Explanation

(a)
An alkyl cyanide is dissolved in ether or better in ethyl formate or ethyl acetate, and reduced with $SnCI_{2}$ and $HCI$ and then steam distilled. The whole process is called Stephen reaction. In this process alkyl cyanide is reduced to aldehyde.
$CH_{3} - C \equiv NH = 2\lbrack H\rbrack + HCI\underset{ether}{\overset{{SnCI}_{2} + HCI}{\rightarrow}}$
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There is no analogous method for the preparation of ketones.