Dear student,
When carboxylic acids react with chlorine or bromine  in the presence of red phosohorous, the  alpha-hydrogen atoms are replaced by halogen atoms. It is called  Hell-Volhard-Zelinsky(H.V.Z) reaction.
 Ex:-
    acetic acid reacts with chlorine in presence of red phosphorus to  form chloro acetic acid, dichloro acetic acid or trichloro acetic acid  depending on the quantity of chlorine reacting.











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HVZ is a reaction used to prepare alpha-halo acids. The essential condition for this reaction is the presence of alpha hydrogen. Thus Formic acid does not undergo this reaction. The reagent used is Red phosphorous/X2   where X is a halogen. In the first step, -OH in acid group is replaced by X. On further reaction, alpha hydrogen is replaced by another X atom. On hydrolysis, the terminal X atom will be replaced by the -OH group. Thus the product obtained is alpha-halo acids.
RCH2COOH + Br2/P ----> RCH2COBr  + Br2/P ----> RCHBrCOBr + H2O ----> RCHBrCOOH ( alpha-halo acid)