Functional Classes II, Reactions - Organic Chemistry Concepts: An EFL Approach - page 129

simply undergo an acid-base reaction with nitrous acid. This gives soluble salts.
As a result, there is no visible reaction.
7.10 AROMATIC COMPOUNDS
We have talked about aromatic compounds simply as structural examples of the
functional classes. Details of aromatic compounds are limited to the simplest
member, benzene, and its derivatives. In general, you saw in Chapter 2 how the
aromatic system Ar- is an alternative for the alkyl R-.
Most functional group reactions of compounds with an aromatic substituent fol-
low those discussed in earlier sections. However, in Chapter 4 we saw that arenes
have a special delocalized system which gives large resonance effects. Because
of this, the reactivity of functional groups which are attached directly to the
aromatic ring may change. We have already seen the example of the increased
acidity of phenols in Chapter 6.
7.10.1 Electrophilic Aromatic Substitution
As with alkenes, the benzene system is electron rich and can act as a nucleophile.
Because of delocalization of the π-electrons, this nucleophile is weak and has
lower reactivity with electrophiles. Very reactive electrophiles and/or Lewis acid
catalysts are used to promote the reaction.
FIGURE 7.48
Aromatic electrophilic addition versus substitution.
However, alkenes react by electrophilic addition, but the benzene ring reacts by
electrophilic substitution. This is because the benzene nucleus wants to keep the
extra aromatic resonance stabilization. As Figure 7.48 shows, this benefit is lost
in an addition reaction because the delocalized system is broken.
The reaction follows a stepwise mechanism. The rate-determining addition of
the electrophile is the first step. Then, a proton is eliminated to complete the
aromatic system again. A wide choice of electrophiles allows many different sub-
stituted aromatic compounds to be prepared in this way. Some of most impor-
tant ones are summarized in Table 7.5 .
The position of electrophilic substitution when substituents are already on the
benzene ring can be difficult to predict. The position of substitution depends on
the relative position and type of the substituents which are already present. How-
ever, this is beyond the scope of this topic.