Benzene, unlike alkenes, will not react with halogens to form addition products.
This exemplifies the extra stability of the double bonds present in the benzene ring. However, benzene can undergo substitution reactions with halogens in the presence of a Lewis acid catalyst. The Lewis acid enhances the electrophilic nature of the halogen, thus enabling the reaction to proceed.
Electrophilic Substitution Reactions
One of the most characteristic reactions of benzene is the electrophilic substitution reaction, in which a hydrogen is replaced by an electrophile. In such reactions, the benzene acts as the nucleophile.
In a typical aromatic substitution reaction, the electrophile (means electron loving) accepts the electron pair from the pi system of benzene, resulting in a carbocation. This cation of benzene is called the cyclohexadienyl cation. Then, the cyclohexadienyl cation loses a proton forming the substitution product.
Benzene can be nitrated by reacting with nitric acid (HNO3). This is usually done in the presence of sulfuric acid (H2SO4).
In Friedel-Crafts reactions, benzene is reacted with acyl or alkyl chlorides, in the presence of metal halides as catalysts. The metal halides act as Lewis acids in these reactions. The two types of Friedel-Crafts reactions are alkylation and acylation.
- Alkylation In Friedel-Crafts alkylation, benzene is reacted with alkyl chlorides in the presence of metal halides (AlCl3, AlBr3) as catalysts. The catalyst serves as Lewis acid and increases the electrophilicity of the alkyl halide. Alkylation is important for the synthesis of alkyl substituted derivatives of benzene. But there is a possibility of rearrangement of the intermediates resulting in undesired products. For example, if primary halides are used in alkylation, they can rearrange to form secondary or tertiary carbocations which are more stable intermediates. This can result in multiple products.
The Mechanism of Alkylation Rearrangement
- Acylation In Friedel-Crafts acylation reactions, benzene is reacted with acyl chlorides or acid anhydrides in the presence of metal halides (Lewis acids). The importance of acylation is that there is no rearrangement, unlike alkylation where there is a possibility of rearrangement of the cation intermediates.
Acyl products that are formed can be reduced by reactions such as Clemmenson or Wolf-Kishner reductions.
Benzene can be halogenated in the presence of Fe. In this reaction, the iron(III) bromide acts as the catalyst, which is formed from the iron and the bromine. The bromine-iron(III) bromide complex that is formed acts as the electrophile which attacks the benzene.
Benzene can be sulfonated by heating with sulfuric acid.