73 Polysubstituted Benzenes
Unsubstituted benzene is seldom encountered in nature or in the laboratory, and you will
find in your studies that most often benzene rings are found as parts of other, more com-
plicated molecules. In order for benzene to react in most situations, it gains or loses some
functionality dependent on which functional groups are attached. Although the simplest
case is to work with benzene that has only one functional group, it is also essential to un-
derstand the interactions and competitions between multiple functional groups attached to
the same benzene ring.
When there is more than one substituent present on a benzene ring the spatial relationship
betwen groups becomes important, which is why the arene substitution patternsortho
,metaandparawere devised. For example three isomers exist for the molecule cresol
because the methyl group and the hydroxyl group can be placed either next to each other
(ortho), one position removed from each other (meta) or two positions removed from each
other (para). Where each group attaches is most often a function of which order they
were attached in, due to the activating/deactivating and directing activities of previously
attached groups.
Figure 169
73.1 Competition Between Functional Groups
When a ring has more than one functional group, the effects of the groups are combined and
their total effect must be taken into account. In general, effects are summed. For example,
toluene (methylbenzene) is weakly activated. But p-nitrotoluene has both a methyl group
and a nitro group. The methyl group is weakly activating and the nitro is pretty strongly
deactivating, so overall, the group is very deactivated. In terms of direction, however, both
substitutents agree on the direction. The methyl group is ortho/para directing. The nitro
group occupies the para position, so the methyl will now want just ortho direction. The
nitro group is meta directing. The positions meta to the nitro are also ortho to the methyl,
so this works out and further substituents will be almost entirely in the positions ortho to
the methyl group.
If two functional groups disagree on direction, the more activating group is the one that
controls direction. That is, if you had m-nitrotoluene, most of your product would tend to
be ortho/para to the toluene and not meta to the nitro, despite the nitro having a stronger
influence on overall activation.