14: Substituent Effects

(9/94)(11,12/96)(11,12/04,01/05) Neuman Chapter 14
Directive Effects of Substituents (14.4B)
When a benzene ring has a single substituent S, the electrophile E+ can react with any of the
remaining five carbon atoms (Figure 14.31).
Figure 14.31
However we have just seen that the 3 possible products do not form in equal amounts. A
comparison of the sets of resonance structures for the carbocation intermediates provides the
basis for explaining the differences in product distribution for different substituents.
Resonance Structures for o, m, and p Reactions. We show in Figure 14.32 the sets of
resonance structures for the different carbocations formed by reaction of E + at an ortho, or
meta, or para position.
Figure 14.32
We only show resonance structures for ortho attack at one of the two ortho positions
because those from attack at the other ortho position are completely equivalent. The same is
true for for reaction at the two meta positions.
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