Giving plausible explanation for each of the following:
i. Cyclohexanone forms cyanohydrin good yield but 2,2,6-trimethylcyclohexanone does not.
ii. There are two `(-NH_(2))` 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 easter should be removed as soon as it is formed.

Explanation

In cyclohexanone, the attack of `CN^(-)` ion (nucleophile) can easily take place at the carbonyl carbon atom. However, in 2, 4, 6-trimethylcyclohexanone, the three `CH_(3)` groups being electron releasing in nature (+1 effect) will considerably incease the electron density on the carbonyl carbon atom and the nucleophile attack does not seem to be feasible. Moreover, the two-`CH_(3)` substituents at the ortho positions will also hinder the attack of nucleophile `CN^(-)` ion on the carbonyl group.
(i) The structural formula of semi-carbazide is `NH_(2)NHCONH_(2)`. Although both the aminogroups have lone electron pair, but one of these is in conjugation with electron withdrawing carbonyl group and acquires positive charge. Therefore, it is not in a position to act as the nuclephile and only one `NH_(2)` group is involved in the formation of semicarbazone.

(iii) The esterification carried in the presence of acid is fo reversible nature and the reverse reaction is called ester hydrolysis.
`underset("Acid")underset()(RCOOH)+underset("Alcohol")underset()(ROH)overset("Esterification")underset("Hydrolysis")hArrunderset("Estar")underset()(RCOOR)+H_(2)O`
In order that the reaction may proceed in the forward direction, ester or water formed in the reaction must be removed. Sulphuric acid added in esterification helps in removing moleculs of `H_(2)O` as it is a dehydrating agent.