ALKYLATION OF BENZENE

Benzene can be alkylated in the presence of a Lewis or a Bronsted acid catalyst. Olefins such as ethylene, propylene, and Cl₂–Cl₄ alpha olefins are used to produce benzene alkylates, which have great commercial value. Alkyl halides such as monochloroparaffins in the Cl₂–Cl₄ range also serve this purpose.

The first step in alkylation is the generation of a carbocation (carbonium ion). When an olefin is the alkylating agent, a carbocation intermediate forms.

Carboncations also form from an alkyl halide when a Lewis acid catalyst is used. Aluminum chloride is the commonly used Friedel-Crafts alkylation catalyst. Friedel-Crafts alkylation reactions have been reviewed by Roberts and Khalaf:

The next step is an attack by the carbocation on the benzene ring, followed by the elimination of a proton and the formation of a benzene alkylate:

Figure 1.1. Important chemicals based on benzene.