(1) Preparation
(i) From ethylene : (a) Through cold dilute alkaline solution of Bayer’s reagent
(i()iid) idl.ilK. OMHnO– 4 || H2CO2OH
–C=C–
||
OH OH
|| ; OH OH
(Syn-hydroxylation)
– C=C – Anti addition(Trans)
|| OH KMnO4/OH–
RCO2OH –C–C– H2O ||
H+
O –C– C– OH OH
OH
|| Syn-addition (cis)
OH Alkaline
KmnO4
(Anti-hydroxylation)
OH cis OH
Conc. H2SO4
HCO3H/H+ OH
trans OH
(b) With O2 in presence of Ag : CH 2 + 1 O2 Catalyst → CH 2 O H2O → CH 2OH
2
|| | |
Ag,200 − 400°C CH 2 dil. HCl CH 2OH
CH 2
Ethylene Ethylene oxide Ethylene glycol
(c) With HOCl followed by hydrolysis : (Industrial method)
CH2 + HOCl → CH2OH NaHCO3 → CH 2OH + NaCl + CO2
|| | |
CH2 CH2OH
CH2Cl Glycol
Ethylene chlorohydrin
(ii) From 1, 2 dibromo ethane [Lab method]: CH2Br + Na2CO3 + H2O → CH2OH + 2NaBr + CO2
| |
CH2Br CH2OH
CH2Br + 2CH3COOK CH3COOH → CH 2OOCCH3 NaOH → CH 2OH + 2CH3COONa
| − 2 KBr | |
CH2OOCCH3 CH2OH
CH2Br Glycol diacetate
Note : Vinyl bromide is formed as by product.
Best yield of glycol can be obtained by heating 1, 2-dibromo ethane with CH3COOK in glacial acetic acid.
(2) Physical properties
(i) It is a colourless, syrupy liquid and sweet in taste. Its boiling point is 197°C.
(ii) It is miscible in water and ethanol in all proportions but is insoluble in ether.
(iii) It is toxic as methanol when taken orally.
(iv) It is widely used as a solvent and as an antifreeze agent.
CH3 CH3 CH3 CH3
Pinacol; 2-3-dimethyl-2, 3 butanediol : || ||
(O+H2O) → H3C C C CH
H3C − C = C − CH3 dil alk. KMnO4 − − − 3
2, 3- Dimethyl - 2- butene | |
OH OH
Pinacol
When pinacol react with mineral acid then pinacolone is formed.
CH3 CH3 CH3
|| |
H3C C− C− CH3 H+ → H3C C CH3 . This arrangement is called as Pinacol-pinacolon rearrangement
− − − C−
| | ||
|
OH OH O CH3
2,3-dimethyl-2, 3-butanediol 3, 3-Dimethyl-2-butanone
(Pinacol) (Pinacolone)
(3) Chemical properties
Na CH2 ONa Na CH2 ONa Per iodic oxidation of polyhdroxy alcohol gives
50°C 160°C
| | following information,
PCl5 (1) There is cleavage of (C – C) bond. When two –OH–
CH2 OH CH2 ONa
PBr3
PI3 Dialkoxide groups or oxo groups are on adjacent carbon positions.
HCl CH2Cl PCl5 CH2 Cl CH3
160°C
CH3COOH | | | Oxidised to ketone
Conc. HNO3 CH2 OH CH2 Cl Oxidised to aldehyde
Conc. H2SO4 1,2 Dichloroethane CH3 – C – OH
heat
600°C CH2Br PBr3 CH2Br |
Conc. HNO3 | | CH2OH
[O] CH2 OH CH2Br
Here cleavage takes place
CH2I C1 H2 d2ib th (2) Both CHO are present in one molecule indicating
cyclic compound.
| I2 || OH CH2
CH2 CHO
CH2I CH2 HIO4
Ethylene iodide Eth l OH CH2 CHO
CH2
( CH2bCl l) HCl CH2Cl
| 200°C (3) On one side presence of –COOH group indicates
CH2OH |
CH2Cl
CH2OOCCH3 CH3COOH CH2OOCCH3 keto group adjacent to (C – OH) group in (C).
| | CH2
CH2OH CH2 CHO
CH2OOCCH3 OH
CH2OH CH2ONO2 Glycoldiacetate
HIO4
| |
O CH2 COOH
CH2OH CH2ONO2 CH2
Ethylene dinitrate
CH2 – CH2 (4) Formation of two molecule of glyoxalic acid indicate that D
is cyclic compound of four C– atom with C = O and C – OH
O groups on both sides.
Ethylene oxide OC – CHOH
||
COOH
OC – CHOH
|
COOH
Oxalic acid
KMnO4/H+ HCOOH
Formic acid
HIO4 or HCHO
(CH3COO)4Pb
Formaldehyde
Conc. H2SO4 CH2 – CH2 – OH CH2 – CH2
O OO
CH2 – CH2 – OH CH2 – CH2
Diethylene glycol Dioxane
Dehydration CH2 Isomerisation CH3CHO Dehydration
ZnCl2
|| Acetaldehyde
CH2OH
Unstable 18OH OH O
H5PO4
CH3CHO CH2O CHCH3 – H2O18
(HCl)
| Due to formation of stable carbocation
CH2O
Cyclic acetal
CH3
O=C
CH3 CH2O CH3
(HCl) C C(1H,3 3Dioxalane)
|
CH2OCyclic ketal
Dioxalane formation provides a path of protecting a carbonyl group in reaction studied in basic medium in
which acetals are not affected. The carbonyl compound may be regenerated by the addition of periodic acid to
aqueous solution of the dioxalane or by acidic hydrolysis.
R C = O + C H 2 OH → R C O − CH 2
R R | HIO4 → R − CO − R + 2HCHO
|
CH 2OH O − CH 2
Aldehyde is more reactive than ketone in dioxalane formation. This part does not react due to steric hindrance
OO
OO
+ CH2OH – CH2OH H3C CH3 CH2 – OH H3C CH3
CHO H;
C |
CH2 – OH
OO O OO
(4) Uses (ii) Used in the manufacture of dacron, dioxan etc.
(i) Used as an antifreeze in car radiators. (iv) As a cooling agent in aeroplanes.
(iii) As a solvent and as a preservatives.
(v) As an explosives in the form of dinitrate.
Trihydric alcohols.
The only important trihydric alcohol is glycerol (propane-1, 2, 3-triol). It occurs as glycosides in almost all
animal and vegetable oils and fats.
(1) Preparation
(i) From oils and fats
CH2OOCR CH2OH CH2OOCR NaOH CH2OH
| NaOH |
CH OOCR + 3H2O | 3RCOOH ; | NaOH Hydrolysis → OH 3RCOONa
steam → Fatty acids CH +
| CH OH + CH OOCR + | Sodium salt of higher fatty acids
CH2OH
CH2OOCR | |
CH2OH CH2OOCR
Oil or fat Glycerol Oil or fat
(ii) By fermentation of sugar : C6 H12C6 Yeast → C3 H8O3 + CH 3CHO + CO2
Glucose Na2SO3 Glycerol
Acetaldehyde
(iii) From propene [Modern method]
CH3 CH2Cl CH2OH CH2OH CH2 −OH
| | | | |
Cl2 → NaOH(dil) → HOCl → aq. NaOH → − OH
CH 600o C CH CH CH Cl CH
|| || || | |
CH2 CH2 CH2 CH2 −OH CH2 −OH
propene Allyl chloride Allyl alcohol β -monochlorohydrin Glycerol
(iv) From propenal : H2 → = CHCH 2OH H2O2 /OH → HOCH 2CHOHCH 2OH
CH 2 = CHCHO catalyst CH 2 Glycerol
(2) Physical properties
(i) It is a colourless, odourless, viscous and hygroscopic liquid.
(ii) It has high boiling point i.e., 290°C. The high viscosity and high boiling point of glycerol are due to
association through hydrogen bonding.
(iii) It is soluble in water and ethyl alcohol but insoluble in ether.
(iv) It is sweet in taste and non toxic in nature.
(3) Chemical properties
CH 2 −OH CH 2ONa C| H2ONa
CH− OH
(i) Reaction with sodium : | Na→ | Na→
CH− OH CH− OH
| Room | Room |
CH2 −OH temperature CH2 −OH temperature CH2ONa
Monosodium glycerol Disodium glycerolate
(ii) Reaction with PCl5, PBr3 and PI3
CH2OH CH2Cl
||
(a) CH OH + 3PCl5 → CH Cl 3POCl3 + 3HCl
| | +
CH2OH CH2Cl
Glyceryl trichloride
(1, 2, 3-Trichloropropane)
CH2OH CH2Br
||
(b) CH OH + PBr3 CH Br + H3 PO3
| →
|
CH2OH CH2Br
1, 2, 3-Tribromopropane
CH2OH CH2I CH2
| CC||HH2II ||
+ PI 3 → → I2
(c) CH OH CH +
|
CH2OH |
(Unstable) CH2I
Allyl iodide
(iii) Reaction with HCl or HBr
CH2OH CH2Cl CH2OH CH2Cl CH2Cl
| | | | |
110oC → + Excess of HCl →
CH OH CH OH CH Cl 110o C CH Cl + CH OH
| + HCl | | | |
CH2OH CH2OH CH2OH CH2OH CH2Cl
α -Glycerol β -Glycerol Glycerol Glycerol
monochlorohydrin monochlorohydrin α , β -dichlorohydrin α ,α ′-dichlorohydrin
(66%) (34%) (56%) (44%)
(iv) Reaction with HI
CH2OH CH2I CH2
|
| Warm → → || I2
CH I
(a) CH OH + 3HI | CH +
| CH2I
CH2OH 1,2,3- Tri- iodopropane |
(Unstable) CH2I
Allyl iodide
CH2 CH3 CH3 CH3
|| | | |
−I2 → HI →
(b) CH + HI → CH I CH CH I
| | || |
CH2I CH2I CH2 CH3
Allyl iodide Unstable Propene Isopropyl iodide
(v) Reaction with oxalic acid
(a) At 110°C Glycerol is formed
O
||
CH2OH CH2OOC COOH CH2O−C−H CH2OH
| | | |
OH + HOOC − COOH 100−110oC → OH −CO2 → H2O → OH + H COOH
CH CH CH− OH CH
| Oxalic acid −H2O | | |
Formic acid
CH2OH CH2OH CH2 −OH CH2OH
Glycerol mono-oxalate Glycerol mono formate Glycerol
(b) At 260°C, allyl alcohol is formed
CH2OH HOOC CH2OOC CH2
| ||
| + −2H2O → || −2CO2 → − CH 2 OH
HOOC CH
CH OH CH OO C
Allyl alcohol
| |
CH2OH CH2OH
CH2OH CH2
| ||
conc. H2SO4 / P2O5/ KHSO4 → + 2H2O
(vi) Dehydration : CH OH CH
| ∆|
CH2OH CHO
Acrolene or
allyl aldehyde
(vii) Oxidation CHO COOH COOH
[O] | Cl | [O] |
dil. HNO3
CHOH CHOH CHOH
| | |
CH2OH CH2OH COOH
Glyceraldehyde Glyceric acid Tartronic acid
CHO CH2OH
CH2OH Fenton’s ||
reagent CHOH + C = O
|
| |
CHOH
CH2OH CH2OH
|
Glyceraldehyde Dihydroxy acetone
CH2OH
Glycerose
CH2OH CH2OH COOH COOH
|
[O] [O] | [O] | [O] | [O] CO2 + H2O
KMnO4 CO
acidified CO CO COOH
|
| | Oxalic acid
CH2OH
COOH COOH
Dihydroxy
acetone Hydroxy Mesoxalic
Pyruvic acid acid
2HIO4 2CH2O
∆ + + 2HIO3 + H2O
COOH
CH2OH CH2ONO2
| |
(viii) Reaction with nitric acid : + 3HNO3 conc. H2SO4 → ONO2 + 3H2O
CH OH CH
| |
CH2OH CH2ONO2
Glyceryl trinitrate (T.N.G.)
Dynamite is prepared from T.N.G.
Dynamite : A mixture of T.N.G. and glyceryl dinitrate absorbed in kieselguhr is called dynamite. It was
discovered by Alfred. Nobel in 1867. It releases large volume of gases and occupy 10,900 times the volume
of nitroglycerine.
C3H5(ONO)3 → 12CO2 + 10H2O + 6N2 + O2
Blasting gelatin : A mixture of glyceryl trinitrate and cellulose nitrate (gun cotton).
Cordite : It is obtained by mixing glyceryl trinitrate with gun cotton and vaseline it is smokeless explosive.
Hydroxy compound and ethers
(4) Uses
(a) As antifreeze in automobile radiator.
(b) In the preparation of good quality of soap-hand lotions shaving creams and tooth pastes.
(c) As a lubricant in watches.
(d) As a preservatives.
(e) As a sweetening agent in confectionary, beverages and medicines being non toxic in nature.
(f) In manufacture of explosives such as dynamite.
(5) Analytical tests of glycerol
(i) Acrolein test : When glycerol is heated with KHSO4 a very offensive smell is produced due to formation
of acrolein. Its aqueous solution restores the colour of schiff’s reagent and reduces Fehling solution and Tollen’s
reagent.
(ii) Dunstan’s test : A drop of phenophthalein is added approximately 5 ml of borax solution. The pink
colour appears on adding 2-3 drops of glycerol, pink colour disappears. The pink colour appears on heating and
disappears on cooling again.
Unsaturated alcohols.
(1) Preparation (Allyl alcohol)
(i) From allyl halide : CH2 = CH − CH2Br + H2O → CH2 = CH − CH2OH + HBr
Allyl alcohol
(ii) By heating glycerol with oxalic acid : C| H2OH + HOOC −2H2O → C| H2OOC| Heat → C|| H2
CH OH | CH OO C CH
| −2CO2 |
| HOOC CH2OH
CH2OH
CH2OH Allyl alcohol
(2) Physical properties
(a) It is colourless, pungent smelling liquid.
