Page 788 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

9

Aromatic Substitution

Introduction

The introduction or replacement of substituents on aromatic rings by substitution
reactions is one of the most fundamental transformations in organic chemistry. On
the basis of the reaction mechanism, these substitution reactions can be divided into
(a) electrophilic, (b) nucleophilic, (c) radical, and (d) transition metal catalyzed. In
this chapter we consider the electrophilic and nucleophilic substitution mechanisms.
Radical substitutions are dealt with in Chapter 11 and transition metal-catalyzed
reactions are discussed in Chapter 9 of Part B.

9.1. Electrophilic Aromatic Substitution Reactions

Electrophilic aromatic substitution (abbreviated EAS in this chapter) reactions
are important for synthetic purposes and are also among the most thoroughly studied
classes of organic reactions from a mechanistic point of view. The synthetic aspects
of these reactions are considered in Chapter 9 of Part B. This section focuses on
the mechanisms of several of the most completely studied reactions. These mecha-
nistic ideas are the foundation for the structure-reactivity relationships in aromatic
electrophilic substitution that are discussed in Section 9.2.
A wide variety of electrophiles can effect aromatic substitution. Usually, it is
a substitution of some other group for hydrogen that is of interest, but this is not
always the case. For example, both silicon and mercury substituents can be replaced
by electrophiles. Scheme 9.1 lists some of the specific electrophiles that are capable of
carrying out substitution of hydrogen. Some indication of the relative reactivity of the
electrophiles is given as well. Many of these electrophiles are not treated in detail until
Part B. Nevertheless, it is important to recognize the very broad scope of electrophilic
aromatic substitution.
The reactivity of a particular electrophile determines which aromatic compounds
can be successfully substituted. The electrophiles grouped in the first category are suffi-
ciently reactive to attack almost all aromatic compounds, even those having strongly
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