2 N+ H2 X− ; R2 N+ H2 X− + NaOH → R2NH + H2O + NaX
dialkyl ammonium salt Secondary amine
(b) Reduction of isonitriles : R − NC + 4[H] Pt → RNHCH 3
Sec. amine
Alkyl isnitrile
Secondary amine formed by this method always possesses one –CH3 group linked directly to nitorgen.
(c) Reaction of p-nitroso-dialkyl aniline with strong alkali solution :
OH H
NH2 RX NR2 HNO2 ON NR2 NaOH ON OH + R2NH
heat
Sec. amine
Aniline Dialkyl aniline p-Nitroso-dialkyl aniline p-Nitroso phenol
This is one of the best method for preparing pure secondary amines.
CaCNalc−iumCN
(d) Hydrolysis of dialkyl cyanamide : cyanamide 2NaOH → Na2 N − CN 2RX → R 2 N − CN
Sodium Dialkyl
cyanamide cyanamide
R2 N − CN + 2HOH H +or → R2 NH + CO2 + NH 3
OH −
Dialkyl amine
(e) Reduction of N-substituted amides : Reduction of N-substituted amides with LiAlH4 yields secondary
amines.
Alkyl β-amino ketones are formed by the action of ketone with formaldehyde and NH3 (or primry or secondary
amines).
The product is referred to as Mannich base and the reaction is called Mannich Reaction.
CH 3COCH 3 + HCHO + RNH 2 heat → CH 3COCH 2CH 2 NHR
Which can be reduced to alkyl amines.
R − CONHR′ + 4[H] LiAlH4 → RCH 2 NHR′+ H2O
N-Alkyl acid amide Sec.amine
(iv) Methods yielding tertiary amines
(a) Reaction of alkylhalides with ammonia
3RX + NH3 → R3 N + 3HX → R3 N+ H X− ; R3 N+ H X− + NaOH → R3 N + NaX + H2O
Trialkyl ammonium salt
(b) Reduction of N, N-disubstituted amides : The carbonyl group is converted into – CH2 group.
RCONR2′ LiAlH4 → RCH 2 NR ′2 + H2O
4[H] ter. amine
N,N -disubstituted
amide
(c) Decomposition of tetra-ammonium hydroxides : The tetra-alkyl ammonium hydroxides are formed when
corresponding halides are treated with moist silver oxide.
R4 N+ I + AgOH → R4 N+ O H− + AgI
The hydroxides thus formed on heating decompose into tertiary amines. Tetramethyl ammonium hydroxide
gives methyl alcohol as one of the products while all other tetra-alkyl ammonium hydroxides give an olefin and
water besides tertiary amines.
(CH 3 )4 NOH → (CH 3 )3 N + CH 3OH ; (R)4 NOH → (R)3 N + olefin + H 2O
(5) Separation of mixture of amines : When the mixture consists of salts of primary, secondary and tertiary
amines along with quaternary salt, it is first distilled with KOH solution. The mixture of three amines distils
over leaving behind non-volatile quaternary salt.
RNH2.HI or RN H+ 3 − I + K+ O H− → RNH2 + KI + H2O
Primary amine
(Volatile), Distillate
R2 NH.HI or R2 N H+ 2 − I + K+ O H− → R2 NH + KI + H2O
R3 N.HI or R3 N H+ − I + K+ O H− → R3 N + KI + H2O
R4 N+ I (non-volatile tetra-alkyl ammonium salt) has no reaction with KOH, however remains as residue.
This mixture is separated into primary, secondary and tertiary amines by the application of following methods.
(i) Fractional distillation : The boiling points of primary, secondary and tertiary amines are quite different,
i.e., the boiling point of C2H5NH2 is 17°C, (C2H5)2NH is 56°C and (C2 H5 )3 N is 95°C and thus, these can be
separated by fractional distillation. This method is used satisfactorily in industry.
(ii) Hofmann's method : The mixture of three amines is treated with diethyl oxalate. The primary amine
forms a soild oxamide, a secondary amine gives a liquid oxamic ester while tertiary amine does not react.
CO OC2H5 H NHR −2C2H5OH → C ONHR ; C| OOC2 H5 HNR2 −C2H5OH → CONH
| COOC2H5
CO OC2H5 + H NHR | + Secondary | 2
amine
Diethyl oxalate Primary CONHR COOC2H5
amine Diethyl oxalate Dialkyl oxamic ester
Dialkyl oxamide (liquid)
(Solid)
Primary amine is recovered when solid oxamide is heated with caustic potash solution and collected as
distillate on distilling the reaction mixture.
CO NHR H OK COOK + 2RNH 2
| |
CO NHR + H OK → COOK Primary amine
Pot.oxalate (Distillate)
The liquid (mixture of oxamic ester+ tertiary amine) is subjected to fractional distillation when tertiary amine
distils over.
The remaining liquid is distilled with KOH to recover secondary amine.
C| ONR2 HOK COOK
COOC2H5 R2 NH + |+ C2 H OH
+ HOK → COOK 5
Secondary
amine
Pot. oxalate
(iii) Hinsberg's method : It involves the treatment of the mixture with benzene sulphonyl chloride, i.e.,
Hinsberg's reagent (C6H5SO2Cl). The solution is then made alkaline with aqueous alkali to form sodium or
potassium salt of monoalkyl benzene sulphonamide (soluble in water).
C6 H 5 SO2Cl + HNHR → C6 H 5 SO2 NHR NaOH → C6 H 5 SO2 N(Na)R
Primary N - Alkyl benzene Soluble salt
amine sulphonamide
The secondary amine forms N,N-dialkyl benzene sulphonamide which does not form any salt with NaOH and
remains as insoluble in alkali solution.
C6 H 5 SO2Cl + HNR2 → C6 H 5 SO2 NR2 NaOH → No reaction
Sec. amine (Insoluble in water,
soluble in ether)
Tertiary amine does not react.
The above alkaline mixture of the amines is extracted with ether.
Two distinct layers are formed. Lower layer, the aqueous layer consists of sodium salt of N-alkyl benzene
sulphonamide (primary amine) and upper layer, the ether layer consists of N,N-dialkyl benzene sulphonamide
(secondary amine) and tertiary amine.
Two layers are separated. The upper layer is fractionally distilled. One fraction obtained is tertiary amine and
the other fraction is treated with concentrated HCl to recover secondary amine hydrochloride which gives free
secondary amine on distillation with NaOH.
C6 H 5 SO2 NR2 + HCl + H 2O → C6 H 5 SO2 .OH + R2 NH.HCl ; R2 NH.HCl + NaOH → R2 NH + NaCl + H 2O
Sec. amine
The aqueous layer is acidified and hydrolysed with dilute HCl. The hydrochloride formed is then distilled with
NaOH when primary amine distils over.
C6 H 5 SO2 N(Na)R + HCl → C6 H 5 SO2 NHR + NaCl
Sulphonamide of
primary amine
C6 H 5 SO2 NHR + HCl + H 2O → C6 H 5 SO2 .OH + RNH 2 .HCl ; RNH 2 .HCl + NaOH → RNH 2 + NaCl + H 2O
Primary amine
hydrochloride
(6) Physical properties
(i) Lower amines are gases or low boiling point liquids and possess a characteristic ammonia like smell
(fishy odour). Higher members are solids.
(ii) The boiling points rise gradually with increase of molecular mass. Amines are polar compounds like NH3
and have comparatively higher boiling points than non-polar compounds of similar molecular masses. This due to
the presence of intermolecular hydrogen bonding.
HHH
|||
H – N : − − −H – N : − − −H – N : − − −
|||
RRR
Hydrogen bonding in amines
(iii) Amines are soluble in water. This is due to hydrogen bonding between amine and water molecules. Amines
are also soluble in benzene and ether.
HH
| |
H – O : − − −H – : − − −H – O : − − −H – : − − −
N N
||||
HRHR
Hydrogen bonding between amine and water molecules
Solubility decreases with increase of molecular mass.
(7) Chemical properties : The main reactions of amines are due to the presence of a lone pair of electrons
on nitrogen atom. Amines are electrophilic reagents as the lone pair of electrons can be donated to electron
seeking reagents, (i.e., electrophiles).
(i) Basic nature of aliphatic amines : Amines like ammonia are basic in nature. The basic nature is due to
the presence of an unshared pair (lone pair) of electrons on nitrogen atom. This lone pair of electrons is available for
the formation of a new bond with a proton or Lewis acids.
H – N – H ; R – N – H ; R – N – H ; R – N – R
| || |
H HRR
Ammonia Primary amine Secondary amine Tertiary amine
Amines are weak bases as they combine partially with the water to form hydroxyl ions.
R − NH 2 + H 2O ⇌ R − N+ H 3 + OH –
Alkyl ammonium ion
Applying law of mass action.
Kb = [R – N+ H 3 ][OH – ] (where Kb is dissociation constant of the base)
[R – NH 2 ]
[Concentration of water is considered constant as it is present in large amounts.]
