Chemistry, Third edition

324 17 · ORGANIC CHEMISTRY: HYDROCARBONS

Note that ‘Ar–’ stands for aryland is used to represent an aromatic group, includ-

ing a benzene ring. For example, Ar–Cl could mean:

The symbol ‘Ph–’ stands specifically for the phenylgroup (C 6 H 5 –), so the com-

pound above might also be written C 6 H 5 – Cl or Ph–Cl.

Physical properties of benzene

Benzene is a colourless liquid which is insoluble in water. It was widely used as a

solvent, but now this use is limited since benzene has been found to be toxic –

inhalation of its vapour can cause leukaemia. Toluene (see next section) is used as a

safer alternative.

Chemical properties of benzene

1. Combustion

Benzene burns in air with a smoky, luminous flame. In addition to carbon dioxide

and water, particles of carbon are formed because of incomplete combustion.

2. Substitution

Benzene undergoes substitution reactionsrather than addition reactions (it does

not behave chemically like the alkenes). In these reactions, hydrogens attached to the

ring carbons are substituted by other atoms or groups. Some examples are shown

below:

●Benzene can be nitratedby reaction with a mixture of concentrated sulfuric and

nitric acids at 60 °C. Nitrobenzene, a pale yellow liquid is formed:

●Benzene is halogenatedat room temperature by reaction with chlorine or

bromine, in the presence of a catalyst. For example, chlorine reacts with benzene

in the presence of iron(III) chloride. A chlorine atom is substituted for hydrogen

in the benzene ring and chlorobenzene is formed:

Cl 2

Cl

FeCl 3

+ +HCl

HNO 3

NO 2

H 2 SO 4

+ +H 2 O

C

Cl

H

H H

HH

C

C C

C

C