Chemguide: Support for CIE A level Chemistry

Learning outcome 30: Hydrocarbons

30.1: Arenes

Learning outcome 30.1.3

This statement is about whether substitution by a halogen in an arene happens in the ring or the side-chain.

Before you go on, you should find and read the statement in your copy of the syllabus.

In fact, this is just a repeat of what you have already come across in 30.1.1(a). Look back at the page about the halogenation of benzene and methylbenzene again. This time just concentrate on the substitution reactions of methylbenzene.

You should remember:

  • In the presence of UV light, and the absence of a catalyst, substitution takes place in the side-chain.

  • In the presence of a catalyst, and the absence of UV light, substitution takes place in the ring.

Note:  There are several possible catalysts for these reactions, but the syllabus specifically asks for aluminium chloride (or aluminium bromide if you are substituting bromine).

You must be very careful not to confuse the two cases. And don't think you can be clever and include both a catalyst and UV light in an answer to the same question. You won't get the marks if you do that - whichever of them is right will be cancelled out by the wrong one.

The easiest way of remembering which is which is to make the comparison with methane and other alkanes. You already know that chlorine or bromine can be substituted into these in the presence of UV light. These are chain compounds, and the side-chains attached to benzene rings are no different.

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© Jim Clark 2020