Chemguide: Support for CIE A level Chemistry

30.1: Arenes

Learning outcome 30.1.2

This statement is about the mechanism for the electrophilic substitution reaction involved in the nitration and bromination of arenes, and the effect of the delocalised electrons in the ring.

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

Important background

Before you look in detail at these reactions, you should first read the introductory page on electrophilic substitution.

Statement 30.1.2(a)

Nitration

You will find this described on the page about the nitration of benzene.

Don't forget to learn the equation for the formation of the electrophile. That is an essential part of the mechanism.

It is essential that you follow the "Help!" link at the bottom of the page where you will find the mechanism explained in detail.

On this second page, you will find the two alternative ways of writing the final stage of the mechanism. CIE mark schemes show that they are expecting what I have described as "a sloppy way" of writing the final step, but obviously they will also accept the proper way of writing it. The choice is yours. If you want an easy life, choose the sloppy way; if you like things to be right, learn what really happens.

Bromination

The mechanisms for chlorination and bromination are explained on the page about the halogenation of benzene. Don't forget to read the "Help" page as well.

These pages actually show the mechanism equations for chlorination, but all you need to do for bromination is literally just replace every Cl by Br. But don't try to do that mentally at the first read-through. Just make sure that you understand it in the chlorine case.

Once you are happy, rewrite the mechanism replacing every Cl by Br.

Statement 30.1.2(b)

This statement is about why the delocalisation of electrons in benzene leads to substitution rather than addition reactions.

Important ideas include:

• It costs about 150 kJ mol^-1 to break the delocalisation of the electrons. If benzene (or any other arene) undergoes addition reactions, that would permanently break the delocalisation, and the energetics would be against this happening. Arenes tend to undergo substitution rather than addition to the ring.

• The delocalised electrons lying above and below the plane of the molecule are very attractive for electrophiles to attack. Therefore, arenes undergo electrophilic substitution.

• The substitution mechanisms all involve a temporary breaking of the delocalisation. That costs energy, even if it is given out again when the delocalisation reforms. Therefore, activation energies are high, and substitution reactions of arenes tend to be relatively slow.

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