Write HVZ reaction.

Explanation

Hell-Volhard Zelinsky reaction. Carboxylic acid (having `alpha`-hydrogen) on heating with chlorin or bromine in the presence of small amount of red phosphorus undergo substitution at `alpha`-carbon atom to form `alpha`-haloacids.
`underset("Carboxylic acid")(R-overset(O)overset(||)(C)-OH)hArr underset("Carboxylate ion")(R-overset(O)overset(||)(C)-O^(-)) + H^(+)`,