Chlorination of benzene in the presence of iron (III) chloride involves mechanism of:

Explanation

Iron three chloride acts as a halogen carrier, polarising the chlorine molecule to generate an electrophile that attacks the electron rich ring, and a hydrogen is then lost so the aromatic system is preserved. Substitution rather than addition is what keeps the delocalised sextet intact. In ultraviolet light and without the catalyst, benzene undergoes free radical addition instead, which is why the conditions matter.