It is \(\alpha \) - subtitution when a carboxylic acid having \(\alpha \) - hydrogens is treated with chlorine or bromine in presence of small amount of red phosphorous.

\(R-C{{H}_{2}}COOH\,+\,{{X}_{2}}\,\xrightarrow{P}\) \(\begin{matrix}
   X\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,  \\
   |\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,  \\
   RCH-COOH+HX  \\
\end{matrix}\)  \((X\,=\,Cl,\,Br)\)