Question

(a) Write a note on Wolff-Kishner reduction.
(b) Boiling point of carboxylic acids are higher than aldehydes and ketones of comparable molecular masses. Why?
(c) Draw the dimer structure of ethanoic acid in vapour state.

Answer 1

(a) Wolff-Kishner reduction:
Carbonyl group is reduced to methylene group by treatment with hydrazine in presence of KOH/ ethylene glycol.
$C{H}_{3}COC{H}_3\underset{-H_{2}O}{\overset{H_{2}N-N{H}_2}{\to }}C{H}_3-C\left(C{H}_3\right)=N-N{H}_2\underset{\Delta n}{\overset{\text{KOH / ethylene glycol}}{\to }}C{H}_3-C{H}_2-C{H}_3+N_2$
This method is suitable for carbonyl compounds that are not suitable to bases.
(b) Boiling point of carboxylic acids are higher than aldehydes and ketones of comparable molecular masses. Carboxylic acids forms dimers due to intermolecular hydrogen bonding. Huge amount of energy is required to break these hydrogen bonds. Hydrogen bonding is not possible in aldehydes and ketones.
(c) The dimer structure of ethanoic acid in vapour state is as shown.