This is a process whereby a hydrogen atom is replaced by a chloromethyl group, CH2Cl. The reaction may be carried out by heating the aromatic hydrocarbon with formalin or paraformaldehyde and hydrochloric acid in the presence of zinc chloride as catalyst, e.g., benzyl chloride from benzene :
C6H6 + CH2O + HCl ........> C6H5CH2Cl + H2O
The mechanism of chloromethylation is uncertain; a possibility is :
2CH2=O + 2HCl + ZnCl2 <......>[ZnCl4] 2- + 2 [+CH2OH] ........>ArCH2OH .......> ArCH2Cl
2- after ZnCl4 and +before CH2OH are charges on these species., first step arrows are for reversible reaction.