The value of Kb describes the relative strength of the bases. Strong bases have higher value of Kb while weak
bases have low values.
NH 3 ;CH 3 NH 2 ;(CH 3 )2 NH;(CH 3 )3 N
Kb ; 1.8×10−5 37×10 – 5 54×10 – 5 6.7×10 – 5
Except the amines containing tertiary butyl group, all lower aliphatic amines are stronger bases than ammonia
because of + I (inductive) effect. The alkyl groups, which are electron releasing groups, increase the electron density
around the nitrogen thereby increasing the availability of the lone pair of electrons to proton or Lewis acids and
making the amine more basic (larger Kb). Thus, it is expected that the basic nature or amines should be in the order
tertiary > secondary > primary, but the observed order in the case of lower members is found to be as secondary
> primary > tertiary. This anomalous behaviour of tertiary amines is due to steric factors, i.e., crowding of
alkyl groups cover nitrogen atom from all sides and thus makes the approach and bonding by a proton relatively
difficult which results the maximum steric strain in tertiary amines. The electrons are there but the path is blocked,
resulting the reduced in its basicity.
The order of basic nature of various amines has been found to vary with nature of alkyl groups.
Alkyl group Relative strength
CH3 – R2NH > RNH2 > R3N > NH3
C2H5 – R2NH > RNH2 > NH3 > R3N
(CH3)2CH – RNH2 > NH3 > R2NH > R3N
(CH3)3C – NH3 > RNH2 > R2NH > R3N
(ii) Basic nature of aromatic amines : In aniline or other aromatic amines, the non-bonding electron
pair is delocalized into benzene ring by resonance.
:NH2 +NH2 +NH2 +NH2
•–• •–• N• • – H Nδ+ – H
•–• | Resonance hybrid |
H H
But anilinium ion is less resonance stabilized than aniline.
N+H3 N+H3
No other resonating structure possible
Thus, electron density is less on N atom due to which aniline or other aromatic amines are less basic than
aliphatic amines.
However, any group which when present on benzene ring has electron withdrawing effect (– NO2, – CN, –
SO3H, – COOHT – Cl, C6H5, etc.) decreases basicity of aniline (Nitroaniline is less basic than aniline as nitro group is
electron withdrawing group (– I group) and aniline is more basic than diphenyl amine), while a group which has
electron repelling effect (– NH2, – OR, R –, etc.) increases basicity of aniline. Toluidine is more basic than aniline as –
CH3 group is electron repelling group (+ I group).
Further greater the value of Kb or lower the value of pKb, stronger will be the base. The basic character of some
amines have the following order,
R2 NH > RNH 2 > C6 H5CH 2 NH 2 > NH 3 > C6 H5 NH 2
N-alkylated anilines are stronger bases than aniline because of steric effect. Ethyl group being bigger than
methyl has more steric effect, so N-ethyl aniline is stronger base than N-methyl aniline. Thus, basic character is,
C6 H5 N(C2 H5 )2 > C6 H5 NHC2 H5 > C6 H5 N(CH 3 )2 > C6 H5 NHCH 3 > C6 H5 NH 2 NH 3 > C6 H5 NHC2 H5
> C6 H5 NHCH 3 > C6 H5 NH 2 > C6 H5 NHC6 H5
In Toluidines –p-isomer > m- > o-
Chloroanilines–p-isomer>m-> o-
Phenylene diamines –p-isomer > m- > o-
Nitroanilines–m-isomer > p- > o-
Note : Aniline is less basic than ammonia. The phenyl group exerts –I (inductive) effect, i.e., it withdraws
electrons. This results to the lower availability of electrons on nitrogen for protonation.
Ethylamine and acetamide both contain an amino group but acetamide does not show basic nature.
This is because lone pair of electrons on nitrogen is delocalised by resonance with the carbonyl group
which makes it less available for protonation.
Not available due to delocalization
O O–
|| | N+
CH3 – NH2 ↔ CH3 = H2
C− −C
The compounds with least 's' character (sp3-hybridized) is most basic and with more ‘s’ character (sp-
hybridized) is least basic. Examples in decreasing order of basicity are,
CH 3 NH 2 > CH 3 – N = CHC H 3 > CH 3 – C ≡ N
(sp3 ) (sp)
(sp2 )
CH 3CH 2CH 2 NH 2 > H 2C = CHCH 2 NH 2 > HC ≡ CCH 2 NH 2
(CH 3 )2 NH > CH 3 NH 2 > NH 3 > C6 H5 NH 2
Electron withdrawing (C6H5 –) groups decrease electron density on nitrogen atom and thereby
decreasing basicity.
(CH 3 )2 NH > CH 3 NH 2 > C6 H5 NHCH 3 > C6 H5 NH 2
CH 3CH 2 NH 2 > HO(CH 2 )3 NH 2 > HO(CH 2 )2 NH 2
Electron withdrawing inductive effect of the –OH group decreases the electron density on nitrogen.
This effect diminishes with distance from the amino group.
CH 3CH 2 NH 2 > C6 H5CONH 2 > CH 3CONH 2
(iii) Salt formation : Amines being basic in nature, combine with mineral acids to form salts.
R − NH2 + HCl → RN+ H3C l ; 2R – NH2 + H2SO4 → (RN+ H3 )2 SO4–
Alkylammonium Alkylammonium sulphate
chloride
(iv) Nature of aqueous solution : Solutions of amines are alkaline in nature.
RNH 2 + HOH ⇌ R N+ H 3OH – ⇌ [RNH 3 ]+ + OH –
R2 NH + HOH ⇌ R2 N+ H 2OH – ⇌ [R2 NH 2 ]+ + OH –
R3 N + HOH ⇌ R3 N+ HOH – ⇌ [R3 NH]+ + OH –
The aqueous solutions of amines behaves like NH4OH and give ferric hydroxide precipitate with ferric chloride
and blue solution with copper sulphate.
3RNH 3OH + FeCl3 → Fe(OH)3 + 3RNH 3Cl
(v) Reaction with alkyl halides (Alkylation)
RNH 2 R′X → RNHR′ R′X → R– NR2′ R′X →(R – N+ R3′ )X –
− HX − HX Quaternary salt
Pri.amine Sec.amine Tert. amine
(vi) Reaction with acetyl chloride (Acylation
RNH 2 + ClOCCH3 –HCl → RNHOCCH 3 ; R2 NH + ClOCCH3 –HCl → R2 NOCCH 3
Pri. amine N-Alkyl acetamide Sec. amine N,N-Dialkyl acetamide
Tertiary amines do not react since they do not have replaceable hydrogen on nitrogen.
Therefore, all these above reactions are used to distinguish between P,S and T-amines.
(vii) Action of sodium
2RNH2 + 2Na ∆ → 2[RNH]– Na+ + H2 ↑ ; 2R2 NH + 2Na ∆ → 2[R2 N]– Na + + H 2 ↑
Sod. salt Sod.salt
(viii) Action of halogens
RNH 2 X2 → RNHX X2 → RNX 2 ; R2 NH X2 → R2 NX
NaOH NaOH NaOH
Alkyl amine Dihalo-alkyl Dialkyl amine Halo-dialkyl
amine amine
(ix) Reaction with Grignard reagent
RNH2 + Mg CH3 → CH4 + RNH − Mg − I ; R2 NH + CH3 – Mg – I → CH4 + R2 N – Mg – I
I
(x) Carbylamine reaction : This reaction is shown by only primary amines. This is a test of primary amines
and is used to distinguish primary amines from secondary and tertiary amines.
RNH2 + CHCl3 + 3KOH → RNC + 3KCl + 3H2O
(Alc.)
Alkyl isocyanide
(carbyl amine)
Isocyanides are bad smelling compounds and can be easily detected.
(xi) Reaction with nitrous acid
(a) Primary amines form alcohols with nitrous acid (NaNO2+ HCl). Nitrogen is eliminated.
RNH 2 + HONO → ROH + N2 + H2O
Pri. amine
Alcohol
Methyl amine is an exception to this reaction, i.e.,
CH3 NH 2 + 2HONO → CH 3 – O–N =O + N2 + 2H 2O
Methyl nitrite
2CH3 NH2 + 2HONO → CH3 – O – CH3 + 2N 2 + 3H2O
Dimethyl ether
(b) Secondary amines form nitrosoamines which are water insoluble yellow oily liquids.
R2 NH + HONO → R2 NNO + H2O
Sec. amine Dialkyl
nitrosoamine
Nitrosoamine on warming with phenol and conc. H2SO4 give a brown or red colour which soon changes to
blue green. The colour changes to red on dilution and further changes to blue or violet with alkali. This colour
change is referred to Liebermann's nitroso reaction and is used for the test of secondary amines.
(c) Tertiary amines react nitrous acid to form nitrite salts which are soluble in water. These salts on heating give
alcohols and nitrosoamines.
