So far, we have added heteroatoms only—bromine, nitrogen, or sulfur. Adding a carbon sub stituent to a reluctant aromatic nucleophile requires reactive carbon electrophiles and that means carbocations, however weak, will react with a carbocation in the SN1 reaction: benzene rings are no exception. The classic SN1 electrophile is the t-butyl cation, which is generated from tert-butanol with acid.

This is, in fact, an unusual way to carry out such reactions. The Friedel–Crafts alkylation, as this is known, usually involves treating benzene with a tertiary alkyl chloride and the Lewis acid AlCl3. Rather in the manner of the reaction with bromine, AlCl3 removes the chlorine atom from t-BuCl and releases the t-Bu cation for the alkylation reaction.

We have not usually bothered with the base that removes the proton from the intermediate. Here it is chloride ion as the by-product is HCl, so you can see that even a very weak base will do. Anything, such as water, chloride, or other counterions of strong acids, will do this job well enough and you need not in general be concerned with the exact agent. A more important variation of this reaction is the Friedel–Crafts acylation with acid chlorides and AlCl₃. Aluminium chloride behaves with acyl chlorides much as it does with alkyl chlorides—it removes chloride to leave behind a cation. In this case the cation is a linear acylium ion, with the carbocation stabilized by the adjacent oxygen lone pair. When the acylium ion attacks the benzene ring it gives an aromatic ketone: the benzene ring has been acylated.

The acylation is better than the alkylation because it does not require any particular structural feature in the acyl chloride—R can be almost anything. In the alkylation step it is essential that the alkyl group can form a cation, otherwise the reaction does not work very well. In addition, for reasons we are about to explore, the acylation stops cleanly after one reaction whereas the alkylation often gives mixtures of products.

● Friedel–Crafts reactions Friedel–Crafts alkylation with t-alkyl chlorides and Lewis acids (usually AlCl₃) gives t-alkyl benzenes. The more reliable Friedel–Crafts acylation with acid chlorides and Lewis acids (usually AlCl₃) gives aryl ketones.

مواضيع ذات صلة

Reaction conditions

Conjugated alkenes can be electrophilic

Alkenes conjugated with carbonyl groups

Two reasons to avoid a Friedel–Crafts alkylation

Two or more substituents may cooperate or compete

Phenols react rapidly with bromine

Sulfonation of benzene

Nitration of benzene

Benzene and its reactions with electrophiles

enols and phenols

Reactions of silyl enol ethers with halogen and sulfur electrophiles

Reactions of enol ethers Hydrolysis of enol ethers