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 presence of acid catalyst, the water or the ester should be removed as soon as it is formed.

Explanation

The yield of second reation is very low because of the presence of three methyle groups at ex-positions with respect to the C = O, the nucleophilic attack by the `CN^(-)` ion does not occur due to steric hinderance. Since there is no such steric hindrance in cyclhexanone. therefore, nucleophilic attack by the `CN^(-)` ion occurs readily and hence cyclohexanone cyanohydrin is obtained in good yield.

Although semicarbazide has two `-NH_(2)` group decreases and hence it does not act as a nucleophille in contrast the other group but one of them (i.e., which is directly attached to C = O) is involved in resonance as shwon above. As a result electron density on N of this `-NH_(2)` group (i.e., attached to NH) is not involved in resonance and hence long pair of electrons present on N atom of this `-NH_(2)` group is available for nucleophilic attack on the C = O group of aldehydes and ketones.
(iii) The formation of esters froma carboxylic acid and an alcohol in presence of an acid catalyst is a reversible reactio.
`RCOOH+R'OHoverset(H_(2)SO_(4))hArrRCOOR'+H_(2)O`
Thus to shift the equilibrium in the forward direction, the water or the ester formed should be removed as fast as it is formed.