R3 N + HONO → [R3 NH]+ NO2– heat → R – OH+ R2 N – N = O
Alcohol Nitrosoamine
Tert.amine Trialkyl ammoniumnitrite
This reaction (nitrous acid test) is used to make distinction between primary, secondary and tertiary amines.
(xii) Reaction with carbon di sulphide : This Hofmann’s mustard oil reaction and is used as a test for
primary amines.
RNH2 S=C=S → S=C NHR HgCl2 → RNC = S + HgS + 2HCl
1° heat SH Black
Alkyl isothiocyanate ppt.
(Mustard oil smell)
Alkyl dithiocarbamic acid
R2NH S=C=S → S = C NR2 HgCl2 → No reaction
2° SH
Dialkyl dithiocarbamic acid
(xiii) Oxidation : All the three types of amines undergo oxidation. The product depends upon the nature of
oxidising agent, class of amine and the nature of the alkyl group.
(a) Oxidation of primary amines
RCH 2 NH 2 [O] → RCH = NH H2O → RCHO+ NH 3
Pri. amine KMnO4 Aldimine Aldehyde
R2 CHNH 2 [O] → R2C = NH H2O → R2CO+ NH 3
KMnO4 Ketimine Ketone
(b) Oxidation of secondary amines : R2 NH [O] → R2 N – NR2 ; R2 NH [O] → R2 NOH
KMnO4 H 2 SO5
Sec. amine Tetra-alkyl hydrazine Dialkyl hydroxylamine
(c) Oxidation of tertiary amines : Tertiary amines are not oxidised by potassium permanganate but are oxidised
by Caro's acid or Fenton's reagent to amine oxides.
R3 N + [O] → [R3 N → O]
Tert. amine Amine oxide
(xiv) Reaction with other electrophilic regents
OO
|| ||
RNH 2 + O = HCR′ RN = HC R′ ; 2RNH 2 Cl – C– Cl RNH – C– NHR+ 2HCl
Pri. amine → Schiff's base + Carbonyl → Dialkyl urea
Aldehyde
chloride (Symmetrical)
OS
|| ||
RNHH + O = C = N – R′ → RNH – C– HNR′ ; RNHH + S = C = N – R′ → RNH – C– NHR′
Isocyanate Dialkyl urea Isothiocyanate Dialkyl thiourea
(Unsymmetrical)
(xv) Ring substitution in aromatic amines : Aniline is more reactive than benzene. The presence of amino
group activates the aromatic ring and directs the incoming group preferably to ortho and para positions.
(a) Halogenation NH2 NH2
+ 3Br2 Br Br
+ 3HBr
This reaction is used as a test for aniline. Br
2, 4, 6-Tri Bromoaniline
(white ppt.)
However, if monosubstituted derivative is desired, aniline is first acetylated with acetic anhydride and then
halogenation is carried out. After halogenation, the acetyl group is removed by hydrolysis and only monosubstituted
halogen derivative is obtained.
It may be noted that – NH2 group directs the attacking group at o- and p-positions and therefore, both o- and
p-derivatives are obtained.
NH2 NHCOCH3 NHCOCH3 NHCOCH3 NH2 NH2
(CH3CO)2O Br H2O, H+, –CH3COOH Br
–CH3COOH +Br2 +
Aniline Acetanilide p-Bromoacetanilide Br o-Bromoaaniline Br
(minor) (minor)
p-Bromoacetanilide p-Bromoaniline
(major) (major)
Acetylation deactivates the ring and controls the reaction to monosubstitution stage only because acetyl group
is electron withdrawing group and therefore, the electron pair of N-atom is withdrawn towards the carbonyl group.
(b) Nitration : Aromatic amines cannot be nitrated directly because they are readily oxidized. This is because,
HNO3 is a strong oxidising agent and results in partial oxidation of the ring to form a black mass.
Therefore, to solve this problem, nitration is carried out by protecting the –NH2 group by acetylation. The
acetylation deactivates the ring and therefore, controls the reaction.
The hydrolysis of nitroacetanilides removes the protecting acyl group and gives back amines.
NH2 O NHCOCH3 NHCOCH3 NHCOCH3 NH2 NH2
|| HNO3, H2SO4 NO2 –CH3COOH NO2
288 K + H2O, H+ +
+ Cl – C – CH3
Acetyl chloride
Aniline Acetaniline o-Nitroacetanilide NO2 o-Nitroaniline NO2
(minor)
(c) Sulphonation p-Nitroacetanilide p-Nitroaniline
NH2 NH3+ (major)
NH3+ HSO4– NH2
+ H2SO4 Heat
453-473 K
Aniline Anilinium hydrogen SO3H SO3–
sulphate
Sulphanic acid (I) Zwitter ion structure (II)
The sulphanilic acid exists as a dipolar ion (structure II) which has acidic and basic groups in the same
molecule. Such ions are called Zwitter ions or inner salts.
(8) Uses :
(i) Ethylamine is used in solvent extraction processes in petroleum refining and as a stabiliser for rubber
latex.
(ii) The quaternary ammonium salts derived from long chain aliphatic tertiary amines are widely used as
detergents.
(iii) Aliphatic amines of low molecular mass are used as solvents.
Distinction between primary, secondary and tertiary amines
Test Primary amine Secondary amine Tertiary amine
Action of CHCl3 and Bad smelling carbylamine No action. No action.
alcoholic KOH. (Isocyanide) is formed.
(Carbylamine test) No action. No action
Action of CS2 and HgCl2. Alkyl isothiocyanate is
(Mustard oil test) formed which has pungent Forms nitrosoamine Forms nitrite in cold
smell like mustard oil. which gives green colour which on heating gives
Action of nitrous acid. Alcohol is formed with with phenol and conc. nitrosoa- mine which
evolution of nitogen. H2SO4 (Liebermann's responds to
test). Liebermann's test.
Action of acetyl Acetyl derivative is Acetyl derivative is No action.
formed. formed.
chloride.
Action of Hinsberg's Monoalkyl sulphonamide Dialkyl sulphonamide is No action.
reagent. is formed which is soluble formed which is insoluble
in KOH. in KOH.
Action of methyl iodide. 3 molecules (moles) of 2 moles of CH3I to form One mole of CH3I to
CH3I to form quaternary quaternary salt with one form quaternary salt with
salt with one mole of one mole of tertiary
mole of secondary amine.
primary amine. amine.
Note : Aniline does not form alcohol with nitrous acid but it forms benzene diazonium chloride which
shows dye test.
Aniline
Aniline was first prepared by Unverdorben (1826) by dry distillation of indigo. In the laboratory, it can
be prepared by the reduction of nitrobenzene with tin and hydrochloric acid.
C6 H 5 NO2 + 6H Sn,HCl → C6 H 5 NH 2 + 2H 2O
Nitrobenzene Aniline
Aniline produced combines with H 2SnCl6 (SnCl4 + 2HCl) to form a double salt.
2C6 H 5 NH 2 + SnCl4 + 2HCl → (C6 H 5 NH 3 )2 SnCl6
Double salt
From double salt, aniline is obtained by treating with conc. caustic soda solution.
(C6 H 5 NH 3 )2 SnCl6 + 8 NaOH → 2C6 H 5 NH 2 + 6NaCl + Na2 SnO3 + 5H 2O
On a commerical scale, aniline is obtained by reducing nitrobenzene with iron filings and hydrochloric acid.
NO2 NH3+Cl– NH2
Fe3/HCl 30% Na2CO3
Aniline is also obtained on a large scale by the action of amine on chlorobenzene at 200°C under 300-400 atm
pressure in presence of cuprous catalyst.
2C6 H5Cl + 2NH 3 + Cu2O 200°C → 2C6 H 5 NH 2 + Cu 2 Cl 2 + H2O
300−400 atm
Properties Aniline when freshly prepared is a colourless oily liquid (b.p. 184°C). It has a characteristic
unpleasant odour and is not poisonous in nature. It is heavier than water and is only slightly soluble. It is soluble in
alcohol, ether and benzene. Its colour changes to dark brown on standing.
It shows all the characteristic reactions discussed earlier.
Uses : (1) It is used in the preparation of diazonium compounds which are used in dye industry.
(2) Anils (Schiff's bases from aniline) are used as antioxidants in rubber industry.
(3) It is used for the manufacture of its some derivatives such as acetamide, sulphanilic acid and sulpha drugs,
etc.
(4) It is used as an accelerator in vulcanizing rubber.
Some important conversionss
(1) Conversion of methylamine to ethylamine (Ascent)
CH 3 NH 2 HNO2 → CH 3OH PI3 → CH 3 I NaCN → CH 3CN LiAlH4 → CH 3CH 2 NH 2
Methylamine Methyl alcohol Methyl iodide Methyl cyanide Ethylamine
(2) Conversion of ethylamine to methylamine. (Descent)
CH3CH2 NH2 HNO2 → CH3CH2OH [O] → CH 3 CHO [O] → CH3COOH SOCl2 → CH3COCl
Ethylamine Ethanol Acetic acid Acetyl chloride
K2Cr2O7 H2SO4 Acetaldehyde
NH3 → CH3CONH2 Br2 → CH3 NH2
Acetamide KOH
Methylamine
(3) Conversion of ethylamine to acetone
C2H5 NH2 HNO2 → C2H5OH K2Cr2O7 → CH 3 CHO K2Cr2O7 → CH 3 COOH Ca(OH)2 → (CH3COO)2 Ca
Ethylamine Ethyl alcohol H2SO4 H2SO4 Acetic acid Calcium acetate
Acetaldehyde
heat → CH 3 COCH 3
Acetone
(4) Conversion of propionic acid to :
(i) Ethylamine, (ii) n-Butylamine.
