Chemistry, Third edition

SUBSTITUTED BENZENE DERIVATIVES 347

The presence of the aromatic ring makes aniline a much weaker base than

ammonia. Aniline is an extremely toxicliquid that readily enters the body through the

skin or by inhalation and should be handled with extreme care.

Aniline undergoes an interesting reaction with sodium nitrite and ice-cold

aqueous acid. The compound formed is called a diazonium salt:

C 6 H 5 NH 2 (l) NaNO 2 (aq) 2HCl(aq)

C 6 H 5 —NN, Cl(aq) NaCl(aq) 2H 2 O(aq)

diazonium salt

Diazonium salts can be reacted to form many classes of compounds but decompose,

even at cold temperatures, so they must be used immediately. They also react with

certain aromatic compounds to form azo compounds. The reaction is called a

coupling reaction, for which the general equation is

Ar—NNArHAr—N==N—ArH

azo compound

The double bond between the nitrogen atoms makes azo compounds strongly

coloured. Because of their intense colours they are widely used as dyes. Many

acid–base indicators are azo dyes.

Some examples of azo dyes are

Benzyl compounds

The benzyl group, C 6 H 5 CH 2 , is an aralkyl group. It contains an aromatic group

(the benzene ring) and an alkyl group. If functional groups are attached to the alkyl

part of the group and notto the ring, the behaviour of the new compound tends to

resemble similar alkyl compounds. For example, benzyl alcohol is classed as an

alcoholand does not behave chemically as a phenol.