Give possible explanation fòr each of the following: i. Cyclohexanone forms cyanohydrin in 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 an acid catalyst, the water or the ester should be removed as soon as it is formed.

Explanation

i. figure
`2,2.6` - trimethylcyclohexanone does not form cyanohydrin with .HCN due to the presence of three methyl groups at the ortho- position w,r.t the carbonyl group. These three-methyl groups cause steric hin- ` ^1` drance to the nucleophilic attack of the -CN- group. Since there is no such steric hindrance in cyclohexanone, hence the nucleophilic attack of the `-CN^-` group occurs readily in cyclohexanone.
ii. figure
Although semi- carbazide has two- `NH_2` groups. but one of them (i.e., directly attached to `C=O` ) is involved in resonance as shown above.. As. a result, electron density on this `NH_2` group decreases and hence it does not act as a nucleophile. In contrast, the lone pair of electrons on the other `NH_2`,group is not
-involved in resonance and hence is available for nucleòphilic attack on the `C=O` group of aldehydes and ketones.
iii. Esterification process being a reversible process, the éstếr so formed will react with water to give back the acid ạnd the alcohol back. . `RCOOH+H stackrelcircrarr OR^prime H^+ R dotCOOR^2+H_2 O`
Hence water should be removed as soon as it is formed.