Write a note on the reaction occurring in the hydrocarbon part of carboxylic acid or write a note on the Hell-Volhard-Zelinsky reaction.

(N/A) Carboxylic acids having an $\alpha$-hydrogen are halogenated at the $\alpha$-position on treatment with chlorine or bromine in the presence of a small amount of red phosphorus to give $\alpha$-halo-carboxylic acids. This reaction is known as the 'Hell-Volhard-Zelinsky' reaction.
$RCH_{2}COOH \xrightarrow[(ii)\ H_{2}O]{(i)\ X_{2}/\text{Red } P} RCH(X)COOH$
Where,$X = Cl, Br$.
This halogenation reaction continues until all $\alpha$-hydrogens are replaced.
Example: Chlorination of ethanoic acid:
$CH_{3}COOH$ $\xrightarrow{Cl_{2}/\text{Red } P} CH_{2}ClCOOH$ $\xrightarrow{Cl_{2}/\text{Red } P} CHCl_{2}COOH$ $\xrightarrow{Cl_{2}/\text{Red } P} CCl_{3}COOH$
Uses: This reaction is utilized to introduce groups like $-OH, -CN, -NH_{2}$ by first substituting the $\alpha$-position with $Cl$ or $Br$.