(b) It is soluble in water, alcohol and ether in all proportion.
(3) Chemical properties H2 CH3CH2CH2OH
Pt
Br2 1-propanol
CH2Br – CHBrCH2OH
2, 3-dibromopropanol-1
HBr CH2BrCH2 CH2OH
3-Bromopropanol-1
HOCl CH2OHCHClCH2OH
Alk. KMnO4 Glycerol β-monochlorohydrin
(O + H2O)
CH2 OH – CHOH – CH2OH
Glycerol
CH2 = CH – CH2OH – Na CH2 = CH – CH2ONa
(Allyl alcohol)
CH3COOH
CH2 = CH – CH2OOCCH3
Allyl acetate
HCl CH2 = CH – CH2Cl
Allyl chloride
Oxidation COOH
| + HCOOH
COOH
Formic acid
Oxalic acid
Br2 CH2 – CH – CH2 [O] CH2 – CH – COOH Zn dust CH2 – CH – COOH
HNO3 (CH3OH)
||| || Acrylic acid
Br Br OH Br Br
Phenol (Carbolic acid), C6H5OH or Hydroxy benzene.
It was discovered by Runge in the middle oil fraction of coal-tar distillation and named it ‘carbolic acid’ (carbo
= coal, oleum = oil) or phenol containing 5% water is liquid at room temperature and is termed as carbolic acid. It
is also present in traces in human urine.
(1) Preparation
(i) From benzene sulphonic acid : Sodium salt of benzene sulphonic acid is fused with sodium hydroxide
(NaOH) when sodium phenoxide is formed. Sodium phenoxide on treatment with dilute acid or carbon dioxide
yields phenol. The start can be made with benzene.
C6 H6 H2SO4(fuming) → C6 H 5 SO3 H NaOH → C6 H5 SO3 Na NaOH → C6 H5ONa H + / H2O → C6 H5OH
Benzene sulphonic acid Fuse or CO2 / H2O Phenol
Benzene Sodium benzene sulphonate Sodium phenoxide
This is one of the laboratory methods for the preparation of phenol. Similarly methyl phenols (cresols) can
be prepared. SO3H OK OH
Solid KOH H+/H2O
Fuse
CH3 CH3 CH3
p-Toluene p-Cresol
sulphonic acid
(ii) From benzene diazonium chloride : When benzene diazonium chloride solution is warmed, phenol is
formed with evolution of nitrogen. The phenol from solution is recovered by steam distillation. In this case also,
benzene can be taken as the starting material. This is also a laboratory method.
C6 H6 HNO3 → C6H5 NO2 Sn /HCl → C6 H5 NH2 NaNO2 → C6H5 N2Cl H2O → C6 H5OH
Nitrobenzene Aniline HCl, 0−5o C Warm Phenol
Benzene H 2SO4 , 45o C Benzene diazonium chloride
NH2 N2Cl OH
HNO2 HO2
m-ToluidineCH3 m-TolueneCH3 m-Cresol CH3
Note : Diazonium salts are diazonium chloride and its derivatives by a process called diazotisation.
obtained from aniline
(iii) From Grignard reagent : Chlorobenzene or bromobenzene is first converted into phenyl magnesium
halide in presence of dry ether. The Grignard reagent on reaction with oxygen and subsequent hydrolysis by a
mineral acid, yields phenol.
C6 H5 Br + Mg Ether → C6 H5 MgBr O2 →C6 H5OMgBr H2O → C6 H5OH
Phenyl magnesium bromide H+ Phenol
Bromobenzene
(iv) From salicylic acid : When salicylic acid or its sodium salt is distilled with soda lime, decarboxylation
occurs and phenol is formed. OH
OH
COOH + Na2CO3 + H2O
+ 2NaOH CaO
Salicylic acid Phenol
(v) Middle oil of coal tar distillation : Middle oil of coal-tar distillation has naphthalene and phenolic
compounds. Phenolic compounds are isolated in following steps.
Step I : Middle oil is washed with H2SO4 . It dissolves basic impurities like pyridine (base).
Step II : Excessive cooling separates naphthalene (a low melting solid)
Step III : Filtrate of step II is treated with aqueous NaOH when phenols dissolve as phenoxides. Carbon
dioxide is then blown through the solution to liberate phenols.
C6 H5OH + NaOH → C6 H5ONa + H 2O CO2, H2O → C6 H5OH + Na2CO3
Step IV : Crude phenol (of step III) is subjected to fractional distillation.
180°C Para
Crude phenols fractional 190°-203°C o, m, p-cresols
distillation 211°-235°C xylols (hydroxy xylenes)
(vi) Raschig’s process : Chlorobenzene is formed by the interaction of benzene, hydrogen chloride and air
at 250°C in presence of catalyst cupric chloride and Ferric chloride it is hydrolysed by superheated steam at 425°C
to form phenol and HCl. This is one of the latest methods for the synthesis of phenol. HCl may be again used to
convert more of benzene into chlorobenzene.
C6 H6 + HCl + 1 O2 CuCl2 /FeCl3 → C6 H5Cl + H2O ; C6 H5Cl + H 2O 425oC → C6 H5OH + HCl
2 250o C
Benzene Chlorobenzene Chlorobenzene steam Phenol
(vii) Dow process : This process involves alkaline hydrolysis of chlorobenzene. Large quantities of phenol
are formed by heating chlorobenzene with a 10% solution of caustic soda or sodium carbonate at 300°C under a
very high pressure (200 atm).
C6 H5Cl + 2NaOH 300oC → C6 H 5 ONa + NaCl + H2O
Chlorobenzene High pressure
sodium phenoxide on treatment with mineral acid yields phenol.
2C6 H5ONa + H2SO4 → 2C6 H5OH + Na2SO4
(viii) Oxidation of benzene : This is the latest method for the manufacture of phenol. The mixture of
benzene and air is passed over vanadium pentaoxide at 315°C. Benzene is directly oxidised to phenol.
2C6 H6 + O2 V2O5 → 2C6 H 5 OH
315o C
(ix) Oxidation of isopropyl benzene [Cumene] : Cumene is oxidised with oxygen or air into cumene
hydroperoxide in presence of a catalyst. This is decomposed by dilute sulphuric acid into phenol and acetone.
H3C CH3 O – OH
+ CH3CH2CH2Cl AlCl3 CH | OH
C(CH3)2
O2 H2O/H+ + (CH3)2CO
Catalyst Acetone
+ CH3CH = CH2 AlCl3
Cumene Cumene Phenol
hydroperoxide
(2) Physical properties
(i) Phenol is a colourless crystalline, deliquescent solid. It attains pink colour on exposure to air and light.
(ii) They are capable of forming intermolecular H-bonding among themselves and with water. Thus, they have
high b.p. and they are soluble in water. HH
Hδ+ – Oδ–-------Hδ+ – O| -------Hδ+ – Oδ–-------Hδ+ – |
O-------
δ– δ–
Hδ+ – Oδ–-------H – O-------Hδ+ – Oδ–-------H – O-------
δ+ δ– δ+ δ–
(crossed intermolecular H-bonding between water
and phenol molecules)
(intermolecular H-bonding among phenol molecules)
• Due to intermolecular H- bonding and high dipole moment, m.p. and b.p. of phenol are much higher than
that of hydrocarbon of comparable molecular weights.
Compounds Molecular weight Boiling point Melting point
Phenol 94 g mol−1 182°C 41°C
Toluene 92 g mol−1 111°C –95°C
(iii) It has a peculiar characteristic smell and a strong corrosive action on skin.
(iv) It is sparingly soluble in water but readily soluble in organic solvents such as alcohol, benzene and ether.
(v) It is poisonous in nature but acts as antiseptic and disinfectant.
(3) Chemical properties
(i) Acidic nature : Phenol is a weak acid. The acidic nature of phenol is due to the formation of stable
phenoxide ion in solution.
C6 H 5 OH + H 2O ⇌ C6 H 5 O − + H 3 O +
Phenoxide ion
The phenoxide ion is stable due to resonance.
O– O OO
.–. .–.
.–.
The negative charge is spread throughout the benzene ring. This charge delocalisation is a stabilising factor in
the phenoxide ion and increase acidity of phenol. [No resonance is possible in alkoxide ions (RO–) derived from
alcohols. The negative charge is localised on oxygen atom. Thus alcohols are not acidic].
Note : Phenols are much more acidic than alcohols but less so than carboxylic acids or even carbonic
acid. This is indicated by the values of ionisation constants. The relative acidity follows the following
order
Ka (approx.) (10−5 ) > (10−7 ) > (10−10 ) > (10−14 ) > (10−18 )
RCOOH H2CO3 C6H5OH HOH ROH
Phenol Water
Carboxylic acid Carbonic acid Alcohols
Effects of substituents on the acidity of phenols : Presence of electron attracting group, (e.g., − NO2 ,
–X, − NR3+ , –CN, –CHO, –COOH) on the benzene ring increases the acidity of phenol as it enables the ring to draw
more electrons from the phenoxy oxygen and thus releasing easily the proton. Further, the particular effect is more
when the substituent is present on o- or p-position than in m-position to the phenolic group.
The relative strengths of some phenols (as acids) are as follows :
p-Nitrophenol > o-Nitrophenol > m- Nitrophenol > Phenol
Presence of electron releasing group, (e.g., − CH3 , − C2H5 , − OCH3, − NR2 ) on the benzene ring decreases
the acidity of phenol as it strengthens the negative charge on phenoxy oxygen and thus proton release becomes
difficult. Thus, cresols are less acidic than phenol.
However, m-methoxy and m-aminophenols are stronger acids than phenol because of –I effect and absence of
+R effect.
m-methoxy phenol > m-amino phenol > phenol > o-methoxy phenol > p-methoxy phenol
Greater the value of Ka or lower the value of pKa , stronger will be the acid. Some other examples of acidic
nature of phenols are,
Chloro phenols : o- > m- > p-
Cresols : m- > p- > o-
Dihydric phenol : m- > p- > o-
The acidic nature of phenol is observed in the following :
(a) Phenol changes blue litmus to red.
(b) Highly electropositive metals react with phenol. 2C6 H5OH + 2Na → 2C6 H5ONa + H2
(c) Phenol reacts with strong alkalies to form phenoxides. C6 H5OH + NaOH → C6 H5ONa + H2O
However, phenol does not decompose sodium carbonate or sodium bicarbonate, i.e., CO2 is not evolved
because phenol is weaker than carbonic acid.
(ii) Reactions of –OH group
(a) Reaction with FeCl3 : Phenol gives violet colouration with ferric chloride solution (neutral) due to the
formation of a coloured iron complex, which is a characteristic to the existence of keto-enol tautomerism in phenols
(predominantly enolic form).
OH O OH
;6 + FeCl3 → 3H+ + Fe O 3–
+ 3HCl
Enol Keto 6
This is the test of phenol.
(b) Ether formation : Phenol reacts with alkyl halides in alkali solution to form phenyl ethers (Williamson’s
synthesis). The phenoxide ion is a nucleophile and will replace halogenation of alkyl halide.
C6 H5OH + NaOH → C6 H5ONa+ H 2O ; C6 H5ONa + ClCH3 → C6 H5OCH3 + NaCl
Sod. phenoxide Methyl phenyl ether (Anisole)
C6 H5OK + IC2H5 → C6 H5 − O − C2H5 + KI ; C6 H5ONa + Cl − HC(CH3 )2 → C6 H5 − O − HC(CH3 )2
Ethoxy benzene (Phenetol) Isopropyl chloride Isopropyl phenyl ether
Ethers are also formed when vapours of phenol and an alcohol are heated over thoria (ThO2 ) or Al2O3 .
C6 H5OH + HOCH3 ∆,ThO2 → C6 H5 − O − CH3
Methoxy benzene
(c) Ester formation : Phenol reacts with acid chlorides (or acid anhydrides) in alkali solution to form
phenylesters (Acylation). This reaction (Benzoylation) is called Schotten-Baumann reaction.
O
||
C6 H5OH + NaOH → C6 H5ONa + H2O ; C6 H5ONa + Cl C CH3 → C6 H5OOCCH3 + NaCl
Sodium phenoxide Acetyl chloride Phenyl acetate
C6 H5OH + (CH 3CO)2 O NaOH → C6 H5OOCCH3 + CH 3COOH ;
Acetic anhydride Phenyl acetate (ester)
OO
|| ||
C6 H5OH + Cl C6 H5 NaOH → C6H5 − O − C6H5 + NaCl + H2O
C− C−
Benzoyl chloride Phenyl benzoate
The phenyl esters on treatment with anhydrous AlCl3 undergoes Fries rearrangement to give o- and p-
hydroxy ketones. OOCCH3 OH OH
AlCl3 (anhydrous) COCH3
heat +
Phenyl acetate o- Cp OCH3
hydroxy acetophenone
(d) Reaction with PCl5 : Phenol reacts with PCl5 to form chlorobenzene. The yield of chlorobenzene is poor
and mainly triphenyl phosphate is formed.
C6 H5OH + PCl5 → C6 H5Cl + POCl3 + HCl ; 3C6 H5OH + POCl3 → (C6 H5 )3 PO4 + 3HCl
(e) Reaction with zinc dust : When phenol is distilled with zinc dust, benzene is obtained.
C6 H5OH + Zn → C6 H6 + ZnO
(f) Reaction with ammonia : Phenol reacts with ammonia in presence of anhydrous zinc chloride at 300°C or
(NH4 )2 SO3 / NH3 at 150°C to form aniline. This conversion of phenol into aniline is called Bucherer reaction.
