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.
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.