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.

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