The reaction coordinate diagram in Figure 15.4 shows that the reaction of benzene
to form a substituted benzenehas a close to zero. The reaction of benzeneto form
the much less stable nonaromatic addition productwould have been a highly ender-
gonic reaction. Consequently, benzene undergoes electrophilic substitution reactions
that preserve aromaticity, rather than electrophilic addition reactions(the reactions
characteristic of alkenes), which would destroy aromaticity.¢G°606 CHAPTER 15 Aromaticity • Reactions of Benzene
YYZY++ HbbaaZ−+carbocation
intermediatea nonaromatic
compoundproduct of
electrophilic additionYan aromatic
compoundproduct of
electrophilic substitution- (^) HZ
Figure 15.3N
Reaction of benzene with an
electrophile. Because the aromatic
product is more stable, the reaction
proceeds as (a) an electrophilic
substitution reaction rather than
(b) an electrophilic addition
reaction.
Progress of the reaction
Free energy
Z− - H
Y+ addition product
substitution product
Y
Y
Z - HZ
- YZ
Figure 15.4N
Reaction coordinate diagrams for
electrophilic substitution of
benzene and electrophilic
addition to benzene.
If the carbocation intermediate formed from the reaction of benzene with an elec-
trophile were to react similarly with a nucleophile (depicted as event bin Figure 15.3),
the addition product would not be aromatic. If, however, the carbocation loses a proton
from the site of electrophilic attack (depicted as event ain Figure 15.3), the aromati-
city of the benzene ring is restored. Because the aromatic product is much more stable
than the nonaromatic addition product, the overall reaction is an electrophilic substitu-
tion reaction rather than an electrophilic addition reaction. In the substitution reaction,
an electrophile substitutes for one of the hydrogens attached to the benzene ring.
PROBLEM 16
If electrophilic addition to benzene is an endergonic reaction overall, how can electrophilic
addition to an alkene be an exergonic reaction overall?