c
Carbonylic acids reacts with \(C l_{2}\) or \(B r_{2}\) in presence of red \(P\) to give exclusively \(\alpha\) -chloro or \(\alpha\) -bromo acids.

This reaction is called Hell-Volhard-Zelinsky \((HVZ)\) reduction. This reaction is example of \(\alpha -H-\)substitution.

\(C{{H}_{3}}C{{H}_{2}}COOH\,\xrightarrow[-\,HBr]{B{{r}_{2}}/P}\) \(\begin{matrix}
   \begin{matrix}
   Br  \\
   |  \\
\end{matrix}\,\,\,\,\,\,  \\
   C{{H}_{3}}-C-COOH  \\
   |\,\,\,\,\,\,\,  \\
   Br\,\,\,\,  \\
\end{matrix}\)