When a benzene ring is a substituent, it is called a phenyl group A benzene ring with a methylene group is called a benzyl group

With the exception of toluene, benzene rings with an alkyl substituent are named as alkyl-substituted benzenes or as phenyl-substituted alkanes Aryl group (Ar) is the general term for either a phenyl group or a substituted phenyl group

Aromatic compounds such as benzene undergo electrophilic aromatic substitution reactions

Recall in Section 3.6…

Benzene is a nucleophile that reacts with an electrophile An electrophilic substitution yields an aromatic product which is significantly more stable than the addition reaction

Reaction Coordinate Diagrams for the Two Benzene Reactions

There are five common electrophilic aromatic substitution reactions Halogenation Nitration Sulfonation Friedel-Crafts acylation Friedel-Crafts alkylation

General Mechanism for Electrophilic Aromatic Substitution of Benzene

Halogenation of Benzene

Lewis acid weakens the Br-Br (or Cl-Cl) bond, which makes the halogen a better electrophile

Mechanism for bromination The catalyst is regenerated

Nitration of Benzene

Sulfonation of Benzene

Sulfonic acid is a strong acid

Sulfonation is reversible