Write the reaction showing $\mathrm{\alpha }$ - halogenation of carboxylic acid and give its name?

Carboxylic acids having an $\mathrm{\alpha }$ - hydrogen are halogenated at the $\mathrm{\alpha }$ - position on treatment with chlorine (or) bromine in the presence of small amount of red phosphorous to give $\mathrm{\alpha }$ - halo carboxylic acids. This reaction is known as Hell-volhard zelinsky (HVZ)reaction