Benzene does not react with chlorine at room temperature without a catalyst. However, it does react with chlorine at elevated temperatures (around 350-400 °C) in the presence of ultraviolet (UV) light. This reaction is a free radical halogenation and results in the formation of chlorobenzene.

Mechanism:

1. Initiation: UV light provides the energy to break the chlorine molecule (Cl₂) into two chlorine radicals (Cl•).

2. Propagation: The chlorine radical attacks the benzene ring, abstracting a hydrogen atom and forming a benzene radical. This radical then reacts with another chlorine molecule to form chlorobenzene and a new chlorine radical, continuing the chain reaction.

3. Termination: The reaction terminates when two radicals combine to form a stable molecule.

Note: This reaction is not very selective and can lead to the formation of multiple chlorinated products.

In the absence of a catalyst and UV light, benzene's high electron density makes it relatively unreactive towards electrophilic attack by chlorine.