C6 H5OH + NH 3 ZnCl2 → C6 H5 NH 2 + H2O
300o C
Aniline
(g) Action of P2S5 : By heating phenol with phosphorus penta sulphide, thiophenols are formed.
5C6 H5OH + P2S5 → 5C6 H5SH + P2O5
Thiophenol
(iii) Reactions of benzene nucleus : The –OH group is ortho and para directing. It activates the benzene
nucleus.
(a) Halogenation : Phenol reacts with bromine in carbon disulphide (or CHCl3 ) at low temperature to form
mixture of ortho and para bromophenol.
OH OH OH OH OH
+ Br2 (CS2) Br ; + 3Br2
+ Br Br
+ 3HBr
o-Bromophenol Br
Br 2, 4, 6-Tribromophenol
p-Bromophenol
Phenol forms a white precipitate with excess of bromine water yielding 2, 4, 6-tribromophenol.
(b) Sulphonation : Phenol reacts with conc. H 2SO4 readily to form mixture of ortho and para hydroxy
benzene sulphonic acids. OH OH OH
(H2SO4) SO3H
+
o-Hydroxybenzene SO3H
sulphonic acid
p-Hydroxybenzene
sulphonic acid
At low temperature (25°C), the ortho-isomer is the major product, whereas at 100°C, it gives mainly the para-
isomer.
(c) Nitration : Phenol reacts with dilute nitric acid at 5-10°C to form ortho and para nitro phenols, but the yield
is poor due to oxidation of phenolic group. The –OH group is activating group, hence nitration is possible with
dilute nitric acid. OH OH OH
HNO3(dil.) NO2
(5-10°C) +
o-Nitrophenol
NO2
p-Nitrophenol
It is believed that the mechanism of the above reaction involves the formation of o- and p- nitroso phenol with
nitrous acid, HNO2(NaNO2 + HCl) at 0-5°C, which gets oxidised to o- and p- nitrophenol with dilute nitric acid.
OH OH OH OH OH
NO NO2
HONO + [O] +
(0-5°C) HNO3 (Dil.)
o-Nitrosophenol o- Np-O2
NO Nitrophenol
p-Nitrosophenol
However, when phenol is treated with concentrated HNO3 in presence of concentrated H 2SO4 , 2,4,6-
trinitrophenol (Picric acid) is formed. OH OH
HNO3 (conc.) O2N NO2
H2SO4(conc.)
NO2, 4, 6-Trinitrop2henol
(picric acid)
To get better yield of picric acid, first sulphonation of phenol is made and then nitrated. Presence of − SO3H
group prevents oxidation of phenol.
(d) Friedel-Craft’s reaction : Phenol when treated with methyl chloride in presence of anhydrous aluminium
chloride, p-cresol is the main product. A very small amount of o-cresol is also formed.
OH OH OH
+ CH3Cl AlCl3 + CH3
o-cresol
CH3
p-Cresol
(major product)
RX and AlCl3 give poor yields because AlCl3 coordinates with O. So Ring alkylation takes place as follows,
C6 H5OH + AlCl3 → C6 H5OAlCl2 + HCl
Thus to carry out successful Friedel-Craft’s reaction with phenol it is necessary to use a large amount of AlCl3 .
The Ring alkylation takes place as follows :
C6 H5OH + CH 3 CH = CH 2 H2SO4 → o - and p - C6 H4 / OH
3 )2 CH − OH or HF \ CH(CH 3 )2
(CH
OH OH OH
+ CH3COCl anhydrous AlCl3 COCH3
Acetyl chloride +
ortho COCH3
P
hydroxy acetophenone
(e) Kolbe-Schmidt reaction (Carbonation) : This involves the interaction of sodium phenoxide with carbon
dioxide at 130-140°C under pressure of 6 atmospheres followed by acid hydrolysis, Salicylic acid (o-Hydroxy
benzoic acid) is formed. OH
COOC6H5
∆
ONa OCOONa OH OH Salol
COOH
+ CO2 130-140°C Rearrangement COONa OCOCH3
6 atm CH3COCl COOH
H+
H2O
Sodium phenyl Sodium salicylate Salicylic acid Aspirin
carbonate
OH
CH3OH COOCH3
The probable mechanism is : C6 H 5 OH → C6 H 5 O − + H + OH Winter green
O– O OH
C – O– COOH
H –H+ H+
+C=O C – O– +H+ ||
|| || O
O O
At higher temperature (250-300°C), p-isomer is obtained.
(f) Reimer-Tiemann reaction : Phenol, on refluxing with chloroform and sodium hydroxide (aq.) followed
by acid hydrolysis yields salicylaldehyde (o-hydroxy benzaldehyde) and a very small amount of p-hydroxy
benzaldehyde. However, when carbon tetrachloride is used, salicylic acid (predominating product) is formed.
CHCl3 OH ONa OH
CHO
OH NaOH(aq.) NaOH H+
60°C H2O
CHCl2
CHO
Salicylaldehyde
CCl4 OH ONa OH
NaOH(aq.) NaOH H+ COOH
60°C H2O
CCl3 Salicylic acid
COONa
The electrophile is dichloromethylene : CCl2 generated from chloroform by the action of a base.
OH − + CHCl3 ⇌ HOH+ : C− Cl3 → Cl − + : CCl2
Dichloro carbene
Reimer-Tiemann reaction involves electrophilic substitution on the highly reactive phenoxide ring.
C6 H5OH → C6 H5O − + H + O– O–
Phenoxide + : CCl2 CHCl2
Attack of electrophile (: CCl2 )
O– O– O– O– OH
CHCl2 CH(OH)2 CHO ; CHO
+ 2NaOH Hydrolysis CHO
–H2O + H+ Salicylaldehyde
(g) Gattermann’s reaction : Phenol, when treated with liquid hydrogen cyanide and hydrochloric acid gas in
presence of anhydrous aluminium chloride yields mainly p-hydroxy benzaldehyde (Formylation).
HCl + HC ≡ N AlCl3 → ClCH = NH
OH OH OH
+ ClCH = NH AlCl3 H2O
HCl –NH3
CH = NH CHO
p-Hydroxy
benzaldehyde
(h) Mercuration : Phenol when heated with mercuric acetate undergoes mercuration to form o- and p-isomers.
OH OH OH
+ (CH3COO)2Hg
HgOCOCH3
+
o-Hydroxy phenyl HgOCOCH3
mercuric acetate
p-Hydroxy phenyl
mercuric acetate
(i) Hydrogenation : Phenol, when hydrogenated in presence of a nickel catalyst at about 150-200°C, forms
cyclohexanol. OH OH
+ 3H2 Ni
150-200°C
(iv) Miscellaneous reactions Phenol Cyclohexanol (C6H11,OH)
(C6H5OH) (used as a good solvent)
(a) Coupling reactions : Phenol couples with benzene diazonium chloride in presence of an alkaline solution to
form a red dye (p-hydroxy azobenzene).
N = NCl + OH NaOH N=N OH
–HCl
p-Hydroxyazobenzene
Benzene diazonium chloride Phenol
Phenol couples with phthalic anhydride in presence of concentrated H2SO4 to form a dye, (phenolphthalein)
used as an indicator.
O ∆ O O
|| H || ||
C – OH C C
O Conc. H2SO4
C – OH O
(–H2O)
|| C
C
O
||Phthalic anhydride
Phthalic acid
O
H
OH OH OH OH
Phenol (2 molecules) Phenolphthalein
(b) Condensation with formaldehyde : Phenol condenses with formaldehyde (excess) in presence of sodium
hydroxide or acid (H + ) for about a week to form a polymer known as bakelite (a resin).
OH OH OH OH OH
CH2 CH2
CH2OH CH2
CH2 CH2
+ CH2O NaOH + Condensation with
HCHO
continues give
o-hydroxy benzyl
alcohol
p-hydrCoxHy b2eOnzyHl
alcohol
Polymer Bakelite (a resin)
(c) Liebermann’s nitroso reaction : When phenol is reacted with NaNO2 and concentrated H 2SO4 , it gives a
deep green or blue colour which changes to red on dilution with water. When made alkaline with NaOH original
green or blue colour is restored. This reaction is known as Liebermann’s nitroso reaction and is used as a test of
phenol.
OH HONO NO OH O NOH
p-Nitrosophenol Quinoxim
O N OH + H OH OH2SO4 N OH NaOH Sod. Salt of indophenol
H2O –H2O (blue)
Indo phenol (Red)
(d) Oxidation : Phenol turns pink or red or brown on exposure to air and light due to slow oxidation. The
colour is probably due to the formation of quinone and phenoquinone.
C6H5OH or OH OO2 by air O C6H5OH OH - - - O O - - - HO
or CrO3
OH p-benzoquinone O
Phenoquinone (pink)
[O] + H2O
CrO2Cl2
Phenol O
p-benzoquinone
But on oxidation with potassium persulphate in alkaline solution, phenol forms 1, 4-dihydroxy benzene
(Quinol). This is known as Elbs persulphate oxidation.
OH OH
K2S2O8 in
alkaline solution
Phenol
OH
Quinol
(4) Uses : Phenol is extensively used in industry. The important applications of phenol are :
(i) As an antiseptic in soaps, lotions and ointments. A powerful antiseptic is “Dettol” which is a phenol
derivative (2, 4-dichloro-3, 5-dimethyl phenol).
(ii) In the manufacture of azo dyes, phenolphthalein, etc.
(iii) In the preparation of picric acid used as an explosive and for dyeing silk and wool.
(iv) In the manufacture of cyclohexanol required for the production of nylon and used as a solvent for rubber
and lacquers.
(v) As a preservative for ink.
(vi) In the manufacture of phenol-formaldehyde plastics such as bakelite.
(vii) In the manufacture of drugs like aspirin, salol, phenacetin, etc.
(viii) For causterising wounds caused by the bite of mad dogs.
(ix) As a starting material for the manufacture of nylon and artificial tannins.
(x) In the preparation of disinfectants, fungicides and bactericides.
(5) Tests of phenol : (i) Aqueous solution of phenol gives a violet colouration with a drop of ferric chloride.
(ii) Aqueous solution of phenol gives a white precipitate of 2, 4, 6-tribromophenol with bromine water.
(iii) Phenol gives Liebermann’s nitroso reaction.
Phenol in conc. sulphuric acid NaNO2 → Red colour NaOH → Blue colour
Excess of water (Excess)
(iv) Phenol combines with phthalic anhydride in presence of conc. H 2SO4 to form phenolphthalein which
gives pink colour with alkali, and used as an indicator.
(v) With ammonia and sodium hypochlorite, phenol gives blue colour.
Difference between phenol and alcohol
Property Phenol (C6H5OH) Alcohol (C2H5OH)
Typical phenolic odour Pleasant alcoholic odour
Odour
Nature, reaction with alkali Acidic, dissolves in sodium Neutral, no reaction with alkalies.
sodium
hydroxide forming
phenoxide.
Reaction with neutral FeCl3 Gives violet colouration due to No reaction.
formation of complex compound.
Reaction with halogen acids No reaction with halogen acids. Forms ethyl halides.
Oxidation
Pink or brown colour due to Undergoes oxidation to give
formation of quinone and acetaldehyde and acetic acid.
phenoquinone.
Reaction with HCHO Forms polymer (bakelite). No reaction.
Liebermann’s nitroso reaction Positive. Does not show.
Forms azo dye. Does not form any dye.
Coupling with benzene diazonium
chloride Mainly forms triphenyl phosphate. Forms ethyl chloride
Does not show. Positive.
Reaction with PCl5
Iodoform test
Derivatives of phenol.
NITROPHENOLS
(1) Preparation
(i) Phenol easily undergoes nitration. Ortho and para nitrophenols are obtained by nitration of phenol with
dilute HNO3 in cold. Ortho isomer is separated by steam distillation as it is steam volatile.
OH OH OH
Dil. HNO3 NO2
+
o-isomer
(steam volatile)
p-iNsoOme2r
(ii) o- and p-forms are also obtained by treating chloro or bromo nit(rnoonb-veonlaztilee)ne with caustic alkali at 120°C.
Cl OH
C6 H4 NaOH → C6 H4
NO2
NO2
o- and p-nitrophenol
o- and p-chloro nitrobenzene
(iii) When heated with solid potassium hydroxide, nitrobenzene produces a mixture of o- and p-nitrophenols.
OH
C6 H5 NO2 Solid KOH → C6 H 4
Nitrobenzene heat
NO2
o- and p-nitrophenol
(iv) m-Nitrophenol is obtained from m-dinitrobenzene. One of the nitrogroup is converted into − NH2 group
which is diazotised. The diazonium compound on boiling yields m-nitrophenol.
NO2 NH2 N2Cl OH
NH4HS NaNO2/HCl H2O
or Na2S 0-5°C
m-DinitrobenzenNe O2 m-NitroanilineNO2 m-NitrobenzenNe O2 m-NitrophenolNO2
diazonium chloride
(2) Properties : o-Nitrophenol is a yellow coloured crystalline compound, while m- and p-isomers are
colourless crystalline compounds.
Isomer ortho meta para
m.pt. (°C) 45 97 114
the lowest melting point of o-isomer is due to intramolecular hydrogen bonding whereas meta and para
isomers possess intermolecular hydrogen bonding and thus, they have higher melting points.
