Chemistry, Third edition

326 17 · ORGANIC CHEMISTRY: HYDROCARBONS

Arenes

Hydrocarbons that contain both aromatic and aliphatic groups are called arenes.

The structures of some arenes are

Notice that there can be different structural isomers where more than one carbon

atom in the ring is attached to separate groups. There are three possible xylenes, for

example, the other two being

Notice that they are named by numbering the different carbon atoms in the ring.

The first carbon atom to be attached to an atom or group is given the number 1:

1

4

3

2

5

6

1,3-dimethylbenzene 1,4-dimethylbenzene

(old name m-xylene) (old name p-xylene)

CH 3

CH 3

CH 3

CH 3

methylbenzene ethylbenzene 1,2-dimethylbenzene

(common name (old name o-xylene)

toluene)

CH 3

CH 3

CH 3 CH 2 CH 3

Stability of benzene

The catalytic hydrogenation of cyclohexene, in the presence of a nickel catalyst, proceeds

according to the equation

(i) If benzene had the Kekulé structure, write an equation that would represent the complete

hydrogenation of benzene.

(ii)According to your equation, what would be the expected value of its standard enthalpy

change of hydrogenation?

(iii)The experimental value of the enthalpy of hydrogenation of benzene to cyclohexane is

208kJmol^1. Why is this value different from the one you have calculated?

H 2

ΔH = –119 kJ mol–1

+

Exercise 17P

Arenes

Write down the structural

formulae of all arenes with

the molecular formula

C 9 H 12 (there are eight).

Exercise 17Q