(i) CH3CH2COOH SOCl2 → CH3CH2COCl NH3 → CH3CH2CONH2 Br2 → CH3CH2 NH 2
KOH Ethylamine
Propionic aicd Propionyl chloride Propionamide
or C2 H 5 COOH N3H → C2 H5 NH 2
H2SO4 (conc.)
(ii) CH 3 CH 2COOH LiAlH4 → CH 3 CH 2CH 2OH PBr5 → CH3CH2CH2Br KCN → CH3CH2CH2CN
Propionic acid Ether n- Propyl alcohol Propyl bromide Propyl cyanide
Na+C2H5OH → CH 3 CH 2CH 2CH 2 NH 2
or LiAlH4 n-Butylamine
(5) Conversion of ethylene to 1,4-diaminobutane :
CH 2 = CH 2 Br2 → CH 2 Br.CH 2 Br NaCN → NCCH 2CH 2CN LiAlH4 → NH 2CH 2CH 2CH 2CH 2 NH 2
Ethylene CCl4 Ethylene bromide Ethylene cyanide 1,4-Diaminobutane
Diazonium saltss
The diazonium salts have the general formula ArN + X – , where X– may be an anion like Cl–, Br– etc. and the
2
group N + (− N ≡ N+) is called diazonium ion group.
2
(1) Nomenclature : The diazonium salts are named by adding the word diazonium to the name of the parent
aromatic compound to which they are related followed by the name of the anion. For example,
Cl HO
N+ ≡ NCl– CH3 N+ ≡ NCl– N+ ≡ NCl– N+ ≡ NBr–
Benzenediazonium chloride p-Toluenediazonium chloride o-chlorobenzenediazonium chloride m-Hydroxybenzenediazonium bromide
The diazonium salt may contain other anions also such as NO – , HSO4– , BF4 etc.
3
O2N N+ ≡ NHSO4–
p-Nitrobenzenediazonium hydrogen sulphate
(2) Preparation of diazonium salts : NaNO2 + HCl → NaCl + HONO
NH2 N2+Cl–
NaNO2 + NaCl + H2O
HCl, 273 K
Aniline Benzene diazonium
chloride
The reaction of converting aromatic primary amine to diazonium salt is called diazotisation.
(3) Physical properties of diazonium salts
(i) Diazonium salts are generally colourless, crystalline solids.
(ii) These are readily soluble in water but less soluble in alcohol.
(iii) They are unstable and explode in dry state. Therefore, they are generally used in solution state.
(iv) Their aqueous solutions are neutral to litmus and conduct electricity due to the presence of ions.
(4) Chemical properties of diazonium salts
(i) Substitution reaction : In substitution or replacement reactions, nitrogen of diazonium salts is lost as N2
and different groups are introduced in its place.
(a) Replacement by –OH group N2+Cl– OH
+ H2O Warm + N2 + HCl
Benzene diazonium Phenol
(b) Replacement by hydrogen chloride
N2+Cl–
+ H3PO2 + H2O + N2 + H3PO3 + HCl
Hypophosphoric acid Benzene
Benzene diazonium
chloride
(c) Replacement by–Cl group N2+Cl– Cl
Cu2Cl2 + N2
This reaction is called Sandmeyer reaction. Chlorobenzene
When the diazonium salt solution is warmed with copper powder and the corresponding halogen acid, the
respective halogen is introduced. The reaction is a modified form of Sandmeyer reaction and is known as
Gattermann reaction. N2+Cl– Cl
Cu + N2
HCl
(d) Replacement by iodo (–I) group I
N2+Cl–
+ KI Heat + N2 + KCl
(e) Replacement by – F group Iodobenzene
N2+Cl–
N2+BF4– F
+ HBF4
Heat
Fluoroboric
acid Fluorobenzene
This reaction is called Schiemann reaction. CN
(f) Replacement by Cyano (– CN) group
N2+Cl–
CuCN + N2
Cyanobenzene
The nitrites can be hydrolysed to acids. COOH
CN
Hydrolysis
Benzoic acid
This method of preparing carboxylic acids is more useful than carbonation of Grignard reagents.
(g) Replacement by – NO2 group N2+BF4– NO2
N2+Cl–
HBF4 NaNO2 + NaBF4 + N2
Cu
Nitrobenzene
Diazonium fluoro borate
SH
(h) Replacement by thio (–SH) group
N2+Cl–
+ KSH + N2 + KCl
Potassium Thiophenol
hydro sulphide
(ii) Coupling reactions : The diazonum ion acts as an electrophile because there is positive charge on
terminal nitrogen. It can react with nucleophilic aromatic compounds (Ar–H) activated by electron donating groups
(– OH and – NH2), which as strong nucleophiles react with aromatic diazonium salts. Therefore, benzene diazonium
chloride couples with electron rich aromatic compounds like phenols and anilines to give azo compounds. The azo
compounds contain –N = N– bond and the reaction is called coupling reaction.
N+ ≡ NCl– + OH Base N=N OH
(pH ≈ 9-10) p-Hydroxyazobenzene (yellow)
273-278 K
Phenol
N+ ≡ NCl– + NH H+(pH ≈ 4.5) N=N NH2
2 273-278 K
p-Aminoazobenzene (orange)
N+ ≡ NCl– + N CH3 H+(pH ≈ 4.5) N=N N CH3
CH3 273-278 K CH3
(orange)
Coupling occurs para to hydroxy or amino group. All azo compounds are strongly coloured and are used as
dyes. Methyl orange is an important dye obtained by coupling the diazonium salt of sulphanilic acid with N, N-
dimethylaniline.
Na+O3–S NH2 NaNO2, HCl Na+O3–S N ≡ NCl
273-278 K
Sod. Salt of sulphanilic acid
Na+O3–S N ≡ N Cl + H N(CH3)2 OH– Na+O3–S N=N N(CH3)2
273-278 K
N, N-Dimethylaniline Methyl orange
Note : Diazonium salts are highly useful intermediates in the synthesis of large variety of aromatic
compounds. These can be used to prepare many classes of organic compounds especially aryl halides
in pure state. For example,. 1, 2, 3-tribromo benzene is not formed in the pure state by direct
bromination of benzene. However, it can be prepared by the following sequence of reaction starting
from p-nitroaniline through the formation of diazonium salts as :
NH2 NH2 N2+Cl– Br
Br Br
Br Br Br Br
Diazotisation CuBr Sn, HCl
Br2
NO2 NO2 NO2 NO2
Br Br
p-Nitroaniline Br
Br Br Br Br Br Br
H3PO2
Diazotisation
NH2 N2+Cl– 1, 2, 3-Tribromo benzene
(5) Uses of diazonium salts
(i) For the manufacture of azo dyes.
(ii) For the industrial preparation of important organic compounds like m-bromotoluene, m-bromophenol, etc.
(iii) For the preparation of a variety of useful halogen substituted arenes.
Biomolecules
All living bodies are composed of several lifeless substances which are present in their cells in a very complex but
highly organised form. These are called biomolecules. Some common examples are carbohydrates, proteins, enzymes,
nucleic acids, lipids, amino acids, fats etc :
Living organisms → Organs → Tissues → Cells → Organelles → Biomolecules.
Carbohydrates.
The carbohydrates are naturally occurring organic substances. They are present in both plants and animals.
The dry mass of plants is composed of 50 to 80% of the polymeric carbohydrate cellulose. Carbohydrates are
formed in the plants by photosynthesis from carbon dioxide and water.
nCO2 + nH2O Light → (CH2O)n + nO2
Chlorophyll
Animals do not synthesise carbohydrates but depends on plants for their supply.
(1) Defination : “Carbohydrates are defined as a class of compounds that include polyhydric aldehydes or
polyhydric ketones and large polymeric compounds that can be broken down (hydrolysed) into polyhydric
aldehydes or ketones.”
Carbohydrates contain > C = O and − OH groups. A carbonyl compound reacts with an alcohol to form
hemiacetal.
H
|
R − C = O + HOR′ → R − C − OH
| Alcohol |
H OR′
Aldehyde Hemiacetal
In carbohydrates, the carbonyl group combine with an alcoholic group of the same molecules to form an
internal hemiacetal thus the correct defination of carbohydrates is as follows
“A polyhydroxy compound that has an aldehydic or a ketonic functional group either free or as hemiacetal or
acetal.” HO H OH
C C|
| CHOH
CHOH |
| CHOH
CHOH |
| O
CHOH CHOH
| |
CH
CHOH |
| CH2OH
CH2OH
An internal hemiacetal
Glucose (C6H12O6)
In general, carbohydrates are white solids, sparingly soluble in organic solvents and (except certain
polysaccharides) are soluble in water. Many carbohydrates of low molecular masses have a sweet taste.
