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.