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

(i) cyclohexanone forms cyanohydrin in in good yield

this is because of the presence of three methy l group at ` alpha ` - position with respect to carbonyl group which hinder the nucleophlic attack of `CN^(-)` group due to steric hindrance . However , there is no such steric hindrance in cyclohexanone and therefore the nucleophilic attack by `CN^(-)` ion occurs readily to fiorm cyanohydrin .
(ii ) semicarbozide has two `-NH_2` group but one of these which is directly attached to`c=o` is involved in resonance as shown below :

As a result of resonance , electron density on `NH_2` group decrease and hence it does not act as a nucleophile . in contrast the lone pair of electrons on the `NH_2` group ( i.e, attached to NH ) is not involved in resonance and is therefore ,available for nucleophilic attack on the C=O group of the aldehydes and ketons
(iii ) The formation of esters from carboxylic acid and an alcohol in the presence of an acid cataylst is a reversible reaction
` underset("Carboxylic acid ") (RCOOH ) + underset(" alcohol ") ( R.OH overset(H^+) hArr underset("Ester ")RCOOR. ) +H_2 O`
therefore to shift the equilibrium in the forward direction the water of formed should be removed as fast as it is formed .