(2) Nomenclature : The name of simpler carbohydrates end is –ose. Carbohydrate with an aldehydic
structure are known as aldoses and those with ketonic structure as ketoses. The number of carbon atom in the
molecule is indicated by Greek prefix.
Number of carbon atoms in the Aldose Ketose
molecule
Aldotriose Ketotriose
3 Aldotetrose Ketotetrose
4 Aldopentose Ketopentose
5 Aldohexose Ketohexose
6 Aldoheptose Ketoheptose
7
(3) Classification : The complete classfication of carbohydrates may be depicted in short in the following
chart :
Carbohydrates
Sugars Non-sugars
(Polysaccharides)
Monosaccharides Oligosaccharides Homopoly – Heteropoly –
saccharides saccharides
Aldoses Ketoses
Disaccharides Trisaccharides Tetrasaccharides ….. ….. …..
Monosaccharides.
These are the simplest one unit non-hydrolysable sugars. They have the general formula CnH2nOn where n
varies from 3 to 9 carbon atoms. About 20 monosaccharides occur in nature. The simplest are trioses (n=3)
H −C = O CH 2OH
| |
C3 H6O3 ; H − C − OH ; C=O
Triose | |
CH 2 OH CH 2OH
Glyceraldehyde Dihydroxyacetone
The most important naturally occurring monosaccharides are pentoses and hexoses. A common pentose is
ribose and two common hexoses are glucose and fructose.
Except ketotriose {dihydroxyacetone}, all aldose and ketoses {monosaccharides} contain asymmetric carbon
atoms and are optically active. Number of isomers depand upon the number of asymmetric carbon atom in the
molecules of monosaccharide and is derived by the formula 2n where n is the number of asymmetric carbon atoms
in the molecules.
Number of carbon atoms No. of asymmetric carbon No. of isomers
atoms
Trioses 3 CH2OHCHOHCHO 1 2
Tetroses 4 CH2OHCOCH2OH – –
Pentoses 5 CH2OH(CHOH)2CHO 2 4
CH2OHCHOHCOCH2OH 1 2
Hexoses 6 CH2OH(CHOH)3CHO 3 8
CH2OH(CHOH)2COCH2O 2 4
H
4 16
CH2OH(CHOH)4CHO 3 8
CH2OH(CHOH)3COCH2O
H
Class Molecular formula Structural formula Examples
Aldoses
Aldotrioses C3H6O3 Glyceraldehyde
Aldotetroses C4H8O4 CH2OHCHOHCHO Erythrose, Threose
Aldopentoses C5H10O5 CH2OH(CHOH)2CHO Arabinose, Ribose, Xylose, Lyxose
Aldohexoses C6H12O6 CH2OH(CHOH)3CHO Glucose, Galactose, Mannose,
CH2OH(CHOH)4CHO Allose, Talose, Gulose, Idose, etc.
Ketotrioses C3H6O3
Ketotetroses C4H8O4 Ketoses Dihydroxyacetone
Ketopentoses C5H10O5 CH2OHCO.CH2OH Erythrulose
CH2OH.CO.CHOH.CH2OH Ribulose, Xylulose
Ketohexoses C6H12O6 CH2OH.CO.(CHOH)2CH2O
H Fructose, Sorbose, Tangatose,
CH2OH.CO(CHOH)3CH2O Psicose
H
(1) D and L-designation : By convention, a molecule is assigned D-configuration if the –OH group
attached to the carbon adjacent to the –CH2OH group (last chiral carbon) is on the right hand side irrespective of
the position of other groups. On the other hand, the molecule is assigned L-configuration if the –OH group attached
to the carbon adjacent to the –CH2OH group is on the left.
However, it may be noted that D- and L- do not represent dextrorotatory or laevorotatory. The optical
activity of the molecule is represented by (+) and (–) which represent the direction of rotation of plane polarized
light whether dextrorotatory or laevorotatory.
(2) Configuration : Configuration of Monosaccharides
(i) Aldotriose : CHO.CHOHCH2OH isomers (2)1 = 2
CHO CHO
| |
H – C – OH OH – C – H
| |
CH2OH CH2OH
D-Glyceraldehyde L-Glyceraldehyde
(ii) Aldotetrose : CHO.CHOH.CHOH.CH2OH isomers (2)2=4
CHO CHO L-Erythrose L-Threose
| |
H – C – OH HO – C – H
| |
H – C – OH H – C – OH
| |
CH2OH CH2OH
D-Erythrose D-Threose
So it has four isomers, i.e., D, L-Erythrose and D, L-Threose.
(iii) Aldopentose : CHO.CHOH.CHOH.CHOH.CH2OH, isomers (2)3 = 8
D-Erythrose D-Threose
CHO CHO CHO CHO
| | | |
H – C – OH OH – C – H H – C – OH HO – C – H
| | | |
H – C – OH H – C – OH HO – C – H HO – C – H
| | | |
H – C – OH H – C – OH H – C – OH H – C – OH
| | | |
CH2OH CH2OH CH2OH CH2OH
D-Ribose D-Arabinose D-Xylose D-Lyxose
So aldopentoses has eight isomers, i.e., D- and L-Ribose, D- and L-Arabinose, D- and L-Xylose and D, L-
Lyxose
(iv) Aldohexose : CHO.(CHOH)4CH2OH, isomers (2)4 = 16
D-Ribose D-Arabinose D-Xylose D-Lyxose
CHO CHO CHO CHO CHO CHO CHO CHO
| | | | | | | |
H – C – OH HO – C – H H – C – OH HO – C – H H – C – OH HO – C – H
| HO – C – H H – C – OH | | || |
H – C – OH || HO – C – H H – C – OH HO – C – H
| H – C – OH HO – C – H | | H – C – OH HO – C – H |
H – C – OH || H – C – OH HO – C – H || HO – C – H
| | | HO – C – H HO – C – H |
H – C – OH H – C – OH H – C – OH H – C – OH H – C – OH || H – C – OH
| || | | |
CH2OH H – C – OH H – C – OH CH2OH CH2OH H – C – OH H – C – OH CH2OH
|| ||
D-Allose CH2OH D-Mannose D-Gulose CH2OH CH2OH D-Talose
CH2OH
D-Altrose D-Glucose D-Idose D-Glactose
(3) Glucose; C6H12O6; Aldo-hexose : Glucose is known as dextrose because it occurs in nature as the
optically active dextrorotatory isomer. It is also called grape sugar as it is found in most sweet fruits especially
grapes. It is present in honey also. It is essential constituent of human blood. The blood normally contains 65 to 110
mg of glucose per 100 mL (hence named Blood sugar). In combined form, it occurs in cane sugar and
polysaccharides such as starch and cellulose. It is also present in various glycosides like amygdalin and salicin.
(i) Preparation
(a) Laboratory method
C12 H 22O11 + H 2O H+ → C6 H12O6 + C6 H12O6
Cane sugar Glucose Fructose
(Sucrose)
Note : HCl (dil.) is used for hydrolysis. Glucose being much less soluble in alcohol than fructose
separates out by crystallising on cooling.
(b) Manufacture : It is obtained on a large scale by the hydrolysis of starch (corn starch or potato starch) with
dilute sulphuric acid or hydrochloric acid.
(C6 H10O5 )n + nH 2O H+ → nC6 H12O6
Starch Glucose
A thin paste of starch is boiled with dilute acid till the hydrolysis is complete. The excess of acid is neutralised
with chalk (calcium carbonate) and the filtrate containing glucose is decolourised with animal charcoal. The solution
is concentrated and evaporated under reduced pressure. Glucose is obtained in crystalline form.
(ii) Physical properties : It is a colourless crystalline solid, melts at 146o C . It is readily soluble in water.
From aqueous solution, it separates as a crystalline monohydrate (C6 H12O6 .H2O) which melts at 86o C . It is
sparingly soluble in alcohol but insoluble in ether. It is less sweet (three-fourth) than cane sugar. It is optically active
and the ordinary naturally occuring form is (+) glucose or dextro form. It shows mutarotation.
(iii) Chemical properties : Glucose is a polyhydroxy aldehyde i.e. aldohexose. It has five – OH groups and
one aldehydic group. It shows characteristics of hydroxyl and aldehydic group. Important chemical reaction of the
glucose are the following :
(a) Alcoholic reaction (Reaction due to –OH group)
• Reaction with acid chlorides and acid anhydride
CHO CHO
||
(CHOH)4 + 5CH3COCl ZnCl2 →(CHOOCCH3 )4 + 5HCl
| Acetyl chloride |
CH 2 OH CH 2 OOCCH 3
Glucose
Glucose penta-acetate
This shows that a molecule of glucose contains 5 – OH groups.
