Functional Classes II, Reactions - Organic Chemistry Concepts: An EFL Approach - page 143

C Remember that an aromatic system gets large resonance stabilization from its
conjugated system. This stabilization is lost if an addition reaction occurs. This happens
because the double bond conjugated system would be broken. Therefore, after the first
step of electrophilic addition, the intermediate carbocation eliminates a proton rather
than adding the nucleophilic half of the addition reagent. This reforms the conjugated
system. See Section 7.10 and Program 24.
PROGRAM 24 Electrophilic Aromatic Substitution
A Many different substituents may be introduced onto the aromatic ring by direct
electrophilic substitution. What is the mechanism for this general reaction? Study the
bromination of benzene. Because the benzene nucleus is electron rich, it can act as
the nucleophile.
Draw diagrams to show this initial nucleophilic attack.
B
This carbocation is stabilized by delocalization.
Draw the resonance forms and hybrid for this carbocation.
C
If the bromide ion simply undergoes an addition reaction as with simple alkenes, a
non-aromatic product would result.
Draw this possible step.