Bonding in Carbon CompoundsIn other words, substitution can take place with overall retention of
aromaticity, addition cannot (cf p. 102).
A rough estimate of the stabilisation conferred on benzene by
delocalisation of its n electrons can be obtained by comparing its
heat of hydrogenation with that of cyclohexene:I^J| +HS -* +28-8 kcal/mole+3Ha -j0 +49-8 kcal/moleThe heat of hydrogenation of three isolated double bonds (i.e.
bonds between which there is no interaction) in such a cyclic system
would thus be 28-8x3 = 86-4 kcal/mole. But when benzene is
hydrogenated only 49*8 kcal/mole are actually evolved. Thus the
infraction of the IT electrons in beazene may be said to result in the
molecule being stabler by 36-6 kcal/mole than if no such interaction
took place (the stabilisation ^arising from similar interaction in
conjugated dienes is only « 6 Real/mole, hence the preference of
benzene for substitution rather than addition reactions, cf. p. 102).
This amount by which benzene is stabilised is referred to ae«*he
delocalisation energy or, more commonly, the resonance energy. The
latter, though more widelpused, is a highly unsatisfactory term as the
v^d resonance immediately conjures up visions of rapid oscillations
between one structure and another, for example the Kekuld struc
tures for benzene, thus entirely misrepresenting the actual state of
affairs.(vi) Conditions necessary for delocalisation
Though the delocalisation viewpoint cannot result in this particular
confusion of thought, it may lead to some loss of facility in the actual
writing of formulae. Thus while benzene may be written as (XI) as
readily as one of the Kekuld structures, the repeated writing of
butadiene as (V) becomes tiresome. This has led to the convention
of representing molecules that cannot adequately be written as a
single classical structure (e.g. (IV)) by a combination of two or more
classical structures linked by double-headed arrows; the way in
TVhich one is derived from another by movement of electron pairs