• Reaction with PCl5 CHO
CHO
||
(CHOH)4 + 5PCl5 → (CHCl)4 + 5POCl3 + 5HCl
||
CH 2OH CH 2Cl
Glucose
Penta-chloroglucose
(Glucose penta-chloride)
• Reaction with metallic hydroxides :
C6 H11O5 — OH + H O — Ca — OH → C6 H11O5 — O — Ca — OH+ H2O
Glucose Calcium hydroxide Calcium glucosate
Note : Glucose behaves as a weak acid. Instead of Ca(OH)2 we can take other metallic hydroxide
like Ba(OH)2,Sr(OH)2,Cu(OH)2 etc to form glucosate which is soluble in water.
• Formation of glycosides : C6 H11O5 — OH + H OCH3 HCl → C6 H11O5OCH3 + H2O
α- and β-Methyl glucoside
H OCH3 CH3O H
C O C O
| |
(C| HOH)3 (C| HOH)3
CH CH
| |
CH2OH CH2OH
α-Methyl glucoside β-Methyl glucoside
This reaction shows the presence of ring structure in glucose.
(b) Reactions of carbonyl group (Aldehydic group)
• Reduction : CH2OH(CHOH)4 CHO+ 2H Na−Hg → CH 2OH(CHOH)4 CH 2OH
Glucose H2O Sorbitol
On prolonged heating with concentrated HI and red phosphorus at 110o C , glucose forms a mixture of 2-
iodohexane and n-hexane.
• Oxidation
Reaction with Fehling solution : CH2OH(CHOH)4 CHO+ 2CuO → CH2OH(CHOH)4 COOH + Cu2O .
Glucose Gluconic acid (red ppt.)
Reaction with Tollen’s reagent : CH 2OH(CHOH)4 CHO + Ag 2O → CH 2OH(CHOH)4 COOH + 2Ag .
(Mirror)
[or black ppt.]
Reaction with Bromine water : CH2OH(CHOH)4 CHO+ [O] Br2/ H2O → CH2OH(CHOH)4 COOH .
Glucose Gluconic acid
Reaction with Nitric acid : CH2OH(CHOH)4 CHO+ 3[O] HNO3 → COOH(CHOH)4 COOH+ H2O .
Glucose (C6 ) Saccharic acid (C6 )
CN
• Reaction with HCN : CH 2OH(CHOH)4 CHO + HCN → CH 2OH(CHOH)4 CH .
Glucose cyanohydrin OH
• Reaction with hydroxyl amine
CH 2OH(CHOH)4 CHO + NH 2OH → CH 2OH(CHOH)4 CH = NOH + H 2O .
Glucose oxime
• Reaction with Phenyl hydrazine (Fischer's mechanism) : When warmed with excess of phenyl hydrazine,
glucose first forms phenylhydrazone by condensation with – CHO group.
CHO + H2NNHC6H5 CH = NNHC6H5
| |
Phenyl hydrazine
CHOH CHOH
|
(C| HOH)3 Warm |
(C| HOH)3
CH2OH CH2OH
Glucose Glucose phenyl hydrazone
The adjacent – CHOH group is then oxidised by a second molecule of phenyl hydrazine.
CH = NNHC6H5 CH = NNHC6H5
| |
CHOH + H2NNHC6H5
| C = O + C6H5NH2 + NH3
|
(C| HOH)3 (C| HOH)3
CH2OH
CH2OH
Keto compound of
Glucose phenyl hydrazone
The resulting carbonyl compounds reacts with a third molecule of phenyl hydrazine to yield glucosazone.
CH = NNHC6H5 CH = NNHC6H5
| |
C = O + H2NNHC6H5
| C = NNHC6H5 + H2O
|
(C| HOH)3 (C| HOH)3
CH2OH
CH2OH
(c) Miscellaneous reactions Glucosazone
• Fermentation : C6 H12O6 Zymase → 2C2H5OH+ 2CO2
Glucose Ethanol
• Dehydration : When heated strongly or when treated with warn concentrated sulphuric acid, glucose is
dehydrated to give a black mass (sugar carbon).
• Reaction with alkalies : When warmed with concentrated alkali, glucose first turns yellow; then brown and
finally gives a resinous mass.
A dilute solution of glucose, when warmed with dilute solution of alkali, some glucose is converted into
fructose and mannose. D-glucose and D-mannose are epimers.
CH = O CH — OH C| H2OH HO — CH CH = O
| || C=O || |
H — C — OH C — OH HO — C — H
HO — C
…
…
…
…
…
Enol Fructose Enol Mannose
Glucose
OH OH
C C
| |
H – C – OH HO – C – H
| |
HO – C – H HO – C – H
| |
H – C – OH H – C – OH
| |
H – C – OH H – C – OH
| |
CH2OH CH2OH
D(+) Glucose D(+) Mannose
Epimers
• Action of concentrated hydrochloric acid
C6 H12O6 Conc.HCl → CH3COCH2CH 2COOH+ HCOOH + H2O
Laevulic acid
On treatment with conc. HCl, glucose can also form hydroxymethyl furfural.
C6H12O6 CH — CH + 3H2O
|| ||
HOCH2 — C C — CHO
O
Hydroxymethyl
furfural
This on acid treatment gives laevulic acid
(iv) Uses
(a) In the preservation of fruits and preparation of jams and jellies.
(b) In the preparation of confectionary and as a sweetening agent.
(c) As a food for patients, invalids and children.
(d) In the form of calcium glucosate as medicine in treatment of calcium deficiency.
(e) As a reducing agent in silvering of mirrors.
(f) As a raw material for alcoholic preparations.
(g) In industrial preparation of vitamin-C.
(h) In the processing of tobacco.
(i) As an intravenous injection to the patients with lower glucose content in blood.
(v) Test of glucose
(a) When heated in a dry test tube, it melts, turns brown and finally black, giving a characteristic smell of burnt
sugar.
(b) When warmed with a little conc. H 2SO4 , it leaves a charred residue of carbon.
(c) When it is boiled with dilute NaOH solution, it first turns yellow and then brown.
(d) Molisch’s test : This is a general test for carbohydrates. A drop or two of alcoholic solution of α-naphthol is
added to 2mL of glucose solution. 1 mL of concentrated H 2SO4 is added carefully along the sides of the test tube.
The formation of a violet ring, at the junction of two liquids confirms the presence of a carbohydrate.
(e) Silver mirror test : A mixture of glucose and ammonical silver nitrate is warmed in a test tube. Appearance
of silver mirror on the inner walls confirms glucose.
(f) Fehling’s solution test : A little glucose is warmed with Fehling’s solution. A red precipitate of cuprous oxide
is formed.
(g) Osazone formation : Glucose on heating with excess of phenyl hydrazine in acetic acid gives a yellow
crystalline compound, m.pt. 205o C .
(vi) Structure of glucose : The structure of glucose has been established as follows
(a) Open chain structure : It is based on the following points :
• Elemental analysis and molecular mass determination show that the molecular formula of glucose is
C6 H12O6 .
• Glucose on complete reduction with HI and red phosphorus finally gives n-hexane. This indicates that it
contains a straight chain of six carbon atoms.
• It reacts with acetic anhydride and forms penta-acetate derivative. This shows the presence of five hydroxyl
groups each linked to a separate carbon atom as the molecule is stable.
• Glucose combines with hydroxyl amine to form a monoxime. It also combines with one mole of HCN to
form a cyanohydrin. These reactions indicate the presence of a carbonyl group, > C = O , in the molecule.
• Mild oxidation of glucose with bromine water gives gluconic acid. This shows the presence of an aldehyde
group.
On the basis of above observations, the following open chain structure can be written for glucose.
OH OH OH OH OH H
| *| *| *| *| |
H—C—C—C—C—C—C=O
| | | ||
H HH H H
There are four asymmetric carbon atoms marked by asterisks (*) in the molecule. This representation is
incomplete, because a definite configuration to these asymmetric centres has not been assigned. The configuration
of D-glucose was proved by Emil Fischer. The structure of D-glucose as elucidated by Emil Fischer is,
HO
C1
|2
H – C – OH
HO – C|3– H
H – C|4– OH
|5
H – C – OH
|6
CH2OH
D-Glucose
Evidence against open chain structure : The open chain formula of glucose accounts for most of the
reactions satisfactorily but fails to explain the following
Even though an aldehyde group is present, the glucose does not react with NaHSO3 and NH3 .
Glucose does not give the Schiff’s test for aldehydes.
Glucose does not react with Grignard reagents.
Glucose penta-acetate does not react with hydroxyl-amine.
Two isomeric methyl glucosides (α and β) are obtained by heating glucose with methyl alcohol in presence
of dry HCl gas.
Glucose exists in two stereoisomeric forms (α and β). α- glucose with specific rotation + 110o is obtained
by crystallizing glucose from alcohol or acetic acid solution, whereas β-glucose with specific rotation + 19.7o is
obtained by crystallizing glucose from pyridine solution.
