Hi,

Carboxylic acids having an α-hydrogen are halogenated at the α-position on treatment with chlorine or bromine in the presence of small amount of red phosphorus to give α- halocarboxylic acids. The reaction is known as Hell-Volhard-Zelinsky reaction. 

For better understanding please refer to this video - https://www.youtube.com/live/cQsl5Pf_aTg?feature=shared&t=5350

Thanks for asking 

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