They are stronger acids than phenol. The order is :
p-isomer > o-isomer > m-isomer > phenol
When reduced, they form corresponding aminophenols. o- and p-Nitrophenols react with bromine water to
form 2, 4, 6-tribromophenol by replacement of nitro group. OH
/ OH Br Br
\ NO2 Br
C6 H 4 Br2 →
o- or p-isomer
2,4,6 Tribromophenol
Picric acid (2, 4, 6-trinitrophenol)
(1) Preparation : It is obtained when phenol is treated with conc. HNO3 . However, the yield is very poor. It
is prepared on an industrial scale :
(i) From chlorobenzene
Cl Cl OH OH
NO2
HNO3 NO2 NO2 HNO3 O2N
H2SO4 Aq. Na2CO3 H2SO4
Chlorobenzene NO2 NO2
2, 4-Dinitrochlorobenzene PicrNic Oaci2d
(2, 4, 6-Trinitrophenol)
(ii) From phenol through disulphonic acid
OH OH OH
NO2
H2SO4 SO3H O2N
HNO3
Phenol
Phenol dSisOulp3hHonic acid PicrNic Oaci2d
(iii) It may be prepared in the laboratory by oxidation of s-trinitrobenzene (TNB) with potassium ferricyanide.
OH
O2N NO2 Ke3Fe(CN)6 O2N NO2
+ [O]
NO2 NO2
Picric acid
(2) Properties : It is a yellow crystalline solid, m.pt. 122°C. it is insoluble in cold water but soluble in hot
water and in ether. It is bitter in taste. Due to the presence of three electronegative nitro groups, it is a stronger acid
than phenol and its properties are comparable to the carboxylic acid. It neutralises alkalies and decomposes
carbonates with evolution of carbon dioxide.
Dry picric acid as well as its potassium or ammonium salts explode violently when detonated. It reacts with
PCl5 to form picryl chloride which on shaking with NH3 yields picramide.
O2N OH O NPHC2Ol5 2 Cl NH3 O2N NH2 NO2
NO2 NO2
NO2 PicrylNchOlor2ide PicrNamOid2e
When distilled with a paste of bleaching powder, it gets decomposed and yields chloropicrin, CCl3 NO2 , as
one of the products and is thus employed for the manufacture of tear gas.
It forms yellow, orange or red coloured molecular compounds called picrates with aromatic hydrocarbons,
amines and phenols which are used for characterisation of these compounds.
Note : Picrates are explosive in nature and explode violently when heated. These are prepared carefully.
(3) Uses : It is used as a yellow dye for silk and wool, as an explosive and as an antiseptic in treatment of burns.
Catechol (1, 2-Dihydroxy benzene)
(1) Preparation
(i) By fusion of chlorosubstituted phenolic acid with caustic soda. or by hydrolysis of o-dichlorobenzene or o-
chlorophenol with dilute NaOH solution at 200°C and in the presence of copper sulphate catalyst.
OH OH OH Cl ONa OH
Cl + CO2 + NaCl ; ONa OH
Catechol Cl H+/H2O
NaOH
NaOH
COOH 200°C, Cu2+
(ii) By fusing alkali salt of o-phenol sulphonic acid with caustic alkali and then hydrolysing the product with
mineral acid. OH OK OH
SO3K OK 2HCl OH
+ 3KOH
(iii) It may be conveniently prepared in the laboratory by treating salicylaldehyde with alkaline H2O2 .
CHO OH
+ H2O2 + NaOH + HCOONa + H2O
OH
OH
Catechol
Salicylaldehyde
(2) Properties : It is a colourless crystalline solid, m.pt. 105°C. it is soluble in water. It is affected on exposure
to air and light. It acts as a reducing agent as it reduces Tollen’s reagent in cold and Fehling’s solution on heating.
With silver oxide it is oxidised to o-benzoquinone. O
OH
OH O + 2Ag + H2O
+ Ag2O
o-Benzoquinone
It forms insoluble lead salt (white ppt.) when treated with lead acetate solution and gives green colour with
FeCl 3 which changes to red on adding Na2CO3 solution. It forms alizarin dye stuff when condensed with phthalic
anhydride in the presence of sulphuric acid.
O OH O OH
|| ||
C OH C OH
Con H2SO4
O+ (–H2O) C
C ||
|| O
O Alizarin
Phthalic anhydride
(iii) Uses : It finds use as photographic developer, in the manufacture of alizarin and adrenaline hormone and
as an antioxidant (inhibitor in auto oxidation) for preserving gasoline.
Resorcinol (1, 3-Dihydroxy benzene)
(1) Preparation : It is prepared by alkali fusion of 1,3, benzene disulphonic acid (Industrial method).
SO3H NaOH ONa HCl OH
SO3H Fuse ONa OH
(2) Properties : It is a colourless crystalline solid, m.pt. 110°C. it is affected on exposure by air and light. It is
soluble in water, alcohol and ether. It shows tautomerism. Its aqueous solution gives violet colour with FeCl3 . It
reduces Fehling’s solution and Tollen’s reagent on warming.
With bromine water, it gives a crystalline precipitate, 2, 4, 6-tribromoresorcinol.
OH OH
Br Br
+ 3Br2 + 3HBr
OH OH
Br
On nitration, it forms 2, 4, 6-trinitro-1, 3-dihydroxybenzene.
OH OH
HNO3 O2N NO2
H2SO4 OH
OH
NO2
Styphnic acid
It condenses with phthalic anhydride and forms fluorescein.
OH OH
O
OH OH Conc. H2SO4 O
O=C C=O+ OH –2H2O O=C C
H
Phthalic OH Fluorescein OH
anhydride
Resorcinol
(2 moles)
With nitrous acid, it forms 2, 4-dinitrosoresorcinol
OH OH O
NOH
HNO2 NO
OH OH O
NO NOH
Resorcinol behaves as a tautomeric compound. This is shown by the fact that it forms a dioxime and a
bisulphite derivative. OH O
(3) Uses OH O
Dienol form Diketo form
(i) It is used as antiseptic and for making dyes.
(ii) It is also used in the treatment of eczema. 2, 4, 6-trinitroresorcinol is used as an explosive.
Hydroquinone or quinol (1, 4-Dihydroxy benzene)
(1) Preparation : It is formed by reduction of p-benzoquinone with sulphurous acid (H2SO3 = H2O + SO2 ).
O O + SO2 + 2H2O HO OH + H2SO4
H2SO3 + H2O Quinol
(p-Benzoquinone is obtained by oxidation of aniline)
NH2 O OH
[O] Fe/H2O
MnO2/H2SO4 [H]
O OH
Quinol
(2) Properties : It is a colourless crystalline solid, m.pt. 170°C. it is soluble in water. It also shows
tautomerism. It gives blue colour with FeCl3 solution.
It acts as a powerful reducing agent as it is easily oxidised to p-benzoquinone. It reduces Tollen’s reagent and
Fehling’s solution. HO OH [O] O O
FeCl3
p-Benzoquinone
Due to this property, it is used as photographic developer.
(3) Uses : It is used as an antiseptic, developer in photography, in the preparation of quinhydrone electrode
and as an antioxidant.
Trihydric Phenols : Three trihydroxy isomeric derivatives of benzene are Pyrogallol (1, 2, 3), hydroxy quinol
(1, 2, 4) and phloroglucinol (1, 3, 5).
Pyrogallol is obtained by heating aqueous solution of gallic acid at 220°C.
OH OH OH
OH + CO2
Pyrogallol
HOOC OH heat OH
220°C
Gallic acid
Phloroglucinol is obtained from trinitrotoluene (TNT) by following sequence of reactions.
CH3 COOH COOH
NO2 NH2 HO
O2N NO2 KMnO4 O2N Fe/HCl H2N OH
[H] H2O/H+ + CO2 + 3NH4Cl
[O] 100°C
NO2, 4, 6-trinitro2toluene NO2 NH2 OH
Phloroglucinol
Hydroxyquinol is prepared by the alkaline fusion of hydroquinone in air.
OH OH
NaOH OH
Fuse
+ ½ O2
OH OH
Quinol Hydroxy Quinol
The three isomers are colourless crystalline compounds. All are soluble in water and their aqueous solutions
give characteristic colour with FeCl3 . For example, Pyrogallol-red; Hydroxy quinol-greenish brown; Phloroglucinol-
bluish violet. Alkaline solutions absorb oxygen rapidly from air.
Uses of pyrogallol
(i) As a developer in photography.
(ii) As a hair dye.
(iii) In treatment of skin diseases like eczema.
(iv) For absorbing unreacted oxygen in gas analysis.
Hydroxy compounds and ethers
Introduction of ether.
Ethers are anhydride of alcohols, they may be obtained by elimination of a water molecule from two alcohol
molecules.
R − OH + HO − R → R −O− R+ H 2O
Ether
General formula is Cn H 2n+2O
(1) Classification : It may be divided in two category.
(i) Aliphatic ethers : Both R of ether are alkyl group.
H
|
Example : CH 3 − O − CH 3 , CH 3 C− O− CH − CH 2 − CH 3 .
Dimethyl ether − 2
|
CH3
Isopropyl propyl ether
(ii) Aromatic ethers : In which either one or both R of ether are aryl groups.
Example : C6 H5 − O − CH 3 or C6 H5 − O − C6 H5
Methyl phenyl ether Diphenyl ether
Aromatic ethers are divided in two category.
(a) Phenolic ethers : Ethers in which one of the group is aryl and other one is alkyl.
Example : C6 H5 − O − CH 3 .
methyl phenyl ether
(b) Diaryl ether : In which both the groups are aryl group.
Example : C6 H5 − O − C6 H5
Diphenyl ether
(iii) Symmetrical and unsymmetrical ethers,
(a) An ether in which both the R are same known as symmetrical ethers.
Example : CH 3 − O − CH 3 , C6 H5 − O − C6 H5 .
Dimethyl ether Diphenyl ether
(b) An ether in which the both R are different called unsymmetrical ethers or mixed ethers.
Example: CH 3 − O − C2 H5 and CH 3 − O − C6 H5 .
Ethyl methyl ether Methyl phenyl ether
(2) Structure : The oxygen atom in ether is sp3 hybridised. Two of the hybrid orbitals overlap with hybrid
orbital (one each) of two carbon atoms to form sigma bonds. C σ O σ C . The bond angle is 110o .
− −
Besides the open chain saturated homologous series, there are numerous organic compounds in which – O –
functional group is present. These compounds are also termed ethers.
O
CH 3 − OCH = CH 2 ; CH2 − CH2 ; H 2C− CH 2 ; ; C6 H5OCH3 LONE PAIRS
Methyl vinyl ether || H2C CH2 Anisole
{unsaturated} O | (Methyl phenyl ether) O
H2C CH2 |
Ethylene oxide O H2C CH2 110°
{cyclic}
RR
Tetra hydro furan O
Dioxan
(3) Nomenclature : There are two systems for naming ethers.
(i) Common names : When both the alkyl group are same, the prefix di- is used. In case of unsymmetrical
ethers two alkyl groups are named in alphabetical order.
Example : CH 3OCH3 , C2 H5OC3 H7 .
Di methyl ether Ethyl propyl ether
(ii) IUPAC system : Ethers are named as alkoxy alkanes.
(a) The ethernal oxygen is taken with smaller alkyl group.
Example : CH 3O ; C2 H5O .
Methoxy Ethoxy group
(b) The R group having the longest carbon chain is chosen as the parent alkane.
Example : CH 3 − O − C2 H5 , OCH3
Methoxy ethane
Methoxy benzene
Note : In case of simple ethers common names are also accepted in IUPAC system.
(4) Isomerism : Ether show the following types of isomerism :
(i) Chain isomerism
CH3
|
Example : CH 3 − O − CH 2CH 2CH 2CH 3 , CH3 − O − CH2 − CH − CH3 .
1-methoxy butane 1-methoxy-2-methyl propane
(ii) Functional isomerism
OCH3 CH2OH
Example : CH3 − O − CH3 ↔ CH3 − CH2 − OH ,
Dimethyl ether Ethanol
(iii) Metamerism Anisole Benzyl alcohol
Example : CH3CH2 − O − CH2CH3 ↔ CH3 − O − CH2 − CH2 − CH3
Ethoxy ethane Methoxy propane
General methods of preparation of ethers.
(1) From alkyl halides
(i) Williamson’s synthesis : It is the laboratory method for the preparation of ethers.
It is a nucleophilic substitution reaction and proceed through SN 2 mechanism.
RONa + R′X → ROR′ + NaX
C2 H5ONa + CH 3 − I → CH 3OC2 H5 + NaI
Sodium ethoxide Ethyl methyl ether
C2 H5ONa + C2 H5 Br → C2 H5OC2 H5 + NaBr
Sodium ethoxide Ethyl bromide Ethoxyethane
(a) Order of reactivity of primary halide is CH 3 X > CH 3CH 2 X > CH 3CH 2CH 2 X .
(b) Tendency of alkyl halide to undergo elimination is 3o > 2o > 1o .
(c) For better yield alkyl halide should be primary and alkoxide should be secondary or tertiary.
C2 H5 Br + NaO C| H3 C| H3
C− CH C2 H 5 O C− CH
Ethyl bromide − 3 → − − 3
| |
CH3 CH3
Sodium salt of Ethyl tert. butyl ether
tert. butyl alcohol
(d) Secondary and tertiary alkyl halides readily undergo E2 elimination in the presence of a strong base to
form alkenes.
CH3 C| H3 CH3 CH3
| − C⊕ + Cl − |
C2H5ONa → |
C− |
CH 3 − Cl CH 3 , CH 3 − C ⊕ + C 2 H 5 O − → CH 3 − C+ C2 H 5 OH
|
CH3 CH3 | ||
CH2 − H CH2
SN 2 mechanism : C2 H5ONa ⇌ C2 H5O− + Na + ,
C2 H5O− + C2 H5 I slow → C2 H5O - - - -C2 H5 - - - -I Fast → C2 H5OC2 H5 + I , Na + + I − ⇌ NaI
Trnsition state
Note : Aryl halide and sodium alkoxide cannot be used for preparing phenolic ethers because aryl halide
are less reactive toward nucleophilic substitution reaction than alkyl halides.