An aqueous solution of glucose shows mutarotation, i.e., its specific rotation gradually decreases from
+ 110o to + 52.5o in case of α-glucose and increases from + 19.7o to + 52.5o in case of β-glucose.
All these observation indicate that free aldehydic group is not present in the molecule.
(b) Cyclic structure of glucose : D-glucose exists in two optically active forms known as α-D-glucose and
β-D-glucose.
α-D-glucose has specific rotation of + 110o and β-D-glucose has specific rotation of + 19.7o . The two
isomers are interconvertible in aqueous solution. The equilibrium rotation is + 52o . The equilibrium mixture has
36% α-glucose, 64% β-glucose. Glucose forms a stable cyclic hemiacetal (according to Fischer) between − CHO
group and the − OH group of the fifth carbon atom in pyranose structure. In this process first carbon atom
becomes asymmetric giving two isomers (I) and (II) which differ only in the configuration of the first asymmetric
carbon.
HO HO H
H OH
C1 1
|2 1
CHOH C
C|3HOH C |2
|4 C|2HOH CHOH
CHOH C|3HOH C|3HOH
|5 C|4HOH |4
CHOH |5 CHOH
|6 CH O |5 O
CH2OH CH
|6
D-Glucose |6 CH2OH
CH2OH
[α]D = + 52.5o β-D-Glucose (II)
α-D-Glucose (I)
[α]D = + 19.7o
[α]D = + 110o
α-Glucose Open chain form β-Glucose
36% 0.02% 64%
Carbon-1 in both configuration (I) and (II) is called an anomeric carbon atom. Due to anomeric carbon,
glucose exists in two forms. Both the forms have different physical properties and are called anomers.
The ring structure explains all the reactions of glucose. The objections against the open chain structure of
glucose have also been satisfactory explained, e.g.,
• α- and β-glucose on treatment with CH3OH in presence of dry HCl gas forms α- and β-methyl glucosides
respectively.
H – C – OH H – C – OCH3
| |
H – C – OH
H – C – OH |
| HO – C – H
O + CH3OH | O
HO – C – H H – C – OH
| (Dry HCl gas) |
H – C – OH H–C
| |
CH2OH
H–C
| α-D-Methyl glucoside
CH2OH
α-D-Glucose (I)
HO – C – H CH3O – C – H
| |
H – C – OH
H – C – OH |
| HO – C – H
O + CH3OH | O
HO – C – H H – C – OH
| (Dry HCl gas) |
H – C – OH H–C
| |
CH2OH
H–C
| β-D-Methyl glucoside
CH2OH
β-D-Glucose (II)
• No reaction with NH3 and NaHSO3 : The glucose ring is not very stable. It is easily broken up by
strong reagents like HCN, NH2OH and C6H5NHNH2, etc., to give the intermediate aldehyde form, which reacts
with them just like an aldehyde.
But weak reagents like NH3 and NaHSO3 are unable to open the chain and cannot react with it. This explains
the inability of glucose to form aldehyde ammonia and bisulphite compound.
• It explains mutarotation : Ordinary glucose is α-glucose, with a fresh aqueous solution has specific
rotation, [α]D + 110o. On keeping the solution for some time; α-glucose slowly changes into an equilibrium mixutre
of α-glucose (36%) and β-glucose (64%) and the mixture has specific rotation + 52.5o.
Similarly a fresh aqueous solution of β-glucose having specific rotation, [α]D + 19.7o , on keeping (standing)
gradually changes into the same equilibrium mixutre (having, specific rotation + 52.7o ). So an aqueous solution of
glucose shows a physical property, known as mutarotation, i.e., a change in the value of specific rotation
(muta=change; rotation = specific rotation) is called mutarotation.
• Methods for determining the size of rings : Fischer and Tollen’s proposed that the ring or the internal
hemiacetal is formed between C1 and C4 . It means the ring is Furan type or 5-membered ring; this is called
Furanose strucutre,
43
CH — CH
C||5H C||2H
1
O
Furan
However according to Haworth and Hirst the ring is formed between C1 and C5 . It means the ring is Pyran
type or 6-membered ring, this is called Pyranose structure.
CH2
H5C 4
3
CH
HC||6 C||2H
1
O
Pyran
The two forms of D-glucose are also shown by Haworth projection formula which are given below;
6CH2OH 6CH2OH
5 OH 5 O OH
H H
4 H H1 4 H H1
OH OH
OH OH
32 OH H
32
H OH
α-D glucose H OH
β-D glucose
The above projection formulae show that the six membered ring is planar but actually the ring has a chain
structure similar to cyclohexane.
In Haworth formula all the OH groups on the right in Fischer’s formula are directed below the plane of the
ring while those on the left go above the plane. The terminal CH 2OH projects above the plane of the ring.
(4) Fructose, fruit sugar C6H12O6, Ketohexose : It is present in abundance in fruits and hence is called
fruit sugar. It is also present in cane sugar and honey alongwith glucose in combined form. The polysaccharide
inulin is a polymer of fructose an gives only fructose on hydrolysis. Since naturally occurring fructose is
laevorotatory, it is also known as laevulose.
(i) Preparation
(a) Hydrolysis of cane sugar : C12 H 22O11 + H2O H2SO4(dil.) → C6 H12O6 + C6 H12O6
Cane sugar Warm D-Glucose D-Fructose
The solution having equal molecules of D-glucose and D-fructose is termed invert sugar and the process is
known as inversion.
Note : The excess of sulphuric acid is neutralised by adding milk of lime. A little more of lime is
added which converts both glucose and fructose into calcium glucosate and calcium fructose respectively.
C6 H11O5 − O − CaOH + CO2 → C6 H12O6 + CaCO3
Calcium fructose Fructose
(b) Hydrolysis of Inulin with dilute sulphuric acid : (C6 H10O5 )n + nH2O H2SO4(dil.) → nC6 H12O6
Inulin Fructose
(ii) Properties : The anhydrous fructose is a colourless crystalline compounds. It melts at 102o C. It is soluble
in water but insoluble in benzene and ether. It is less soluble in water than glucose. It is the sweetest* of all sugars
and its solution is laevorotatory. Like glucose, it also shows mutarotation.
Fructose is a pentahydroxy ketone and its open-chain and closed-chain structures can be represented as :
C| H2OH HOH2C OH OH CH2OH
C= O
| C| H2OH C C
HO – C – H | |
| HO – C – H HO – C – H
H – C – OH C= O | |
| | H – C – OH O H – C – OH
H – C – OH or (CHOH)3 | | O
| H – C – OH
CH2OH | | H – C – OH
CH2
D-Fructose CH2OH |
[α]D = – 92o α- D- Fructose CH2
[α]D = – 21o
β- D- Fructose
[α]D = – 133o
(5) Comparison between glucose and fructose
Property Glucose Fructose
C6H12O6
Molecular formula C6H12O6 Polyhydroxy ketone
Nature Polyhydroxy aldehyde. 102oC
Laevorotatory
Melting point 146oC More soluble
Optical activity of natural form Dextrorotatory No reaction
Mixture of glycollic acid, tartaric acid
With ethyl alcohol Almost insoluble and trihydroxy glutaric acid
Mixture of sorbitol and mannitol
Oxidation Forms calcium fructosate, insoluble in
water
(a) With bromine water Gluconic acid Forms a violet ring
Gives red precipitate
(b) With nitric acid Saccharic acid (Glucaric acid) Forms silver mirror
Reduction Sorbitol
Calcium hydroxide Forms calcium glucosate, soluble in
water
Molisch’s reagent Forms a violet ring
Fehling’s solution
Tollen’s reagent Gives red precipitate
Forms silver mirror
Phenyl hydrazine Forms osazone Forms osazone or
No colouration
Resorcinol + HCl (dil.) Light blue colour Gives red or brown colour
(Selivanoff’s test) precipitate
No colouration
Freshly prepared ammonium Bluish green colour on heating
molybdate sol. + few drops of
acetic acid (Pinoff’s test). A purple colour (violet) on boiling
Alcoholic α-naphthol + HCl
(conc.) (Furfural test)
Note : Fructose gives reactions similar to glucose. The difference in properties is due to the fact that
it contains a ketonic group while glucose contains an aldehydic group.
(6) Interconversions
(i) Chain Lengthening of Aldoses (Killiani-Fischer synthesis) : The conversion of an aldose to the next
higher member involves the following steps :
(a) Formation of a cyanohydrin.
(b) Hydrolysis of – CN to – COOH forming aldonic acid.
(c) Conversion of aldonic acid into lactone by heating.
(d) The lactone is finally reduced with sodium amalgam or sodium borohydride to give the higher aldose.
