Page 392 - Mechanism and Theory in Organic Chemistry

I Electrophilic Aromatic Substitution 379
i
T-overlap.lZ8 Nucleophilic additions in which the second step, protonation of the
intermediate carbanion, is rate-determining are also known.lZ9

7.4 ELECTROPHILIC AROMATIC SUBSTITUTION130
The substitution of an electrophile for another group on an aromatic ring is
electrophilic aromatic substitution (Equation 7.55). Although the leaving group
is most often H+, it may also be another Lewis acid. Perrin has found that the
order of leaving group abilities is H+ >> I + > Br + > NO, + > Cl', which is

El E,
also the order of the ability of the group to bear a positive charge. When El in
Equation 7.55 is not H-that is, when an electrophile attacks a substituted aro-
matic ring, not ortho, meta, or para,to the substituerit but directly at the position
bearing the substituent-then attack is at the ips0 position.131
After a brief discussion of the nature of the attacking species in some of the
most important types of electrophilic aromatic substitution, we shall examine the
mechanism and the effect of substituents on rates and products.
Substitution by halogen may be carried out in thi-ee ways: (1) by molecular
halogenation, in which polarized X, itself acts as the electrophile (Equation
7.56) ; (2) by molecular halogenation with a catalyst, in which the role of the
catalyst is to polarize the halogen molecule; and (3) by positive halogenation in
which the halogen is the cation of a salt.132

Iodination by molecular iodine is slow and operates only when the aromatic
substrate is particularly reactive. Iodination can, however, be effected by using
0 0
It II
ICl, CH3COI or CF3COI as reagents. Addition of zinc chloride to an iodination
reaction in which IC1 is the reagent increases the rate by assisting in breaking the
I-C1 bond.133 Usually positive I + is the attacking reagent in these reactions.
Bromination with molecular bromine takes place readily. The reaction is

See note 121, p. 377.
lag L. A. Kaplan and H. B. Pickard, J. Amer. Chm. Soc., 93, 3447 (1971).
130 For reviews, see: (a) L. Stock, Aromatic Substitution Reactions, Prentice-Hall, Englewood Cliffs,
New Jersey, 1968; (b) R. 0. C. Norman and R. Taylor, Electrojhilic Substitution in Benzenoid Com-
poud, Elsevier, Amsterdam, 1965; (c) E. Berliner, Prog. Phys. Org. Chm., 2, 253 (1964); (d) L. M.
Stock and H. C. Brown, Adu. Phys. Org. Chem., 1, 35 (1963). For deviations from the "normal"
mechanism, see P. B. D. de la Mare, Accts. Chem. Res., 7, 361 (1974).
131 (a) C. L. Perrin, J. Org. Chem., 36, 420 (1971); (b) C. L. Perrin and G. A. Skinner, J. Amer.
Chem. Soc., 93, 3389 (1971).
'3a R. M. Keefer and L. J. Andrews, J. Amer. Chem. Sac., 78, 5623 (1956).
133 J. R. Barnett, L. J. Andrews, and R. M. Keefer, J. Amer. Chem. Soc., 94, 6129 (197?).