(ii) By heating alkyl halide with dry silver oxide
2RX + Ag 2O heat → R − O − R + 2AgX , 2C2 H5 Br+ Ag 2O heat → C2 H5OC2 H5 + 2AgBr
Ethyl bromide Diethyl ether
(2) From alcohols
(i) By dehydration of alcohols
(a) With conc. H2SO4 at 140° C : ROH + HOR H2SO4(conc.) → ROR+ H 2O .
140o C
2 molecules of alcohol Ether
Note : In this reaction alcohol must be present in excess.
This reaction is mainly applicable for the dehydration of primary alcohols. Secondary and tertiary
alcohols form alkenes mainly.
When this reaction is carried out between different alcohols then there is a mixture of different ethers
is obtained.
Example : CH 3OH+ C2 H5OH H2SO4(conc.) → CH 3 −O − C2H5 + CH 3 − O− CH 3 + C2 H 5 −O − C2H5
140o C
Methanol Ethanol
(b) With Al2O3 at 250° C : 2ROH Al2O3 → R − O − R + H2O
250o C
(ii) By the action of diazomethane on alcohols : This reaction is in presence of catalyst, boron trifluoride
or HBF4 .
ROH + CH 2 N 2 BF3 → R − O − CH 3 + N 2
(iii) Addition of alcohols to alkenes : CH 2 = CH 2 + HOR H2SO4 → CH 3 − CH 2 − OR .
Ether
Alcohol
The intermediate is carbonium ion.
CH 2 = CH 2 + H+ ⇌ CH 3 C+ H 2 , CH3 C+ H2 + HOR → CH3CH2 − O+ − R ,
|
H
CH 3CH 2 − O+ − R −H+ → CH 3CH 2 − O − R.
Ether
|
H
(a) This method is very useful for preparing mixed ethers.
(b) In higher cases, there can be 1, 2-hydride or 1, 2-methyl shift to form more stable carbonium ion.
(3) Alkoxy mercuration-demercuration
> C=C < +R − OH + Hg[OOCCF3 ]2 → − | | NaBH4 → | |
alkene
Mercuric trifluoro acetate C− C −C− C
|
| | OR |
OR HgOOCCF3 H
Ether
Note : This is the best method for the preparation of t-ethers.
(4) Reaction of lower halogenated ether with grignard reagent
ROCH2 X + XMgR′ → ROCH2 R′+ MgX2
Halogenated Grignard Higher
ether reagant ether
(i) Higher members can be prepared by the action of grignard reagent on lower halogenated ethers.
(ii) Ether form soluble coordinated complexes with grignard reagent.
General properties of ethers.
Physical properties
(1) Physical state : Methoxy methane and methoxy ethane are gases while other members are volatile liquid
with pleasant smell.
(2) Dipole moment (D.M.) : Bond angle of ether is due to sp3 hybridisation of oxygen atom. Since C – O
bond is a polar bond, hence ether possess a net dipole moment, even if they are symmetrical.
D.M. of dimethyl ether is 1.3 D
D.M. of di ethyl ether is 1.18 D
Note : The larger bond angle may be because of greater repulsive interaction between bulkier alkyl
groups as compared to smaller H-atoms in water.
(3) Boiling points : Boiling points of ethers are much lower than those of isomeric alcohols, but closer to
alkanes having comparable mass. This is due to the absence of hydrogen bonding in ethers.
(4) Solubility : Solubilities of ethers in water are comparable with those of alcohols.
Example : Di ethyl ether and n-butyl alcohol have approximately the same solubility in water. This is because,
ether form hydrogen bond with water much in the same way as alcohol do with water.
R O.......... H O R
R R
H.......... .O
Ether Water
Ether
Note : Solubility of ether in water decreases with the size of alkyl groups.
(5) Hydrogen bonding : There is no hydrogen directly attach (bonded) to oxygen in ethers, so ethers do not
show any intermolecular hydrogen bonding.
RRR
|||
H − O- - - H − O- - - H − O- - - R − O − R
hydrogenbonding in alcohols No hydrogen bond in ether
(6) Density : Ethers are lighter than water.
Chemical properties : Ethers are quite stable compounds. These are not easily attacked by alkalies, dilute
mineral acids, active metals, reducing agents or oxidising agents under ordinary conditions.
(1) Reaction due to alkyl group
(i) Halogenation : CH 3CH 2OCH 2CH 3 Cl2 → CH 3CHClOCH 2CH 3
Diethyl ether dark
(α -Monochlorodiethyl ether )
CH3CH2OCH2CH3 Cl2 → CH3CHClOCHClCH 3
Diethyl ether dark
(α,α ′-Dichlorodiethyl ether)
C2H5OC2H5 + 10Cl2 Cl2 → C2Cl5OC2Cl5 + 10 HCl
light
(Perchlorodiethyl ether )
(ii) Burning : Ethers are highly inflammable. They burn like alkanes.
C2 H5 − O − C2 H5 + 6O2 → 4CO2 + 5H 2O
(2) Reaction due to ethernal oxygen
.. .. ..
(i) Peroxide formation : C2H5 O.. C2H5 + O.. :→ C2H5 O C2 H 5 or (C2H5)2O → O .
↓..
:O.. :
(a) The boiling point of peroxide is higher than that of ether. It is left as residue in the distillation of ether and
may cause explosion. Therefore ether may never be evaporated to dryness.
(b) Absolute ether can be prepared by distillation of ordinary ether from conc. H2SO4 and subsequent storing
over metallic sodium.
Note : Formation of peroxide can be prevented by adding small amount of Cu2O to ether.
With strong oxidising agent like acid, dichromate ethers are oxidised to aldehydes.
CH3CH2OCH2CH3 2[O] → 2CH3CHO+ H2O
Acetaldehyde
The presence of peroxide can be indicated by the formation of blood red colour complex in the
following reaction.
Peroxide + Fe +2 → Fe +3 SCN− →[Fe(SCN)n]3−n
Blood red colour
(n=1 to 6)
(ii) Oxidation with K2Cr2O7 / H⊕ : α α′ R′
R′
R − CH2 − O − CH
(a) Oxidation of ether can only be possible if any one of the alkyl groups of ether has hydrogen on α-carbon.
(b) α-carbon having two hydrogens converts in carboxylic group and α-carbon having only one hydrogen
converts into keto group.
CH 3 − α O − α′ CH 2 − CH 3 K2Cr2O7 → CH 3 − COOH + CH 3 − CH 2 − COOH
H⊕ / ∆
CH 2 − CH 2 −
CH3 O
CH3 ||
CH3 − CH2 − O − CH K2Cr2O7 → CH 3 − COOH + CH3 − CH3
H⊕ /∆ C−
(iii) Salt formation : Due to lone pair of electrons on oxygen atom. Ether behaves as Lewis base and form
stable oxonium salt with strong inorganic acids at low temperature.
H
|
C2 H5OC2 H5 + HCl → (C2 H5 )2 O.. + Cl − or [(C2 H5 )2 O ⋅ H]+ Cl −
Diethyl oxonium chloride
..
C2H5OC2H5 + H2SO4 → (C 2 H5 )2 O+ + HSO4− or [(C2 H5 )2 O ⋅ H]+ HSO4−
|
H
Diethyl oxonium hydrogen sulphate
The oxonium salts are soluble in acid solution and ethers can be recovered from the oxonium salts by
treatment with water. (C2 H5 )2 O Cl H2O →(C2 H5 )2 O+ HCl
Diethyl ether
|
H
Oxonium salt
Note : The formation of oxonium salt is similar to the formation of ammonium salts from ammonia and
acids.
Ether is removed from alkyl halides by shaking with conc. H 2SO4 .
Ethers can be distinguished from alkanes with the help of this reaction.
(iv) Reaction with Lewis acids : Being Lewis bases, ethers form complexes with Lewis acids such as BF3 ,
AlCl3 , FeCl3 , etc. These complexes are called etherates. CH 3 CH 2 .. → CH 3 CH 2 ..
CH 3 CH 2 O : +BF3 CH 3 CH 2 O → BF3
Boron trifluoride etherate (complex)
Similarly, diethyl ether reacts with Grignard reagent forming Grignard reagent etherate.
2(CH3CH2)2 O + RMgX → R Mg O(CH 2CH 3 )2
(CH3CH2)2 O X
Grignard reagent etherate
Due to the formation of the etherate, Grignard reagents dissolve in ether. That is why Grignard reagents are
usually prepared in ethers. However, they cannot be prepared in benzene, because benzene has no lone pair of
electrons and therefore, cannot form complexes with them.
(3) Reaction involving cleavage of carbon-oxygen bond
(i) Hydrolysis
(a) With dil. H 2SO4 : ROR + H 2O H2SO4 → 2ROH
C2 H5OC2 H5 + H 2O H2SO4 → 2C2 H5OH
Diethyl ether Ethanol
(b) With conc. H 2SO4 : C2H5OCC2H2H5O5 H+ +HH2S2SOO44→→CC22HH55HOSHO+4 +CH22HO5 HSO4
C2H5OC2H5 +2H2SO4 → 2C2H5 HSO4 +H2O
Diethyl ether Ethyl hydrogen sulphate
(ii) Action of hydroiodic acid
(a) With cold HI : C2H5OC2H5 + HI Cold → C2H5 I + C2H5OH
Diethyl ether Ethyl iodide Ethyl alcohol
Note : Same reaction are observed with HBr and HCl. The order of reactivity of halogen acid
HI>HBr>HCl. OC2H5 OH
+ HBr → + C2H5Br
Phenyl ethyl ether Phenol Ethyl bromide
Mechanism of the reaction
R − O − R+ H − X → .+. X − + R − O.+. R → R−X + R − OH
Ether R − O− R + X− , H Alcohol
Alkyl halide
| Nucleophile
H
Protonated ether
• This can be explained on the basis of steric hinderence.
• The above reaction is a SN ′ reaction.
• The ether molecule gets protonated by the hydrogen of the acid to form protonated ether and the
protonated ether undergoes nucleophilic attack by halide ion [X − ] and form alkyl alcohol and alkyl
halide.
(b) With hot HI : R − O − R′ + 2HI heat → RI + R′I + H 2O
(iii) Zeisel method : RI + AgNO3 (alc.) → AgI ↓ +RNO3
Note : The silver iodide thus form can be detected and estimated. This formed the basis of Zeisel method
for the detection and estimation of alkoxy group in a compound.
(iv) Action of PCl5 : R − O − R + PCl5 heat → 2RCl + POCl3 . There is no reaction in cold.
(v) Reaction with acetyl chloride : CH 3COCl+ C2 H5 ⋅ O ⋅ C2 H5 ZnCl2 → CH 3 COOC2 H 5
heat
Acetyl chloride Diethyl ether Ethyl acetate
(vi) Reaction with acid anhydride : CH3CO ⋅ O ⋅ OCCH3 + C2 H5 ⋅ O ⋅ C2 H5 ZnCl2 → 2CH 3 COOC2 H 5
heat Ethyl acetate
Acetic anhydride Diethyl ether
(vii) Dehydration : C2 H5OC2 H5 Al2O3 → 2CH 2 = CH 2 + H2O
300o C
(viii) Reaction with carbon mono oxide : C2 H5OC2 H 5 + CO BF3/ 150oC → C2 H5COOC2 H 5
Diethyl ether 500 atm.
Ethyl propionate
(ix) Action of bases : L+ i C− H3 + H − CH2 − CH2 − O − CH2 − CH3 → CH4 + CH2 = CH2 + L+ i O− C2 H5
(4) Ring substitution in aromatic ethers : Alkoxy group is ortho and para directing and it directs the
incoming groups to ortho and para position. It activates the aromatic ring towards electrophilic substitution reaction.
.. .. .. .. ..
:OR :O – R +OR +OR +OR
.. :
.. V
I II III IV
III, Iv and V show high electron density at ortho and para position.
(i) Halogenation : Phenyl alkyl ethers undergo usual halogenation in benzene ring.
For example, Bromination of anisole gives ortho and para bromo derivative even in the absence of iron (III)
bromide catalyst. OCH3 OCH3 OCH3
Br2 → Br
CS2
+
Anisole o-Bromoanisole Br
Para isomer is obtained in 90% yield. p-Bromoanisole
(ii) Friedel craft reaction
OCH3 OCH3 OCH3 OCH3 OCH3 OCH3
COCH3
+ CH3Cl AlCl3 → CH3 ; + CH 3COCl AlCl3 → COCH3
Methyl chloride +
+ p Methoxy
acetophenone
Anisole Ortho Anisole o-Methoxy
acetophenone
CH3
OCH3 OCH3
Para
(iii) Nitration : HNO3 /H2SO4 →
OCH3
NO3
+
Methyl phenyl Methyl-2 NO3
ether nitrophenyl ether
(o-Nitroanisole) Methyl-4
(Anisole)
nitrophenyl ether
Note : (p-Nitroanisole)
Ethers are relatively less reactive than phenol towards electrophilic substitution reaction.
Some commercially important compounds of ethers.
(1) Di methyl ether : It is the simplest ether.
(i) Preparation
(a) It can be prepared by using any of the general method of preparation.
(b) 2CH 3 OH ∆,Alumina→ CH 3 − O − CH3 + H2O
350o −400o C,15 atm
(c) It can be manufactured by dehydration of methyl alcohol with conc. H2SO4 at 140o C .
(ii) Properties
(a) It is colourless highly inflammable gas having B.P. of − 24.8o C .
(b) It is soluble in water, alcohols and other organic solvents and gives all the characteristic reactions of ethers.
(iii) Uses
(a) It is used in the form of compressed liquid as a refrigerant, low temperature solvent and propellant for
sprays.
(b) It is used for storing food stuffs by freezing on direct contact because it doe not leave any undesirable taste
or smell.
