A Guidebook to Mechanism in Organic Chemistry

Structure, Reactivity and Mechanism

2 3

Bond Order

1 -20

but it will be seen that the relationship is not a linear one and that

the bonds in benzene are not midway between double and single bonds

in length. The influence of the layer of negative Charge on the type of

reagents that will attack benzene fi discussed below (p. 101).

The relative unreactivity of benzene, as compared with the highly

unsaturated system implied in its usual representation and actually

observed in a non-cyclic conjugated triene, arises from the stability

conferred by the cyclic delocalisation of the IT electrons over the six

carbon atoms coupled with the fact tfeat the angle between the plane

trigonal a bonds is at its optimum vafUe of 120°. The stability conferred

by such cyclic delocalisation also explains why the characteristic

regions of aromatic systems are substitutions rather than- the

addition reactions that might, from the classical Kekule structures,

be expected and which are indeed realised with non-cyclic conjugated

trienes. For addition would lead to a product in which delocalisatifea,

though still possible, could now involve only four carbon atoms and

would have lost its characteristic cyclic character (IX; cf. butadiene),

whereas substitution results in the retention of delocalisation essen­

tially similar to that in benzene with all that it implies (X):

Br, Br,

: +HBr

Addition

(IX)

Substitution

(XI) (X)

* This symboi has, where appropriate, been used to represent the benzene

nucleus as it conveys an excellent impression of the closed, delocalised orbitals

from which its characteristic aromaticity stems.