Aldehydes, Ketones and Carboxylic Acids Test 4

NaBH₄ brings about reduction of carbonyls by hydrid e transfer mechanism at planar sp² carbon. Hence, if a chiral carbon is generated, racemic mixture is always produced.

LiAIH₄ reduces both carbonyls and carboxylic groups but does not reduce olefinic bond.

In the above reaction, four stereoisomers, two pairs of enantiomers are formed. Each member of a pair of enantiomer is diastereomer of each member of other pair of enantiomer. Hence, fractional crystallisation would give two fractions, each containing racemic mixture.

Clemmensen reduction is suitable for reductio n of carbonyls containing additional acidic functional group.

Wolff-Kishner reduction is suitable for reduction of carbonyls containing olefinic bonds. If Clemmensen reduction is done, HCI also attacks olefinic bonds.

With aluminium isopropoxide (MPV reduction), carbonyls are selectively reduced to alcohols leaving other groups intact.

If possible, intramolecular reaction is preferred over intermolecular reaction.

(i) It has a carbonyl carbon that turns chiral on reduction with hydrides. Hence, with LiAIH₄ or aluminium isopropoxide, gives racemic mixture. Also, it has no other functional groups, either Clemmensen reduction or Wolff-Kishner reduction can be used.
(ii) It has an olefinic double bond, Clemmensen reduction would not be suitable for selective reduction of carbonyl group.
(iii) LiAlH₄ also reduces primary halide but aluminium isopropoxide does not. Wolff-Kishner reduction would not be suitable because HO^- reacts with halide group (E_N2 or E2).
(iv) Same reasons as with (i).