(2) Diethyl ether (Sulphuric ether) : It is the most important member of the ether series and is known as
ether. It may also be regarded as an anhydride of C2 H5OH (ethanol).
(i) Preparation : Laboratory method : C2 H5OH + H 2SO4 heat → C2 H5 HSO4 + H 2O
C2 H5 HSO4 + C2 H5OH → C2 H5OC2 H5 + H 2SO4
[Excess] Diethyl ether
It is also known as Williamson’s etheral continuous process.
(ii) Properties
(a) It is a colourless, highly volatile and inflammable liquid having boiling point 34.5o C .
(b) It has a pleasant smell and burning taste.
(c) It is only slightly soluble in water but readily soluble in organic solvents.
(d) Di ethyl ether is itself a very good solvent even for fats and oils.
(e) It produces unconsciousness when inhaled.
(iii) Uses
(a) It is used as a refrigerant.
(b) It is used as a reaction medium in LiAlH4 reduction and grignard synthesis.
(c) It is used as an extracting solvent in laboratory.
(d) Mixture of alcohol and ether is used as petrol substitute under the trade name Natalite.
(e) It is used in perfumary and in the manufacture of smokeless powder.
(3) Di-isopropyl ether
(i) Preparation : 2CH 3 CH = CH 2 + H2O H+ →(CH 3 ) 2 CHOCH(CH 3 )2
3−7 atm
75−125o C
(ii) Properties : It is a colourless liquid with a pear like odour having B.P. 68.5o C .
(iii) Uses : It is used for reducing knocking of petrol.
(4) Divinyl ether
(i) Preparation : CH 2ClCH2OCH2ClCH2 + 2KOH ∆ → CH 2 = CH 2 + 2KCl + H2O
= CHO− CH
(ii) Properties
(a) It is highly inflammable liquid.
(b) It is colourless and boils at 28.3o C .
(iii) Uses : It is better anaesthetic than di ethyl ether because of its rapid action and rapidrecovery from
anaesthesia.
||
(5) Epoxides [oxirane] or cyclic ether − C− C−
O
(i) Preparation
(a) By oxidation of ethylene with oxygen : 2CH 2 = CH 2 + O2 Ag → 2CH 2 − CH 2
250o C
O
(b) By oxidation of ethylene with peroxy acids : RCH = CHR C6H5CO3H → R − CH − CH − R .
or CF3CO3H
O
(c) By treatment of ethylene chlorohydrin with NaOH
CH2 = CH2 HOCl → ClCH2 − CH2OH NaOH → CH2 − CH2
Ethylene chlorohydrin
O
(ii) Properties
(a) It is a poisonous, flammable gas.
(b) Its B.P. is 14 o C .
(c) It is very reactive compound because of its strained configuration.
(d) An epoxide is converted into protonated epoxide by acid which can undergo attack by any nucleophilic
reagent.
|| H+ − | | HOH → − | − | − + H+
− C− C C− C− C C
O ||
O+ OH OH
| Glycol
H
(Protonated epoxide)
Some of the chemical reactions
(a) Case I : Base catalysed ring opening : In this case nucleophile attack on less hindered carbon of the
oxirane ring and reaction is SN 2 mechanism.
R C− CH − R R OCH3
R O R
CH3 ONa⊕/ CH3OH → |
C − CH− R .
|
OH
(b) Case II : Acid catalysed ring opening : Nucleophile attacks on carbon of oxirane ring which is highly
substituted. SN 2 reaction is there.
R C− CH − R CH3OH/ H⊕ → R OCH3
R O R
|
C− CH − R
|
OH
Some reactions of oxirane are given below
CH2 – CH2 CH2 – CH2OH
O CH3OH/H |
(Ethylene oxide) HX OCH3
NH3 2-Methoxy ethanol
CH3ONa CH2OH – CH2X
CH3OH Ethylene halohydrin
CH2OH – CH2NH2
2-Amino ethanol
OCH3
|
CH2 – CH2OH
2-Methoxy ethanol
–δ +δ R – CH2 – CH2 – OH
(i) RMgX Alcohol
(ii) HOH
R – CH2 – CH2 – OH
(i) R2CuLi
(ii) HOH
Uses δ+ δ– CH3 – CH2 – OH
(i) LiAlH4 Ethanol
⊕
(ii) HOH/H
(a) It is used in the manufacture of ethylene glycol.
(b) It is used in the manufacture of dacron.
(c) It is used in the manufacture of solvent ethoxy ethanol used in varnishes and enamels for quick drying.
(6) Crown ethers : The crown ethers are heterocyclic poly-ethers usually with at least four oxygen atoms.
These are called crown ethers because they have crown like shape.
18-crown-6 means compound is eighteen member ring compound out of which [18 – 6 = 12] 12 atoms are
carbon [i.e. six ethylene group − CH 2 − CH 2 − ] and six atoms of oxygen.
Crown ethers have remarkable affinity for metal ions.
Example : 12-crown-4 has affinity with Li⊕ and 18-crown-6 has affinity with K ⊕ .
Example : − X 18-crown-6 → R − CH 2 − CN + KX
benzene 100% yield
K ⊕ C N + R − CH 2
C6 H5 − CH 2 − Cl + K⊕F 18-crown-6 → C6 H 5 − CH 2 − F+ K ⊕Cl
acetonitrile
100% yield
O
OO OO
OO OO
O
18-crown-6 12-crown-6
A crown ether binds certain metal ions depending on the size of the cavity.
OO OO
O O.... O + Na+ → O Na+ O
O..
Host Inclusion compound
In this reaction, the crown ether is the ‘host’ and the species it binds is the ‘guest’. The crown-guest complex is
called an inclusion compound.
(7) Anisole (Methyl phenyl ether) C6H5OCH3 or (methoxy benzene)
(i) Preparation
(a) By the action of methyl iodide on sodium phenoxide.
C6 H5ONa + ICH3 → C6 H5OCH3 + NaI
(b) By passing vapours of phenol and methyl alcohol over heated thoria.
C6 H5OH + CH3OH ThO2 → C6 H5OCH3 + H2O
(c) By methylation of phenol with diazomethane.
C6 H5OH + CH 2 N 2 → C6 H5OCH3 + N 2
(ii) Properties : It is a pleasant smelling liquid. It is used as a solvent in some organic reactions. Anisole
undergoes electrophilic substitution reactions. − OCH3 group is o- and p- directing.
OCH3 OCH3 OCH3
+ E+ E+ + H+
E
Anisole is decomposed by conc. hydroiodic acid again into phenol.
OCH3 OH
+ HI heat + CH3I
Aldehydes & Ketones
Introduction.
Carbonyl compounds are of two types, aldehydes and ketones. Both have a carbon-oxygen double bond
often called as carbonyl group. O
C
Carbonyl group
Both aldehyde and ketones possess the same general formula CnH2nO .
Aldehyde Ketone
Aldehydes may be considered as derivatives of Ketones may be considered as derivatives of
hydrocarbons in which two hydrogen atoms of the end hydrocarbons in which the two hydrogen atoms of a carbon
carbon atom have been replaced by a bivalent oxygen atom present in the middle of carbon chain have been
atom. replaced by a bivalent oxygen atom.
HH HO
| | | ||
H − − H −2H → H − = O CH 3 − − CH 3 −2H → CH 3
C C C − C − CH3
| + [O] Formaldehyde + [O]
| Acetone
H H
Methane Propane
Aldehydes contain the monovalent group Ketones contain the divalent group
H C = O (ketonic group) linked to two alkyl groups,
| same or different. Hence, the general formula of the
ketones is represented as,
− C = O (aldehydic group) linked to a hydrogen atom
or an alkyl group. Hence, the general formula of the
H RR
C = O or C=O
| R R'
aldehydes is represented as, R − C = O (R may be H or Ketones can also be regarded as the first oxidation
alkyl group). products of secondary alcohols.
Aldehydes can also be regarded as the first
oxidation products of primary alcohols.
CH3 CHOH [O] → CH3
RCH 2OH [O] → RCHO + H2O C = O+ H2O
Primary alcohol Aldehyde
CH3 CH3 Acetone
Isopropyl alcohol
Classification, Structure, Nomenclature and Isomerism.
(1) Classification
Aldehydic group is always terminal. Aldehydes can be Ketonic group is never terminal. Ketones can be classified
classified into three categories, into three categories,
O O O
|| || ||
(i) Aliphatic aldehydes : R − C − H (i) Aliphatic ketones : R − C − R R − C − R'
Symmetrical
O Unsymmetrical
|| (ii) Aromatic ketones : In aromatic ketones both
substituents are aryl
(ii) Aromatic aldehydes : Ar − C − H O
(iii) Unsymmetrical aldehydes : All aldehydes except ||
formaldehyde are unsymmetrical. O – C – C6H5
||
O O CH3 –
C6 H6 − C− C6 H5 ,
|| ||
(iii) Mixed ketones : In mixed ketones one substituent is
H −C− H R−C−H O
Formaldehyde
Both substituents are different ||
aryl and other is alkyl.ArCyl 6grHou5p − C −AlCkyHl gr3oup
(2) Structure : Carbonyl carbon atom is joined to three atoms by sigma
bonds. Since these bonds utilise sp 2 -orbitals, they lie in the same plane and are 120° σ-bond
π-bond
120° apart. The carbon-oxygen double bond is different than carbon-carbon double 120° C
bond. Since, 120°
Oxygen is more electronegative, the electrons of the bond are attracted towards
oxygen. Consequently, oxygen attains a partial negative charge and carbon a partial
positive charge making the bond polar. The high values of dipole moment, δC+ = δO−
(2.3 - 2.8D) cannot be explained only on the basis of inductive effect and thus, it is proposed that carbonyl
group is a resonance hybrid of the following two structures. C = O ←→ C+ − O−
(3) Nomenclature
(i) Aldehyde : There are two systems of naming aldehydes,
(a) Common system : In the common system, aldehydes are named according to the name of the
corresponding acid which they form on oxidation. The suffix –ic acid the name of the acid is replaced by aldehyde.
For example, CH3CHO derived from acetic acid (CH3COOH) is named as acetaldehyde.
H
|
CH 3 COOH −ic acid → O Acetic acid + aldehyde = acetaldehyde
Acetic acid + aldehyde CH3 − C =
Acetaldehyde
Branching in the aldehyde chain, if any, is indicated by the Greek letters α, β, γ, δ etc. The carbon attached to
the − CHO group is α as :
γ βα CH 3 CH 2 CHCHO
For example, − C− C− C− CHO |
CH3
α−Methyl butyraldehyde
(b) IUPAC system : In the IUPAC system, the aldehydes are known as alkanals. The name of aldehyde is
derived by replacing the terminal –e of the name of corresponding alkane by al.
For example, HCHO CH3CHO C2 H 5 CHO Alkane –e + al = Alkanal
Methanal Ethanal Propanal
(ii) Ketone : There are two systems of naming ketone,
(a) Common system : In the common system, ketones are named by using the names of alkyl group present in
the molecule. For example,
CH 3 COCH 3 CH 3 COCH 2 CH 3 CH3CH2COCH2CH3
Dimethyl ketone Ethyl methyl ketone Diethyl ketone
CH3COCH2CH2CH3 CH 3COCH(CH 3 )2 C6 H5CH 2COCH 3
Methyl n− propylketone Methyl isopropyl ketone Benzyl methyl ketone
Some of the ketones are known by their old popular names as well. For example, dimethyl ketone,
CH3COCH3 is still popularly known as acetone.
(b) IUPAC system : In this system, longest chain containing the ketonic group is taken as the parent chain. In
naming the ketone corresponding to the chain, the following procedure is adopted.
Root word + ane –e + one i.e., Alkanone
The positions of the ketonic group and the substituents are indicated by the locants.
CH3COCH3 Propanone
CH3COCH2CH3 Butanone-2
CH3CH2COCH2CH3 Pantanone-3
CH3 − CHCOCH2CH2CH3 2-Methylhexanone-3
|
CH3
(4) Isomerism : Aldehydes show chain and functional isomerism.
Chain isomers : CH3CH2CH2CHO (CH3)2 CHCHO
n − Butanal
2-Methylpropanal (iso −Butanal)
(i) Functional isomers: CH3CH2CHO ; CH3COCH3 ; CH2 = CHCH2OH ; CH3 CH − CH2 ; CH2 = CH.O.CH3
Propanal Acetone Allyl alcohol Methyl vinyl ether
O
α , β −Propylene oxide
Ketones show chain, functional and metamerism. Examples of functional isomerism is given above in
aldehydes.
OO
|| ||
(ii) Chain isomers : CH3CH2CH2 − C − CH3 ; (CH3)2 CH − C − CH3
Methylpropyl ketone Methylisopropyl ketone
O O
|| ||
(iii) Metamers : CH3CH2CH2 − C − CH3 ; CH3CH2 − C − CH2CH3
Methylpropyl ketone Diethyl ketone
Preparation of carbonyl compounds.
Preparation of only aliphatic or aliphatic as well as aromatic carbonyl compounds.
(1) From alcohols
(i) Primary and secondary alcohols on oxidation give aldehydes and ketones respectively.
OH O
| ||
R − − R' Mildoxidising → R − − R'
CH agents C
O
||
R − CH2 − OH Mildoxidising → R − − H
agents C
Mild oxidising agents are :
(a) X2 (b) Fenton reagent (c) K2Cr2O7 / H⊕ (d) Jones reagent
(e) Sarret reagent (f) MnO2 (g) Aluminium tertiary butoxide
Note: When the secondary alcohols can be oxidised to ketones by aluminium tert-butoxide,
[(CH3 )3 CO]3 Al the reaction is known as oppenauer oxidation. Unsaturated secondary alcohols can
also be oxidised to unsaturated ketones (without affecting double bond) by this reagent.
