The chlorination of o-xylene (1,2-dimethylbenzene) with molecular chlorine in the presence of anhydrous iron(III) chloride proceeds through Lewis acid-catalyzed electrophilic aromatic substitution. The FeCl₃ polarizes the Cl–Cl bond and generates an electrophilic chlorine donor, while the two methyl substituents activate the aromatic ring and direct substitution away from positions that would be meta to both methyl groups. The o-xylene molecule contains four aromatic hydrogen atoms at ring positions 3, 4, 5, and 6. A symmetry operation interchanges positions 3 and 6, and a separate symmetry operation interchanges positions 4 and 5. Chlorination at position 3 or 6 therefore gives the same constitutional isomer, 3-chloro-1,2-dimethylbenzene, while chlorination at position 4 or 5 gives 4-chloro-1,2-dimethylbenzene. Accordingly, the number of distinct monochlorinated products formed under controlled FeCl₃-catalyzed conditions is 2.

Mechanistically, the reaction proceeds through formation of a polarized chlorine–iron(III) chloride complex, followed by nucleophilic attack of the aromatic π-system on the terminal chlorine atom. This generates a sigma complex, or arenium ion, in which positive charge is delocalized across the ring. The methyl substituents stabilize the arenium ion most effectively when the positive charge can be accommodated at a methyl-bearing carbon atom, which occurs when substitution takes place at positions ortho or para to one of the methyl groups. In o-xylene, both unique monochlorination sites receive activation from at least one methyl group, and neither site is meta to both methyl groups. The 4-position is para to the C1 methyl group and meta to the C2 methyl group, whereas the 3-position is ortho to the C2 methyl group and meta to the C1 methyl group. Both positions are therefore electronically activated, but the 3-position experiences greater steric hindrance because it lies adjacent to the C2 methyl substituent.

Under ordinary FeCl₃-catalyzed conditions in the dark or in the absence of radical initiators, side-chain chlorination at the methyl groups is not observed. The Lewis acid pathway suppresses the homolytic dissociation of Cl₂ that would be required for benzylic free-radical substitution, and the reaction instead proceeds through the polar aromatic substitution manifold. The two monochloro derivatives are commonly obtained as a mixture, with the less hindered 4-chloro-1,2-dimethylbenzene generally predominating over 3-chloro-1,2-dimethylbenzene. The exact ratio varies with solvent polarity, temperature, Cl₂ feed rate, and catalyst loading; published data for a universal selectivity ratio across all possible process configurations is limited. The constitutional isomer count, however, remains fixed by the symmetry of the starting arene.

If the chlorination is performed with excess chlorine and without selective monochlorination control, the initially formed 3- and 4-chloro-o-xylenes can undergo further electrophilic substitution to give dichlorinated, trichlorinated, and tetrachlorinated derivatives. In that case, the total number of chlorination products depends on the stoichiometry, reaction time, and degree of feed control. For the standard interpretation of the query as a monochlorination reaction with Cl₂/FeCl₃, the symmetry of o-xylene reduces the four aromatic C–H sites to two non-equivalent substitution positions. The number of distinct chlorination products is therefore 2.