Chemguide: Support for CIE A level Chemistry

31.1: Halogen compounds

Learning outcome 31.1.1

This is about reactions in which halogenoarenes are produced in a substitution reaction between benzene or methylbenzene and chlorine (in the presence of the catalyst AlCl₃) or bromine (in the presence of the catalyst AlBr₃).

This is a repeat of what you met in statement 30.1.1(a). You will find this described on the page about the halogenation of benzene and methylbenzene.

You can ignore the addition reactions of both benzene and methylbenzene. Side chain substitution has already been covered in statement 30.1.3 and isn't relevant to this statement.

Learning outcome 31.1.2

This statement deals with the relative lack of reactivity of compounds like chlorobenzene compared with halogenoalkanes

The lack of reactivity of compounds like chlorobenzene is covered on the page about reactions of aryl halides. It is important that you follow the link in the section about the strength of the carbon-halogen bond to the introductory page about aryl halides. You can ignore the stuff about physical properties on that second page.

For exam purposes, CIE will ask you about the lack of reactivity of chlorobenzene in terms of the extra strength of the carbon-chlorine bonds. Don't use the additional explanation about the repulsion between the incoming nucleophile and the ring electrons.

If you are asked why chlorobenzene is unreactive towards nucleophiles, say "Delocalisation of a lone pair on a chlorine atom over the benzene ring strengthens the carbon-chlorine bond."

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