The yield of aldehydes is usually low by this methods. The alcohols can be converted to aldehydes
stage by treating with oxidising agent pyridinium chloro-chromate (C5 H5 NH +CrO3Cl − ). It is
abbreviated as PCC and is called Collin's reagent. This reagent is used in non-aqueous solvents
like CH2Cl2 (dichloro methane). It is prepared by mixing pyridine, CrO3 and HCl in
dichloromethane. This is a very good reagent because it checks the further oxidation of aldehydes to
carboxylic acids.
(ii) Dehydrogenation of 1° and 2° alcohols by Cu/300° or Ag/300°C.
O
||
R − CH 2OH Cu /300°C → R − − H+ H2
C
OH O
| ||
R − − R' Cu /300°C → R − − R' + H2
CH C
(2) From carboxylic acids
(i) Distillation of Ca, Ba, Sr or Th salts of monobasic acids: Salt of monobasic acids on distillation give
carbonyl compounds. Reaction takes place as follows,
O
||
(RCOO)2 Ca + (R' COO)2 Ca ∆ → 2R − C − R'+ 2CaCO3
Thus in the product, one alkyl group comes from one carboxylic acid and other alkyl group from other
carboxylic acid.
O
||
(HCOO)2 Ca + (HCOO)2 Ca ∆ → H − C − H
O
||
(RCOO)2 Ca + (HCOO)2 Ca ∆ → R − C − H
(Equimolar amount)
O
||
(RCOO)2 Ca + (RCOO)2 Ca ∆ → R − C − R'
(Equimolar amount)
O
||
(C6 H5COO)2 Ca + (HCOO)2 Ca ∆ → C6 H5 − C − H
O
||
(CH3COO)2 Ca + (C6 H5COO)2 Ca ∆ → C6 H5 − C − CH3
Calcium salts of dibasic acid (1, 4 and higher) on distillation give cyclic ketones.
O
||
C H2 C O
| − − Ca ++ Distillation → O
CH 2 − C− O Cyclopropanone
|| O
O
O
O − || − COO Ca++
− (CH 2 )5 Distillation →
C
Cyclohexanone
(ii) Catalytic decomposition of carboxylic acids or Decarboxylation and Dehydration of acids by MnO/ 300°C.
(a) This reaction takes place between two molecules of carboxylic acids. Both may be the same or different.
(b) If one of the carboxylic acids is HCOOH then this acid undergoes decarboxylation because this acid is the
only monobasic acid which undergoes decarboxylation even in the absence of catalyst.
Case I : When both are HCOOH
O O
||
|| MnO → CO2 + HOH + H − − H
300°C C
H − C − OH + H COO H
Case II : When only one is formic acid.
OO
|| ||
MnO/ 300°C → R − − H + CO2 + HOH
R − C − OH + H − COO H C
Case III : When none is formic acid.
OO
|| ||
MnO/ 300°C → R − − R + CO2 + HOH
R − C − OH + R COO H C
Or
O
||
RCOOH + R' COOH MnO/ 300°C → R − − R' + CO2 + HOH
C
(3) From gem dihalides : Gem dihalides on hydrolysis give carbonyl compounds
(i) R − CHX2 HOH / OH → R − CHO
XO
| ||
(ii) R − C − R' HOH/ OH → R − C − R'
|
X
Note : This method is not used much since aldehydes are affected by alkali and dihalides are usually
prepared from the carbonyl compounds.
(4) From alkenes
(i) Ozonolysis : Alkenes on reductive ozonolysis give carbonyl compounds
R − CH = CH − R (i) O3 R − CHO + RCHO
(ii) H2O/ Zn →
R C=C R' (i) O3 R − O + O − R'
R R'
(ii) H2O/ Zn → || ||
C−R R' − C
Note : This method is used only for aliphatic carbonyl compounds.
(ii) Oxo process : This method converts terminal alkenes into aldehydes.
R − CH = CH 2 + CO + H2 CO2 (CO)8 R − CH 2 − CH 2 − CHO
150°C,300atm →
Note : Oxo process is used only for the preparation of aldehydes.
(iii) Wacker process : This reaction converts alkenes in carbonyl compounds.
(a) CH 2 = CH 2 PdCl2 /HOH → CH 3 − CHO
air / Cu2Cl2
O
||
(b) R − CH = CH 2 PdCl2 /HOH → R − − CH 3
air/Cu2Cl2 C
(5) From alkynes : Alkynes on hydration and on boration – oxidation give carbonyl compounds.
H2O/HgSO4 /H2SO4 O
R – C – CH3
R–C≡C–H
(i) Sia2 BH R – CH2 – CHO
(ii) H2O2/ OH
(6) From Grignard reagents : Carbonyl compounds can be prepared from Grignard reagents by following
reactions:
O O (Only ketone)
R' – C – Cl R' – C – R (Aldehyde)
(Ketone)
HCOOC2H5 O
H–C–R
R' COOC2H5
O
R – C – R'
R – MgX O
R–C–H
(Excess)
O
(i) HCN R – C – R'
⊕
(ii) H2O/H
(i) R' – CN
⊕
(ii) H2O/H
O O
CH2 = CH – C – H R – CH2 – CH2 – C – H
(7) From acid chloride
(i) Acid chlorides give nucleophilic substitution reaction with dialkyl cadmium and dialkyl lithium cuprate to
give ketones. This is one of the most important method for the preparation of ketones from acid chlorides.
OO
|| ||
R'2Cd → R − − R'
R − C − Cl C
OO
|| ||
R'2CuLi → R − − R' (Only used for the preparation of ketones)
R − C − Cl C
In this method product is always ketone because R ≠ H and also R' ≠ H .
(ii) Rosenmunds reduction : Acid chlorides on partial reduction give aldehydes. This reduction takes place
in the presence of Lindlars catalyst.
OO
|| ||
R − − Cl H2 /Pd−BaSO4−CaCO3 → R − − H
C Xylene C
OO
|| ||
Ar − − Cl H2 /Pd−BaSO4−CaCO3 → Ar − H (Only used for aldehydes)
C Xylene −C
(8) From cyanides
(i) Stephen aldehyde synthesis : Conversion of cyanides into aldehydes by partial reduction with
SnCl2 / HCl , followed by hydrolysis, is known as Stephens aldehyde synthesis.
R − C ≡ N (i) SnCl2 / HCl / ether→ R − CHO (Only used for aldehydes)
(ii) H2O / ∆ or steam distillation
Example :
CN (i) SnCl2 / HCl → CHO
(ii) H2O / ∆
Cyclopentane nitrile
Cyclopentanaldehyde
CN CHO
Benzenenitrile (i) SnCl2 / HCl →
(ii) H2O / ∆
Benzaldehyde
(9) From vic diols : Vic diols on periodate oxidation give carbonyl compounds.
OH OH O
| | ||
R − − R HIO4 → RCHO + R− − R
CH C− C
|
R
Note : Pb(OCOCH3 )4 also gives similar oxidation products.
(10) From Alkyl halides and benzyl halides : These compounds on oxidation give carbonyl compounds.
Cl O
| ||
R − CH 2Cl DMSO → R − CHO ; R− H − R DMSO →
C R − C− R
C6 H5 − CH2Cl DMSO or (i) (CH2 )6 N4 C6 H5 − CHO
(ii) H2O/ H⊕ orCu(NO3 )2or Pb(NO3)2 →
(11) From nitro alkanes : Nitro alkanes having at least one α -hydrogen atom give carbonyl compounds on
treatment with conc NaOH followed by 70% H 2SO4 . The reaction is known as Nef carbonyl synthesis.
R − CH 2 − N O NaOH R − C O = HN2 OH
O H 70%H2SO4 → R − CHO
Tautomerisation →
O
R O O
||
CH − N (i) NaOH → R − − R
R O (ii) H2SO4 C
(12) Reaction with excess alkyl lithium : Carboxylic acids react with excess of organo lithium to give
lithium salt of gem diols which on hydrolysis give ketones.
O O
||
|| (i) R−Li(excess) → R' − − R
(ii) HOH / H ⊕ C
R' − C − OH
Preparation of only aromatic carbonyl compounds
(1) From methyl arenes : Methyl (ia) rCerOn2eCsl2 can be converted into aldehydes by the following reagents
C6H5CHO
(ii) HOH
C6H5 – CH3 (i) CrO3 /(CH3CO)2O/CH3COOH C6H5 CHO
(ii) H2O
Air/MnO C6H5 CHO
500°C
(2) From chloro methyl arenes : Chloromethyl arenes on oxidation give aromatic aldehydes.
Cu(NO3)2/∆ C6H5CHO
C6H5 – CH2Cl Pb(NO3)2/∆ C6H5 – CHO
(i) (CH2)6N4 /∆ C6H5 – CHO
(ii) H2O
(3) Gattermann – Koch formylation : This reaction is mainly given by aromatic hydrocarbons and
halobenzenes. CHO
CO/HCl /Anhy. ZnCl2
CH3 CH3 CH3
CHO
CO/HCl /Anhy. ZnCl2 +
Cl Cl CHO
CHO Cl
CO/HCl /Anhy. ZnCl2 +
CHO
(4) Gattermann formylation : This reaction is mainly given by alkyl benzenes, phenols and phenolic ethers.
CH3 CH3 CH3
(i) Zn(CN)2 /HCl gas CHO
(ii) H2O/∆ +
OH OH CHO
CHO OH
+
(i) Zn(CN)2 /HCl gas
(ii) H2O/∆
OCH3 OCH3 CHO
CHO OCH3
(i) Zn(CN)2 /HCl gas
(ii) H2O/∆ +
CHO
(5) Houben – Hoesch reaction : This reaction is given by di and polyhydric benzenes.
OH OH
(i) RCN/HCl gas/Anhy.ZnCl2
(ii) H2O OH
COR
OH OH
OH
HO (i) RCN/HCl gas/Anhy.ZnCl2 HO OH
(ii) H2O COR
OH
(6) Reimer – Tiemann reaction : Phenol gives 0- and p- hydroxy benzaldehyde in this reaction.
OH OH OH
(i) CHCl3 /Alc.KOH/∆ CHO
⊕ +
(ii) H2O/H
CHO
Physical properties of carbonyl compounds.
The important physical properties of aldehydes and ketones are given below,
(1) Physical state : Methanal is a pungent smell gas. Ethanal is a volatile liquid, b.p. 294 K. Other aldehydes
and ketones containing up to eleven carbon atoms are colourless liquids while still higher members are solids.
(2) Smell : With the exception of lower aldehydes which have unpleasant odours, aldehydes and ketones
have generally pleasant smell. As the size of the molecule increases, the odour becomes less pungent and more
fragrant. In fact, many naturally occurring aldehydes and ketones have been used in blending of perfumes and
flavouring agents.
(3) Solubility : Aldehydes and ketones upto four carbon atoms are miscible with water. This is due to the
presence of hydrogen bonding between the polar carbonyl group and water molecules as shown below :
δ–
δ+ δ– Oδ+ δ+ δ– δ+
COH H O=C
With the increase in the size of alkyl group, the solubility decreases and the compounds with more than four
carbon atom are practically insoluble in water. All aldehydes and ketones are, however, soluble in organic solvents
such as ether, alcohol, etc. The ketones are good solvents themselves.
(4) Boiling points : The boiling points of aldehydes and ketones are higher than those of non polar
compounds (hydrocarbons) or weakly polar compounds (such as ethers) of comparable molecular masses.
However, their boiling points are lower than those of corresponding alcohols or carboxylic acids. This is because
aldehydes and ketones are polar compounds having sufficient intermolecular dipole-dipole interactions between the
opposite ends of C = O dipoles.
Cδ + = δ− Cδ + = δ− Cδ + = δ−
O O O
However, these dipole-dipole interactions are weaker than the intermolecular hydrogen bonding in alcohols
and carboxylic acids. Therefore, boiling points of aldehydes and ketones are relatively lower than the alcohols and
carboxylic acids of comparable molecular masses.
Compounds CH3CH2CH2CH2CH3 CH 3CH 2OCH 2CH 3 CH3CH2CH2CH2OH CH 3CH 2CH 2CHO CH 3COCH 2CH 3
Molecular mass Pentane Ethoxyethane Butan - 1-ol Butanal Butan-2-one
Boiling point (K)
72 74 74 72 72
309 308 391 349 353
Among the carbonyl compounds, ketones have slightly higher boiling points than the isomeric aldehydes. This
is due t0.o the presence of two electrons releasing groups around the carbonyl carbon, which makes them more
polar.
CH3 .. CH3 ..
C = O: C = O:
H CH3
Acetone
Acetaldehyde µ = 2.88 D
µ = 2.52 D b.pt =329 K
b.pt.=322 K
(5) Density : Density of aldehydes and ketones is less than that of water.
Chemical properties of carbonyl compounds.
Carbonyl compounds give chemical reactions due to carbonyl compounds group and α-hydrogens.
Chemical reactions of carbonyl compounds can be classified into following categories.
(1) Nucleophilic addition reactions (2) Addition followed by elimination reactions
(3) Oxidation (4) Reduction (5) Reactions due to α-hydrogen
(6) Condensation reactions and (7) Miscellaneous reactions
(1) Nucleophilic addition reactions
(i) Carbonyl compounds give nucleophilic addition reaction with those reagents which on dissociation give
electrophile as well as nucleophile.
(ii) If nucleophile is weak then addition reaction is carried out in the presence of acid as catalyst.
(iii) Product of addition reactions can be written as follows,
−Oδ OH
|| |
+ H+δ − N−δu Addition → R'
R − C − R' R − C−
+δ |
Nu
Adduct
In addition reactions nucleophile adds on carbonyl carbon and electrophile on carbonyl oxygen to give
adduct.
