Notes on Aldehydes and Ketones - JEE Main 2014 by ednexa.com

1. Aldehydes and Ketones
Introduction
Carbonyl compounds are of two types, aldehydes and ketones. Both
have a carbon-oxygen double bond often called as carbonyl group.
Both aldehyde and ketones possess the same general formula C
2n
n
H
O.
Structure
Carbonyl carbon atom is joined to three atoms by sigma bonds.
Since these bonds utilize sp2 -orbitals, they lie in the same plane
and are 120° apart. The carbon-oxygen double bond is different
than carbon-carbon double bond. Since, 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, C  O
(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.

2. Preparation of carbonyl compounds
1.
From alcohols
(i)
By oxidation.
OH
O
|
Mild oxidising
||
R  CH  R '  R  C  R '

agents
Ketone
Secondary alcohol
O
||
Mild oxidising
R  CH 2  OH  R  C  H

agents
Aldehyde
Primary alcohol
Mild oxidising agents are
(a)
X2 (Halogen)
(b)
Fenton reagent (FeSO4 + H2O2)
(c)
K 2 Cr2 O7 / H
(d)
Jones reagent
(e)
Sarret reagent (f) MnO2
(g)
Aluminium tertiary butoxide [AI(- O – C(CH3)3)]

∙ When the secondary alcohols can be oxidised to
ketones by aluminium tert-butoxide, [C(CH3)3
CO]3AI the

3. 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 allylic alcohols can be converted to aldehydes by
treating with oxidising agent pyridinium chlorochromate (C5H5NH+CrO3Cl-). It is abbreviated as PCC and
is
called Collin's reagent. This reagent is used in nonaqueous 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
and is suitable method for preparing α, β - unsaturated
aldehydes.
(ii)
Dehydrogenation of 1° and 2° alcohols by Cu/300° or
Ag/300°C.
O
Cu/300C
||
R  CH2 OH  R  C  H H 2

OH
O
|
||
Cu/300C
R  CH  R '  R  C  R '  H 2

2.
From carboxylic acids
(i)
Distillation of Ca, Ba, Sr or Th salts of monobasic acids

4. 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.
Calcium salts of dibasic acid (1, 4 and higher) on
distillation give cyclic ketones.
(ii)
Decarboxylation or 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 molecules are HCOOH
Case II : When only one molecule is formic acid.

5. Case III : When none of the molecule is formic acid.
3.
From gem dihalides
Gem dihalides on hydrolysis give carbonyl compounds
(i)
(ii)
∙ 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
(i)O3
R  CH  CH  R  R  CHO  RCHO
(ii)H2O/Zn
Alkene

6. ∙ This method is used only for aliphatic carbonyl
compounds.
(ii)
Oxo process
CO2 (CO)8
R  CH  CH2  CO  H2  R  CH2  CH2  CHO

150C, 300 atm
∙ Oxo process is used only for the preparation of
aldehydes.
(iii)
Wacker process
(a)
PdCl /HOH
air/Cu 2Cl2
2
CH 2  CH 2  CH3  CHO

Ethene
(b)
O
||
PdCl2 /HOH
R  CH  CH 2  R C  CH3

Alkyl ethene
air/Cu2Cl2
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