Give reasons for the following:
a) Carboxylic acids do not give the characteristic reactions of the carbonyl group.
b) Treatment of benzaldehyde with HCN gives a mixture of two isomers which cannot be separated even by careful fractional distillation.
c) Sodium bisulphite is used for the purification of aldehydes and ketones.
a) Carboxylic acids do not give the characteristic reactions of the carbonyl group.
b) Treatment of benzaldehyde with HCN gives a mixture of two isomers which cannot be separated even by careful fractional distillation.
c) Sodium bisulphite is used for the purification of aldehydes and ketones.
Solution
(a) In –COOH the carbonyl π-electrons are delocalised with the –OH lone pair across O–C–O; the 'carbonyl' is no longer a simple C=O, so no 2,4-DNP/bisulphite-type reactions.
(b) HCN adds to the planar C=O from both faces equally: CHCH(OH)CN is formed as d- and l-enantiomers (racemic mixture). Enantiomers have identical boiling points — fractional distillation cannot separate them.
(c) RC=O + NaHSO → crystalline RC(OH)SONa; the solid is filtered off from impurities and the pure carbonyl regenerated with dilute acid or NaCO.
(b) HCN adds to the planar C=O from both faces equally: CHCH(OH)CN is formed as d- and l-enantiomers (racemic mixture). Enantiomers have identical boiling points — fractional distillation cannot separate them.
(c) RC=O + NaHSO → crystalline RC(OH)SONa; the solid is filtered off from impurities and the pure carbonyl regenerated with dilute acid or NaCO.
CBSE Class 12 Board · 2005 · Delhi Compartment · Subjective