Chlorination of toluene in the presence of sunlight produces which compound?

When toluene (C₆H₅CH₃) is treated with chlorine (Cl₂) in the presence of sunlight or ultraviolet (UV) light, the main product formed is benzyl chloride (C₆H₅CH₂Cl). The reaction occurs by a free radical substitution mechanism at the side chain (methyl group) of toluene, rather than on the aromatic ring.

In the presence of sunlight, chlorine molecules split into chlorine free radicals (Cl·) through photochemical homolysis. These radicals are highly reactive and abstract a hydrogen atom from the methyl group of toluene, forming benzyl radicals (C₆H₅CH₂·). The benzyl radical then combines with another chlorine radical to produce benzyl chloride. This reaction continues step by step, and if excess chlorine is used, further substitution can occur to yield benzal chloride (C₆H₅CHCl₂) and benzotrichloride (C₆H₅CCl₃).

The key feature of this reaction is that it takes place outside the benzene ring, meaning the chlorine replaces hydrogen atoms in the side chain, not on the ring itself. This type of chlorination is known as side-chain chlorination. The aromatic ring remains unaffected because the reaction conditions (presence of sunlight) favor free radical formation rather than electrophilic substitution.

If the reaction were carried out without sunlight and in the presence of a Lewis acid catalyst (like FeCl₃), chlorination would occur on the ring, forming o-chlorotoluene and p-chlorotoluene instead.

Thus, under sunlight, toluene specifically gives benzyl chloride, an important compound used in the synthesis of dyes, resins, and pharmaceuticals.