Chemistry, Third edition

AROMATIC HYDROCARBONS

This was usually abbreviated to:

In fact, none of these structures correctly describes benzene. All the carbon-

carbon bond lengths in benzene have been found to be the same, but the structures

above do not predict this since double bonds are shorter than single bonds between

the same atoms. Also, benzene is relatively unreactive towards addition, which we

would not expect in a compound that contains three double bonds. These days the

benzene ring is written as:

This structure represents the fact that electrons from the carbon–carbon double

bonds are ‘spread out’ or delocalizedover the whole molecule, which makes the

molecule more stable. A Lewis structure is not sophisticated enough to describe the

bonding in benzene. In order to obtain a more satisfactory description of the bond-

ing, the electrons must be treated as ‘clouds’ (rather than ‘crosses’ or ‘dots’). For a

brief description of the bonding model, see Box 17.8.

or

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BOX 17.8

Bonding in benzene

The observed properties of benzene can be

explained using a more sophisticated model,

than that proposed by Kekulé, for the bonding

in benzene. Each carbon atom in the ring is

bonded to a hydrogen and to one carbon on

either side of it by bonds. Every carbon has

one unused porbital, containing a single

electron, perpendicular to the plane. This

gives a planar‘skeleton’ as shown in Fig. 17.5.

Theporbitals overlap sideways with their

neighbours to form clouds or ‘doughnuts’

of electron density, above and below the

molecule, as shown in Fig. 17.6. The

electrons no longer ‘belong’ to the carbon

atom from which they originate; they are free

to ‘wander’ around the clouds and are

delocalized.

Delocalization of the ‘left over’ pelectrons is

believed to make benzene more stable than

would be expected if the Kekulé structures

were correct, and explains why benzene does

not react as if it had three double bonds. The

model predicts that all the carbon–carbon

bond lengths in benzene would be of the

same length, which they are. The length of

each bond is between those normally

observed for C=C and C–C.

Fig. 17.5Carbon skeleton in benzene

(hydrogen atoms have been omitted for

clarity). Fig. 17.6The bonds in benzene.