Which is the mechanism for this conversion? A - AQA - A-Level Chemistry - Question 23 - 2020 - Paper 3

The conversion from toluene (CH₃ attached to a benzene ring) to benzyl chloride (CH₂Cl attached to a benzene ring) suggests that a free-radical substitution mechanism is involved.

In free-radical substitution, the benzyl carbon undergoes a reaction where a chlorine radical replaces a hydrogen atom, typically facilitated by a chlorine gas (Cl₂) in the presence of UV light or heat. This process can be summarized in the following steps:

1. Initiation:

   The Cl₂ molecules dissociate into two chlorine radicals (Cl•) upon exposure to heat or light.

2. Propagation:

   The chlorine radicals attack the benzyl group, replacing a hydrogen atom and forming a benzyl radical (C₆H₅CH₂•).

   The benzyl radical can then react with another chlorine molecule to form benzyl chloride and regenerate another chlorine radical, continuing the cycle.

3. Termination:

   Two radicals can combine to terminate the reaction.

Overall, this identifies free-radical substitution as the correct mechanism for the conversion.