Electrophilic Attack on Benzeneaddition compound, would no longer be aromatic with all that that
implies. By expelling H®, i.e. by undergoing substitution rather than
addition, the complete delocalised n orbitals are regained in the
product (IV) and characteristic aromatic stability recovered:
H D H D
V ci v
A1CI.G (r\\ A1C1.0
H <-( + CIO) (-H©)
(III) (II) (IV)
* Addition SubstitutionThe gain in stabilisation in going from (II) -(IV) helps to provide
the energy required to break the strong C—H bond that expul
sion of H® necessitates; in the reaction of, for example, HC1 with
alkenes (p. 141) there is no such factor promoting substitution and
addition reactions are therefore the rule.
In the face of the concentration of negative charge presented to
an attacking reagent it might be expected that the substitution of
benzene by the common electrophiles (i.e. halogehation, nitration,
sulphonation and the Friedel-Crafts reaction) would be extremely
easy. Though the electrophilic substitution of be'Bzene is not difficult,
that it is not easier than it is, is due to the enftTgy barrier to l^ur-
mounted in converting the very readily formed IT complex to a a
complex in which actual bonding of the reagent to a ring-carbon
atom has taken place. For in the IT complex, the aromatic nature of
the nucleus (i.e. the delocalised n orbitals) is largely undisturbed,
while in the a.complex some of the characteristic stabilisation has
been lost as the orbitals now only involve five carbon atoms. The loss
of stabilisation involved is greater than might be expected as the ir
orbitals are now no longer symmetrical; it is the symmetry of the
orbitals in the intact aromatic nucleus that underlies its characteristic
stability and relative unwillingness to undergo change. The regaining
of this symmetry (or near symmetry, for the orbitals will be deformed
to a certain extent by the introduction of any substituent other than
hydrogen into the nucleus) is responsible for the ease with which the
relatively strong C—H bond undergoes fission in order to allow the
conversion of (H)-(IV).
How this basic theory is borne out in the common electrophilic
substitution reactions of benzene will now be considered.