72 Monosubstituted Benzenes
Benzene is a very important basic structure which is useful for analysis and synthesis in
most aspects of organic chemistry. The benzene ring itself is not the most interesting or
useful feature of the molecule; which substitutents and where they are placed on the ring
can be considered the most critical aspect of benzene chemistry in general.
72.1 Effects of Different Substituents
Depending on the type of substituent, atoms or groups of atoms may serve to make the
benzene ring either more reactive or less reactive. If the atom or group makes the ring more
reactive, it is calledactivating, and if less, then it is calleddeactivating.
Generally, the terms activating and deactivating are in terms of the reactions that fall into
the category of Electrophilic Aromatic Substitution (EAS). These are the most common
forms of reactions with aromatic rings. Aromatic rings can undergo other types of reac-
tions, however, and in the case of Nucleophilic Aromatic Substitution, the activating and
deactivating nature of substituents on the rings is reversed. In EAS, a hydroxyl groups is
strongly activating, but in Nucleophilic Aromatic Substitution, a hydroxyl group is strongly
deactivating. But since EAS is the most common reaction with aromatic rings, when dis-
cussing activation and deactivation, it’s normally done in terms of the EAS.
In addition to activating or deactivating, all groups and/or substituent atoms on a benzene
ring aredirecting. An atom or group may encourage additional atoms or groups to add
or not to add to certain other carbons in relation to the carbon connected to the directing
group. This concept will be further discussed in the next chapter, but when memorizing
the groups below it is helpful to also memorize whether it is O (ortho), M (meta) or P
(para)-directing.
Another factor that heavily influences direction, however, is steric hindrance. If, for ex-
ample, you have a tert-butyl substituent on the ring, despite the fact that it is ortho/para
directing, the ortho positions will be largely blocked by the tert-butyl group and thus nearly
all the product would be para.
72.2 Activating Substituents
Activating substituents make benzene either slightly more reactive or very much more reac-
tive, dependingonthegrouporatominquestion. Ingeneral, ifoneofthemajorheteroatoms
(nitrogen or oxygen) isdirectlyattached to the carbon ring then the result is probably acti-
vation. This is merely a rule of thumb, and many exceptions exist, so it is best to memorize
the groups listed below instead of counting on a quick and dirty rule of thumb.