(a)
In this reaction $\alpha$-H is replaced by chlorine.
$CH_{3}COOH + Cl_{2}\overset{\ \ \ \ \ \ p\ \ \ \ \ \ \ }{\rightarrow}CH_{2}COOH + HCl$
$\ \ \ \ \ \ \ \ \ |$
$\ \ \ \ \ \ \ \ Cl$
This reaction is called the Hell-Volhard-Zelinsky reaction.