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The Reaction of Methylbenzene and Chlorine

A Free Radical Substitution Reaction

This page gives you the facts and a simple, uncluttered mechanism for the free radical substitution reaction between methylbenzene (previously known as toluene) and chlorine. If you want the mechanism explained to you in detail, there is a link at the bottom of the page.

Methylbenzene has a methyl group attached to a benzene ring. The hexagon with the circle inside is the standard symbol for this ring. There is a carbon atom at each corner of the hexagon, and a hydrogen atom on each carbon apart from the one with the methyl group attached.

Note: There is no need to worry about the bonding in the benzene ring at this point. If you are interested, you can follow the link – but it isn't important for now.

The Facts

The reaction we are going to explore happens between methylbenzene and chlorine in the presence of ultraviolet light – typically sunlight. This is a good example of a photochemical reaction – a reaction brought about by light.

Note: These reactions are sometimes described as examples of photocatalysis – reactions catalysed by light. It is better to use the term "photochemical" and keep the word "catalysis" for reactions speeded up by actual substances rather than light.

The organic product is (chloromethyl)benzene. The brackets in the name emphasise that the chlorine is part of the attached methyl group, and isn't on the ring.

One of the hydrogen atoms in the methyl group has been replaced by a chlorine atom, so this is a substitution reaction. However, the reaction doesn't stop there, and all three hydrogens in the methyl group can in turn be replaced by chlorine atoms. Multiple substitution is dealt with on a separate page, and you will find a link to that at the bottom of this page.

Important! There is another reaction which happens between methylbenzene and chlorine in the absence of light and in the presence of a number of possible catalysts. In that one, substitution happens in the benzene ring instead of in the methyl group. You will find this reaction discussed under electrophilic substitution reactions.

The Mechanism

The mechanism involves a chain reaction. During a chain reaction, for every reactive species you start off with, a new one is generated at the end – and this keeps the process going.

Species: a useful word which is used in chemistry to mean any sort of particle you want it to mean. It covers molecules, ions, atoms, or (in this case) free radicals.

The over-all process is known as free radical substitution, or as a free radical chain reaction.

Note: If you aren't sure about the words free radical or substitution, read the page What is free radical substitution?.

Chain initiation

The chain is initiated (started) by UV light breaking a chlorine molecule into free radicals.

\text{Cl}_2 \longrightarrow 2\text{Cl}{\bullet}

Chain propagation reactions

These are the reactions which keep the chain going.

Chain termination reactions

These are reactions which remove free radicals from the system without replacing them by new ones. If any two free radicals collide, they will join together without producing any new radicals.

The simplest example of this is a collision between two chlorine radicals.

2\text{Cl}{\bullet} \longrightarrow \text{Cl}_2