Chemistry, Third edition

AROMATIC HYDROCARBONS

●Benzene is alkylatedby reaction with an halogenoalkane, in the presence of a

catalyst. For example, chloroethane reacts with benzene in the presence of the

catalyst aluminium chloride to form ethylbenzene:

This type of reaction is often referred to a Friedel–Crafts alkylation.

●Benzene is acylatedby reaction with an acid halide in the presence of a catalyst.

An example of an acyl group is

RCO

|

where R is an alkyl group such as methyl, ethyl etc. The reaction of ethanoyl

chloride with benzene in the presence of aluminium chloride gives phenyl-

ethanone:

This type of reaction is called a Friedel–Crafts acylation.

3. Addition

Benzene does undergo some addition reactions:

●It can be hydrogenated to cyclohexane by reaction with hydrogen in the presence

of a nickel catalyst, but at a higher temperature than that needed to hydrogenate

simple alkenes.

●In ultraviolet light, chlorine adds to benzene and eventually 1,2,3,4,5,6-

hexachlorocyclohexane is formed. The numbers show that one chlorine atom is

attached to each carbon atom of the ring.

Sources of benzene

The main source of benzene is petroleum, although it can be obtained from coal tar.

Cl

Cl

Cl Cl

Cl Cl

+ 3 H 2

CH 3 COCl

COCH 3

AlCl 3

+ +HCl

C 2 H 5 Cl

C 2 H 5

AlCl 3

+ +HCl

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