Cyclohexanone forms cyanohydrin in good yield but 2,2,6 trimethylcyclohexanone does not.

Question

Give plausible explanation for each of the following:
i. Cyclohexanone forms cyanohydrin in good yield but 2,2,6 trimethylcyclohexanone does not.

ii. There are two -NH₂ groups in semicarbazide. However, only one is involved in the formation of semicarbazones.

iii. During the preparation of esters from a carboxylic acid and an alcohol in the presence of an acid catalyst, the water or the ester should be removed as soon as it is formed.

Answer 1

i. In cyclohexanone, the carbonyl carbon is not hindered. Hence, the nucleophile CN^-ion can easily attack the carbonyl carbon. However in 2,2,6-trimethyl cyclohexanone, the carbonyl carbon is sterically hindered due to presence of methyl groups. Hence, the nucleophile CN^- ion cannot easily attack the carbonyl carbon.

ii. Out of two -NH₂ groups in semicarbazide, one is a part of amide functional group. In this, the lone pair on nitrogen atom is involved in the resonance with carbonyl group. Hence, this -NH₂ group cannot act as a nucleophile. Hence, it is not involved in the formation of semicarbazones.

iii. The formation of esters from a carboxylic acid and alcohol in the presence of an acid catalyst is a reversible reaction. If water or ester is not removed as soon as it is formed, the ester will hydrolyse to give starting materials carboxylic acid and alcohol.