groups
in semicarbazide. However, only one is involved in the formation of
semicarbazones.
(iii) During the preparation of
esters from a carboxylic acid and analcohol in the presence -of an acid
catalyst, the water or the ester should be removed as soon as it is formed.
(i) 
effect
due to methyl groups: steric hindrance, make a carbonyl compound less reactive.
(ii) Lone pair of electrons
involved in conjugation with CO.
(iii) Consider the reversible nature of
the reaction.
Answer:
(i)
In 2,2,6 trimethyl cyclohexanone, three
methyl groups arepresents at a-position with respect to the ketonic 
group.
Therefore, these groups cause
steric hindrance during thenucleophilic attack of 
ion so
cyanohydrin is not formed. Dueto the absence of methyl groups in cyclohexanone,
there is nosteric hindrance and cyanohydrin is formed.
(ii) Resonance in
semicarbazide
Semicarbazide has two amino 
groups,
out of which one isinvolved in resonance as shown above. Electron-density on
this
decreases
and it does not act as a nucleophile.
But the other group
(attached to NH) has lone pair ofelectrons which, are not involved in
resonance. So, this pair isavailable for the nucleophilic attack on carbonyl
group of aldehydes and/or
ketones.
(iii) Esterification is a reversible
reaction.
When the sufficient amount of products
is formed, the rate offorward reaction decreases and the reverse reaction
begins. Toavoid this condition i.e., in order to shift the equilibrium in
forwarddirection, the concentration of products (ester and/or water)should be
decreased. (Le-Chatelier's principle). So, water shouldbe removed from time to
time.
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