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9. Aromatic Hydrocarbons eLearn.Punjab

e

(a), (b) were proposed by Kekule and c, d, e, were proposed by Dewar.
The stability of a molecule increases with increase in the number
of its resonance structures. Thus molecule of benzene is chemically
quite stable.

In Dewar structure the carbon atoms at opposite positions 1-4,
2-5 and 3-6 are at larger distances than those in the adjacent
positions 1-2,2-3,3-4,4-5,5-6 and 6 -1. Therefore the bondings
between C1-C4, C2-C5 and C3-C6 are not favourable energetically.

Hence the Dewar structures for benzene have minor
contribution towards the actual structure of benzene.

Infact, the structure of benzene is a resonance hybrid of all the five
structures (a), (b), (c), (d) and (e) in which the Kekule’s structure (a)
and (b) have the larger contribution. Therefore, benzene molecule
can be represented by either of the two Kekule’s structure.

The three alternate single and
double bonds in the Kekule’s
structures are called conjugate
bonds or resonating bonds.

Since the structure of benzene is a resonance hybrid, therefore all the C-C
bond lengths are equal but different from those in alkanes, alkenes and
alkynes.

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