(iv) Relative reactivity of aldehydes and ketones : Aldehydes and ketones readily undergo nucleophilic
addition reactions. However, ketones are less reactive than aldehydes. This is due to electronic and stearic effects as
explained below:
(a) Inductive effect : The relative reactivities of aldehydes and ketones in nucleophilic addition reactions may
be attributed to the amount of positive charge on the carbon. A greater positive charge means a higher reactivity. If
the positive charge is dispersed throughout the molecule, the carbonyl compound becomes more stable and its
reactivity decreases. Now, alkyl group is an electron releasing group (+I inductive effect). Therefore, electron
releasing power of two alkyl groups in ketones is more than that of one in aldehyde. As a result, the electron
deficiency of carbon atom in the carbonyl group is satisfied more in ketones than in aldehydes. Therefore, the
reduced positive charge on carbon in case of ketones discourages the attack of nucleophiles. Hence ketones are less
reactive than aldehydes. Formaldehyde with no alkyl groups is the most reactive of the aldehydes and ketones.
Thus, the order of reactivity is:
H > R > R
C=O C=O
C=O
H R
H
Formaldehyde Ketone
Aldehyde
(b) Stearic effect : The size of the alkyl group is more than that of hydrogen. In aldehydes, there is one alkyl
group but in ketones, there are two alkyl groups attached to the carbonyl group. The alkyl groups are larger than a
hydrogen atom and these cause hindrance to the attacking group. This is called stearic hindrance. As the number
and size of the alkyl groups increase, the hindrance to the attack of nucleophile also increases and reactivity
decreases. The lack of hindrance in nucleophilic attack is another reason for the greater reactivity of formaldehyde.
Thus, the reactivity follows the order:
H CH3 CH3 C = O > (CH3)2 CH C=O > (CH 3 )3 C
C=O > C=O >
H H CH3 (CH3)2 CH C=O
Acetone Di-isopropyl ketone
Formaldehyde Acetaldehyde (CH 3 )3 C
Di-tert. butyl ketone
In general, aromatic aldehydes and ketones are less reactive than the corresponding aliphatic
analogues. For example, benzaldehyde is less reactive than aliphatic aldehydes. This can be easily understood
from the resonating structures of benzaldehyde as shown below:
H O. .: H O.. ..:– H O.. ..:– H O.. ..:– HO
C C C C C
⊕ ⊕
I II ⊕ IV V
III
It is clear from the resonating structures that due to electron releasing (+I effect) of the benzene ring, the
magnitude of the positive charge on the carbonyl group decreases and consequently it becomes less susceptible to
the nucleophilic attack. Thus, aromatic aldehydes and ketones are less reactive than the corresponding aliphatic
aldehyde and ketones. The order of reactivity of aromatic aldehydes and ketones is,
C6 H5CHO > C6 H5COCH3 > C6 H5COC6 H5
Benzaldehyde Acetophenone Benzophenone
Some important examples of nucleophilic addition reactions
Some important nucleophilic addition reactions of aldehydes and ketones are given below,
Addition of HCN : Carbonyl compounds react with HCN to form cyanohydrins. This reaction is catalysed
by base.
O OH
|| |
R − − H + HCN
C OH → R − C− CN
|
H
Cyanohydrin
O OH
|| |
C6 H5 − H + HCN C6 H5 CN
C− OH → − C−
|
H
Note : Because HCN is a toxic gas, the best way to carry out this reaction, to generate hydrogen cyanide
during the reaction by adding HCl to a mixture of the carbonyl compound and excess of NaCN.
Benzophenone does not react with HCN.
Except formaldehyde, all other aldehydes gives optically active cyanohydrin (racemic mixture).
This reaction is synthetically useful reaction for the preparation of α-hydroxy acids, β-amino alcohols and α-
hydroxy aldehydes.
OH H2O/H⊕/∆ OH (If R is CH3 then product is lactic acid).
H2/Pt
| |
R − CH− CN R − C H − COOH
α -Hydroxy acid
OH
|
R − C H − CH2 − NH2
β -Amino alcohol
(i) SnCl2/HCl OH
(ii) HOH/∆
|
R − C H − CHO
Addition of sodium bisulphite : Sodium bisulphite dissociates as follows:
NaHSO3 → H⊕ + SO3 Na
Electrophile Nucleophile
(i) All types of aldehydes give addition reaction with this reagent. The adduct of aldehyde is white crystalline
compound which again converts into aldehyde on treatment with acid, base or HCHO.
O OH ⊕ O
||
−H HSO3Na → | H or OHor → R − || H
R−C
R−C−H C−
| HCHO
SO3Na
Adduct; white
crystalline in nature
(ii) Only aliphatic methyl ketones give addition reaction with sodium bisulphite.
O OH ⊕ O
|| | ||
R CH 3 HSO3Na → R C CH 3 H or OHor → R CH 3
− C− − − HCHO −C −
|
SO3 Na
Colourless crystalline
product
Note : This reagent can be used for differentiation between ketones and aliphatic methyl ketones, e.g.
O O
|| ||
CH3 − CH2 − C − CH2 − CH3 and CH3 − CH2 − CH2 − C − CH3
O O
|| ||
C6 H5 − C − CH3 and CH3 − CH2 − C − CH3
This reagent can be used for the separation of aldehydes and aliphatic methyl ketones from the
mixture, e.g.
CH3 − CH2 − CHO O
||
and CH3 − CH2 − C − CH2 − CH3
These two compounds can be separated from their mixture by the use of NaHSO3. Higher aliphatic
ketones and aromatic ketones do not react with NaHSO3.
Addition of alcohols : Carbonyl compounds give addition reaction with alcohols. This reaction is catalysed
by acid and base. Nature of product depends on the catalyst.
Case I : Addition catalysed by base : In the presence of base one equivalent of an alcohol reacts with
only one equivalent of the carbonyl compound. The product is called hemiacetal (in case of aldehyde) and
hemiketal (in case of ketone). The reaction is reversible. There is always equilibrium between reactants and product.
O OH
|| HO |
CH 3 H CH 3 −Oδ − +Hδ CH 3 C H
− C − + − − −
|
OCH3
Hemiacetal
O OH
HO |
|| CH 3 C CH 3
− −
CH3 − C − CH3 + CH3 − O − H |
OCH3
Hemiketal
Hemiacetals and hemiketals are α-alkoxy alcohols.
Case II : Addition catalysed by acid : In the presence of acid one equivalent of carbonyl compound
reacts with two equivalents of alcohol. Product of the reaction is acetal (in case of aldehyde) or ketal (in case of
ketone).
O ⊕ OCH3
H |
|| R C H H2O
− − +
R − C − H + 2CH3OH |
OCH3
Acetal
O OCH3
|
|| R− C −R H2O
+
R − C − R + 2CH3OH |
OCH3
Ketal
(i) Formation of acetals and ketals can be shown as follows:
R H − O − CH3 ⊕ R OCH 3
C=O + H C + H2O
R H − O − CH3 R OCH 3
(ii) Acetals and ketals are gem dialkoxy compounds.
(iii) High yield of acetals or ketals are obtained if the water eliminated from the reaction is removed as it
formed because the reaction is reversible.
(iv) Acetals and ketals can be transformed back to corresponding aldehyde or ketone in the presence of excess
of water.
OCH3 O
| ⊕ ||
R H2O R − − R + 2CH 3OH
−C−R+ H → C
| (Excess)
OCH3
Ketal
This reaction is very useful reaction for the protection of carbonyl group which can be deprotected by
hydrolysis. Glycol is used for this purpose. Suppose we want to carry out the given conversion by LiAlH4 .
OO
|| ||
CH 3 − − CH 2 − COOC2 H5 LiAlH4 →
C CH 3 − C − CH 2 − CH 2OH
This can be achieved by protection of C = O group and then by deprotection
O
|| ⊕
CH 3 − CH 2 COOC2 H5 CH2OH−CH2OH /H → CH C CH 2 COOC2 H5
− C − Protection 3 − − −
O| O|
CH2 −CH2
O
⊕ ||
LiAlH4 → CH 3 − C − CH 2 − CH 2OH − − CH 2 − CH 2OH
HOH /H → CH 3 C
OO
||
CH2 −CH2
Addition of Grignard reagents : Grignard reagents react with carbonyl compounds to give alcohols.
Nature of alcohol depends on the nature of carbonyl compound.
O
(i) H – C – H R – CH2OH 1°-alcohol
(ii) HOH/⊕H
RMgX O OH 2°-alcohol
(i) R' – C – H |
(ii) HOH/⊕H
R' – CH – R
O OH
(i) R' – C – R' | 3°-alcohol
(ii) HOH/⊕H
R' – C – R'
|
R
Addition of water : Carbonyl compounds react with water to give gem diols. This reaction is catalysed by
acid. The reaction is reversible reaction.
O OH
|| |
R − C − R' + HOH R − C − R'
|
OH
Gem diols are highly unstable compounds hence equilibrium favours the backward direction. The extent to
which an aldehyde or ketone is hydrated depends on the stability of gem diol.
Stability of gem diols depend on the following factors:
(i) Steric hindrance by +I group around α-carbon decreases the stability of gem diols. +I group decreases
stability of gem diol and hence decreases extent of hydration.
O OH
|| |
H − C − H + HOH H−C−H
0.1% |
OH
99.9%
O OH
|
|| CH 3 −C H
| −
CH 3 − C − H + HOH
42% OH
58%
O OH
|
|| CH 3 −C CH 3
| −
CH 3 − C − CH 3 + HOH
99.8% OH
0.2%
(i) +I power of +I group is in increasing order
(ii) Stability in decreasing order
(ii) Stability of gem diols mainly depends on the presence of –I group on α-carbon. More is the –I power of the
group more will be stability of gem diols.
O OH
|| |
CF3 −C −H HOH CF3 C H
+ → − −
|
OH
O OH
|| |
CCl 3 C H HOH CCl 3 C H
− − + → − −
|
OH
O OH
|| |
CF3 C − CF3 HOH CF3 C CF3
− + → − −
|
OH
These gem diols are highly stable due to the presence of –I group on α-carbon.
(iii) Intramolecular hydrogen bonding increases stability of gem diols. –I groups present on carbon having gem
diol group increases strength of hydrogen bond.
Strength of hydrogen bond α–I power of the group.
More is the strength of hydrogen bond more will be the stability of gem diol.
Addition of terminal alkynes : Sodium salt of terminal alkynes react with carbonyl compounds to give
alkynol. This reaction is known as ethinylation.
O N⊕ a OH
|
O
|| | ⊕ − C−
R − C ≡ N⊕a + R' − − R′′ → R − C ≡ C ≡ C R"
C −C− R" HOH /H → R − C
C ||
R' R'
Some examples are,
CH3 −C ≡ ⊕ (i) HCHO →CH 3 −C ≡ C − CH2OH
⊕
CNa
(ii) HOH / H
OH
⊕ |
CH 3 −C ≡ (i) CH3CHO → CH 3 −C ≡ − CH 3
CNa ⊕ C − CH
(ii) HOH / H
⊕ (i) O
CH 3 −C ≡ CNa →CH 3 −C ≡C−
⊕ HO
(ii) HOH / H
(2) Addition followed by elimination reactions : This reaction is given by ammonia derivatives
(NH2 − Z) .
(i) In nucleophilic addition reactions poor nucleophile such as ammonia and ammonia derivatives requires
acid as catalyst.
(ii) If the attacking atom of the nucleophile has a lone pair of electrons in the addition product, water will be
eliminated from the addition product. This is called a nucleophilic addition elimination.
Primary amines and derivatives of ammonia react with carbonyl compounds to give adduct.
In adduct nucleophilic group has lone pair of electrons. It undergoes elimination to give product known as
imine. An imine is a compound with a carbon-nitrogen double bond.
R − O − R + +Hδ − −.δ. H − Z ⊕ OH R C=N−Z
N −HOH → An imine
|| H → |
C R−C−R
| R
N HZ
..
The overall reaction can be shown as follows:
R .. ⊕ R
C = O + N H 2 − Z H → H 2O + C= N−R
RR
An imine
Aldehydes & Ketones
Different imine formation with NH 2 − Z is given below R
C = N – R'
⊕
R' – NH2 /H/∆ R
Imine from p-amine (Schiff base)
⊕ R
NH2 – OH /H/∆ C = N – OH
R
Oxime
⊕ R
NH2 – NH2 /H/∆ C = N – NH2
O ⊕ R
R–C–R C6H5 – NH – NH2 /H
Hydrazone
NO2 ⊕
NH2 – NH NO2 /H/∆ R
C = N – NNHC6H5
NO2
R
Phenylhydrozone
(Brown coloured)
NO2
R
C = N – NH
R
2,4-Dinitrophenylhydrazone
(Red coloured)
O O
⊕ R
NH2 – NH – C – NH2 /H/∆ C = N – NH – C – NH2
R
Semicarbazide
Semicarbazone
Ketoxime when treated with acid at 0°C it undergoes rearrangement known as Beckmann rearrangement.
Thus acid catalysed conversion of ketoximes to N-substituted amides is called Beckmann rearrangement. Acid
catalyst used are proton acids(H 2SO4 , HCl, H3 PO4 ) and Lewis acids (PCl5 , SOCl2, PhSO2Cl, RCOCl, SO3 , BF3
etc.)
O
||
C6 H5 − C − CH 3 (i) PCl5 → CH 3 − − NH − C6 H5
(ii) H2O C
||
N −OH
O
||
CH 3 − C − C6 H 5 (i) PCl5 → C6 H 5 − − NH − CH 3
(ii) H2O C
||
N −OH
In short product of the rearrangement can be obtained as follows:
R R' O
C ||
→ R'− C− O − H Tautomerisation → R' − − NH − R
|| || C
N −OH R−N
(3) Oxidation of carbonyl compounds
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