Aliphatic carboxylic acids having alpha hydrogen atom react with chlorine (Cl2) or bromine (Br2) in the presence of small amount of red P (or PCl3 or PBr3) to give alpha chloro or alpha bromo acids. The reaction is called Hell-Volhard-Zelinsky (or HVZ) reaction. These alpha halo acids are important intermediates to form many alpha substituted carboxylic acid (by reaction with NaOH, NaCN etc.). I is a good leaving group so it is difficult to prepare alpha iodoacid by this method. To prepare alpha iodoacid, acid is first dehydrated and then I2 is added.