Page 789 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

772 EWG substituents. Those in the second group react readily with benzene and deriva-
tives having ERG substituents but are not generally reactive toward aromatic rings
CHAPTER 9 with EWG substituents. Those classified in the third group are reactive only toward
Aromatic Substitution aromatic compounds that are much more reactive than benzene. These groupings can
provide a general guide to the feasibility of a given EAS reaction.
Despite the wide range of electrophilic species and aromatic ring systems that
can undergo substitution, a single broad mechanistic picture encompasses most EAS
reactions. The identity of the rate-determining step and the shape of the reaction energy
profile are specific to individual reactions, but the sequence of steps and the nature
of the intermediates are very similar across a wide range of reactivity. This permits
discussion of EAS reactions in terms of the general mechanism that is outlined in
Scheme 9.2.
A complexation of the electrophile with the electron system of the aromatic
ring is the first step. This species, called the complex, may or may not be involved

Scheme 9.1. Electrophiles Active in Aromatic Substitution
Electrophile Typical means of generation
A. Electrophiles capable of substituting both activated and deactivated aromatic rings
–
+
4
3
2
1 a O N + O 2 H SO + HNO NO + 2 HSO + H O +
4
2
3
2 b Br or Br – MX n Br + MX n Br – MX n
2
2
2
2
+
+
3 b BrO H 2 BrOH + H O + BrO H 2
3
4 b Cl or Cl – MX n Cl + MX n Cl – MX n
2
2
2
2
+
5 b ClO H 2 ClOH + H O + ClO H 2
+
3
+
+
–
6 c SO or SO O H H S O 7 HSO + SO O H
2 2
2
3
4
2
+
7 d RSO 2 + RSO Cl + AlCl 3 RSO + AlCl 4 –
2
2
B. Electrophiles capable of substituting activated but not deactivated aromatic rings
+
8 e R C + R CX + MX n R C + [MX n+1 ] –
3
3
3
+
9 f R C + R COH + H + R C + H O
2
3
3
3
+
+
10 g R C CHR' 2 R C CR' + H + R C CHR' 2
2
2
2
2
11 e RCH X – MX n RCH X + MX n RCH X MX n
2
2
2
+
12 h RC O + RCOX + MX n RC O + [MX n+1 ] –
13 h RCOX MX n RCOX + MX n RCOX MX n
+
+
14 i RC + O H RCOX + MX + H + RC + O H + [MX n+1 ] –
n
+
15 j H + HX H + X –
+
+
16 k R C O H R C O + H + R 2 C O H
2
2
–
–
17 k R C O + – M X n R C O + MX n R 2 C O + – M X n
2
2
+
+
18 i HC + N H 2 HC N + 2H + HC + N H 2