Aromatic compounds have unique reactions that can add substituent groups to their structures. The most common is the electrophilic aromatic substitution where a variety of functional groups are added to a benzene ring through an electrophilic attack. These compounds form an intermediate ion called the arenium ion, which is sometimes called the sigma complex. Substituent groups already on the benzene will direct where the electrophilic attack occurs. These groups are called directing groups, and if the group is an electron withdrawing group (EWG), the attack occurs at the meta positions. If the group is an electron donating group (EDG), the attack occurs at the ortho or para position. These directing groups are also classified as activating groups or deactivating groups.
The carbon that is adjacent to the benzene ring is called the benzylic carbon and is reactive because of the stability of the arenium ion/sigma complex. Another reaction type that can occur is the reduction of benzene, which can occur with the help of a catalyst in catalyzed hydrogenation or through Birch reduction. In addition to electrophilic aromatic substitution reactions, groups can also be added to aromatic rings using nucleophilic reactions, SNAr and elimination-addition.
At A Glance
- Electrophilic aromatic substitutions are the most common type of reaction for benzene and can be used to add a variety of functional groups to a benzene ring.
- Substituent groups direct where the electrophilic attacks will occur; electron donating groups (EDG) direct the attack to the ortho or para position, and electron withdrawing groups (EWG) direct the attack to the meta position. EDG are activating groups, and EWG are deactivating groups.
- The benzylic carbon is very reactive because of resonance stability of the benzylic position. Radical halogenation will preferentially occur on a benzylic carbon over other carbon atoms.
- Reduction reactions of benzene include catalytic hydrogenation, which forms cyclohexane, and Birch reduction, which forms 1,4-cyclohexadiene.
- Nucleophilic substitution reactions can substitute a hydrogen with a functional group by using nucleophiles instead of electrophiles. These reactions are classified as nucleophilic substitution reactions (SNAr) and elimination-addition reactions.