CHO CN COOH O=C– O=C–H
| || | |
(CHOH)3 CHOH
| HCN CHOH H2O/H+ | heat CHOH Na – Hg CHOH
| Ba(OH)2 (C|HOH)3 –H2O | |
CH2OH (C|HOH)3 CHOH O in acid solution (C|HOH)3
|
Arabinose CH2OH CH2OH CH
(Aldopentose) | CH2OH
Gluconic acid
Glucose
CHOH (Aldohexose)
|
CH2OH
γ-Lactone
(ii) Chain Shortening of Aldoses (Ruff Degradation)
(a) An aldose can be converted to the next lower member by Ruff Degradation. It involves two steps:
• Oxidation of the aldose to aldonic acid by using bromine water.
• The aldonic acid is treated with CaCO3 to give the calcium salt which is then oxidised by Fenton’s reagent
( H2O2 + ferric sulphate) to form the next lower aldose.
CHO COOH
| |
CHOH Br2 CHOH Ca- salt CHO
| H2O | H2O2+Fe3+ |
(CHOH)3 (CHOH)3 (CHOH)3
| | |
CH2OH CH2OH
CH2OH
Aldohexose Aldopentose
(D-Glucose) Aldonic acid (D-Arabinose)
(b) By Wohl’s method : It involves the following steps
• Formation of oxime with hydroxyl amine.
• Heating of oxime with acetic anhydride undergoes dehydration into cyano compound, whereas the
hydroxyl groups get acetylated.
• The acetyl derivative is warmed with ammonical silver nitrate which removes the acetyl group by hydrolysis
and eliminates a molecule of HCN.
CH = O CH = NOH CN CN
| | | | CHO
CHOH
| H2NOH CHOH (CH3CO)2O C| HO.COCH3 AgOH CHO H – HCN |
(CHOH)3 | (CHO.COCH3)3 warm | (CHOH)3
| | |
(CHOH)3 (CHOH)3
CH2OH | CH2O.COCH3 |
CH2OH CH2OH
Glucose CH2OH
(Aldohexose) Aldopentose
Oxime
(iii) Conversion of an Aldose to the isomeric Ketose : Three steps are involved :
(a) Treatment of aldose with excess of phenyl hydrazine to form osazone.
(b) Hydrolysis of osazone with dil. HCl to form osone.
(c) Reduction of osone with zinc and acetic acid to form ketose.
CHO HC| =NNHC6H5 HC=O C| H2OH
| |
CHOH C6H5NHNH2 C| =NNHC6H5 2H2O/H+ C=O 2H C=O
| (Excess) (CHOH)3 (–2C6H5NHNH2) | Zn/CH3COOH |
(CHOH)3 (CHOH)3 (CHOH)3
| | | |
CH2OH
CH2OH CH2OH CH2OH
Osone
Glucose Osazone Fructose
(iv) Conversion of a Ketose to the isomeric Aldose : Two steps are involved,
(a) Reduction of a ketose with H 2 / Ni to form polyhydric alcohol.
(b) Oxidation with Fenton’s reagent to form aldose.
C| H2OH H2/Ni C| H2OH [O] CHO
C=O CHOH H2O2 + Fe3+ |
| | CHOH
(CHOH)3 (CHOH)3 |
| | (CHOH)3
|
CH2OH CH2OH
CH2OH
Fructose
Glucose
Disaccharides.
The disaccharides yield on hydrolysis two monosaccharides. Those disaccharides which yield two hexoses on
hydrolysis have a general formula C12H22O11. The hexoses obtained on hydrolysis may be same or different.
C12 H 22O11 H2O → C6 H12O6 + C6 H12O6
Sucrose H+ Glucose Fructose
Lactose H2O → Glucose + Galactose
H+
Maltose H2O → Glucose + Glucose
H+
The hydrolysis is done by dilute acids or enzymes. The enzymes which bring hydrolysis of sucrose, lactose and
maltose are invertase, lactase and maltase, respectively. Out of the three disacchrides, sucrose (cane-sugar) is the
most important as it is an essential constituent of our diet.
In disaccharides, the two monosaccharides are joined together by glycoside linkage. A glycoside bond is
formed when hydroxy group of the hemiacetal carbon of one monosaccharide condenses with a hydroxy group of
another monosaccharide giving – O– bond.
(1) Sucrose; Cane-sugar [C12H22O11] : It is our common table sugar. It is obtained from sugar cane and
sugarbeets. It is actually found in all photosynthetic plants.
(i) Properties : It is a colourless, odourless, crystalline compound. It melts at 185 – 186oC. It is very soluble
in water, slightly soluble in alcohol and insoluble in ether. It is dextrorotatory but does not show mutarotation. It is
a non-reducing sugar as it does not reduce Tollen’s or Fehling’s reagent. Sucrose, on heating slowly and carefully,
melts and then if allowed to cool, it solidifies to pale yellow glassy mass called ‘Barley sugar’. When heated to
200oC, it loses water to form brown amorphous mass called Caramel. On strong heating, it chars to almost pure
carbon giving smell of burnt sugar. It is composed of α-D-glucopyranose unit and a β-D-fructofuranose unit. These
units are joined by α-β-glycosidic linkage between C –1 of the glucose unit and C – 2 of the fructose unit.
6CH2OH Glycoside
5O linkage
Glycoside linkage 1
2
H O 12||CCH2OH 4 OH
OH OH
1
3
C|
5(C| HOH)3 H(5C|CHOH)2 O or
HC O
6C| H2OH O 6C| H2OH 6CH2OH O
5 OH 2
4 3 1CH2OH
(ii) Uses Structure of sucrose OH
(a) As a sweetening agent for various food preparations, jams, syrups sweets, etc.
(b) In the manufacture of sucrose octa-acetate required to denature alcohol, to make paper transparent and to
make anhydrous adhesives.
(2) Inversion of cane-sugar : The hydrolysis of sucrose by boiling with a mineral acid or by enzyme
invertase, produces a mixture of equal molecules of D-glucose and D-fructose.
C12 H 22O11 + H 2O H+ → C6 H12O6 + C6 H12O6
Sucrose (ThiDs-mGliuxctousere is laeDv-Forruocttoasteory)
Sucrose solution is dextrorotatory. Its specific rotation is + 66.5o. But on hydrolysis, it becomes laevorotatory.
The specific rotation of D-glucose is + 52o and of D-fructose is − 92o. Therefore, the net specific rotation of an
equimolar mixture of D-glucose and D-fructose is.
+ 52o − 92o = −20o
2
Thus, in the process of hydrolysis of sucrose, the specific rotation changes from + 66.5o to − 20o , i.e., from
dextro it becomes laevo and it is said that inversion has taken place. The process of hydrolysis of sucrose is thus
termed as inversion of sugar and the hydrolysed mixture having equal molar quantities of D-glucose and D-
fructose is called invert sugar. The enzyme that brings the inversion is named as invertase.
(3) Distinction between glucose and sucrose
Test Glucose Sucrose
With conc. H2SO4 in cold No effect Charring occurs and turns black
Molisch’s reagent Violet ring is formed Violet ring is formed
With NaOH Turns yellow No effect
With Tollen’s Solution Gives silver mirror No effect
With Fehling’s solution Gives red precipitate of Cu2O No effect
On heating with phenyl hydrazine
Gives yellow precipitate of No effect, i.e., does not form
glucosazone osazone
Aqueous solution + resorcinol + No effect Reddish-brown precipitate which
HCl (conc.) dissolves in ethanol.
Polysaccharide : [Starch and cellulose].
Polysaccharides are polymers of monosaccharides. The most important polysaccharides are starch and
cellulose. They have a general formula (C6H10O5)n. Starch (Amylum) is most widely distributed in vegetable
kingdom. It is found in the leaves, stems, fruits, roots and seeds. Concentrated form of starch is present in wheat,
corn, barley, rice, potatoes, nuts, etc. It is the most important food source of carbohydrates.
(1) Starch and its derivatives : Starch is a white amorphous substance with no taste or smell. When heated
to a temperature between 200 − 250o C, it changes into dextrin. At higher temperature charring occurs. When
boiled with dilute acid, starch ultimately yields glucose.
(C6H10O5 )n →(C6H10O5 )n1 → C12H22O11 → C6H12O6
Starch Dextrin Maltose Glucose
Both n and n1, are unknown, but n is believed to be greater than n1 .
When treated with enzyme, diastase, it yields maltose.
2(C6H10O5 )n + nH2O → nC12H22O11
Maltose
Starch solution gives a blue colour with a drop of iodine which disappears on heating to 75 − 80o C and
reappears on cooling. The exact chemical nature of starch varies from source to source. Even the starch obtained
from same source consists of two fractions (i) amylose and (ii) amylopectin.
Amylose is a linear polymer while amylopectin is a highly branched polymer. Both are composed of α-D-
glucose units linked by glycosidic linkages. The number of D-glucose units in amylose range from 60 – 300. It is
soluble in hot water, Amylopectin consists of D-glucose units from 300 – 600. It is insoluble in water.
CH2OH CH2OH CH2OH
O O O
OH O OH O OH O
O
OH OH OH n
α-1, 4-Glycoside bonds Repeating monomer
Structure of amylose
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