(i) Consider the following reactions :Reaction (a) is an example of cyanohydrin formation by the nucleophilic attack of cyanide ion on the carbonyl carbon. In cyclohexanone, this attack takes place easily and cyanohydrin is formed. But in II, attack of the Nu– is sterically hindered due tothe presence of the three methyl groups. As a result, the reaction does not occur.
(ii) Semicarbazide has the following resonance structures arising due to the electron withdrawing nature of the O atom.From the above structures we find that the lone pair of electrons on N atoms on either side of =C=O group are delocalised, involved in resonance. Therefore, they are not available for carrying out the nucleophilic attack on the aldehyde or ketone.
But, the –NH2 (terminal one) is intact, i.e. it is not involved in resonance and thus its, lone pair is available for attack on carbonyl compound. This is why the semicarbazone is formed through this N atom only.The esterification reaction is a reversible reaction. If water and the ester are allowed to remain in the reaction mixture the hydrolysis of the ester would take place. Hence, water or ester should be removed as soon as it is formed.