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Saturday, 25 February 2017

Aldehydes, Ketones and Carboxylic acid - I


Aldehydes
Aldehydes are the organic compounds in which carbonyl group is attached to one hydrogen atom and one alkyl or aryl group.

Preparation of aldehydes:

a) By oxidation of alcohols:

Oxidation of primary alcohols in presence of oxidizing agent like K2Cr2O7/H2SO4,
KMnO4, CrO3 gives aldehydes.
b) By dehydrogenation of alcohols:
When the vapours of primary alcohol passed through heated copper at 573 K, it forms aldehyde.
c) By hydration of alkynes:
Ethyne on hydration with HgSO4/dil.H2SO4, at 333 K forms acetaldehyde.

d) By Rosenmund reduction:
Hydrogenation of acyl chloride over palladium on barium sulphate gives aldehyde.
e) By reduction of nitriles:

(i) Stephen Reaction: Reduction of nitriles in presence of stannous chloride in presence of HCl gives imine which on hydrolysis gives corresponding aldehyde.

(ii) Nitriles are selectively reduced by DIBAL-H (Diisobutylaluminium hydride) to aldehydes

f) By reduction of ester:

Esters are reduced to aldehydes in presence of DIBAL-H (Diisobutylaluminium hydride)

g) From Hydrocarbons:


(i) By oxidation of methyl benzene:

Etard Reaction: Chromyl chloride (CrO2Cl2) oxidizes methyl group to a chromium complex, which on hydrolysis gives corresponding benzaldehyde
Using chromium oxide(CrO3): Toluene or substituted toluene is converted to benzaldehyde in presence of chromic oxide in acetic anhydride.
(ii) By side chain chlorination followed by hydrolysis:
Halogenation of toluene: Side chain halogenation of toluene gives benzal chloride which on hydrolysis gives Benzaldehyde.
(iii) Gatterman –Koch reaction:
Benzene or its derivatives on treatment with carbon monoxide and HCl in presence of anhydrous aluminium chloride or cuprous chloride (CuCl) gives benzaldehyde or substituted benzaldehydes


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