Toluene (methylbenzene) - Methyl side chain with chlorine or bromine (Free Radical Substitution)

The halogenation of methylbenzene (toluene) can proceed through a homolytic reaction mechanism, particularly under conditions favoring the formation of free radicals. This process is typically observed in the free radical halogenation of the methyl group in methylbenzene, rather than the aromatic ring itself. The reaction involves the generation of free radicals and is initiated by heat or light. 

The homolytic halogenation of methylbenzene primarily targets the methyl group, leading to the formation of benzyl halides. This reaction mechanism is distinguished by the generation of free radicals and involves initiation, propagation, and termination steps. The preference for halogenation at the methyl group over the aromatic ring is influenced by the stability of the generated benzyl radicals and the relative reactivity of the methyl hydrogen atoms compared to the hydrogen atoms directly attached to the aromatic ring.

Here’s a simplified overview of the homolytic halogenation of methylbenzene, commonly with bromine.

Oxidation of the methyl group in methylbenzene

The powerful oxidising agents acidified potassium manganate(VII) and acidified potassium dichromate(VI) will oxidise the side chain, — CH3, to the carboxylic acid group, —COOH. The reaction mixture must be refluxed for several hours.

 A milder oxidising agent, manganese(IV) oxide, MnO2, or chromium dichloride dioxide, CrO2Cl2, oxidises —CH3 to the aldehyde group, —CHO