Friedel-Crafts Alkylation Reaction

The reaction described in the question is known as the Friedel-Crafts alkylation reaction. This reaction allows the introduction of an alkyl group onto a benzene ring, resulting in the formation of an alkylbenzene. In this case, benzene is treated with an alkyl halide (such as an alkyl chloride or alkyl bromide) in the presence of anhydrous aluminum chloride (AlCl3) as a catalyst.

1. Friedel-Crafts Alkylation Reaction Introduction
The Friedel-Crafts alkylation reaction is a type of electrophilic aromatic substitution (EAS) reaction. It involves the substitution of a hydrogen atom on an aromatic ring with an alkyl group, resulting in the formation of a new carbon-carbon bond. This reaction is widely used in organic synthesis to introduce alkyl groups onto aromatic compounds.

2. Mechanism of the Friedel-Crafts Alkylation Reaction
The reaction mechanism begins with the formation of a complex between the alkyl halide and the Lewis acid catalyst, aluminum chloride. The aluminum chloride acts as a Lewis acid by accepting a pair of electrons from the alkyl halide, forming a more electrophilic alkyl cation and a chloride ion.

Next, the electrophilic alkyl cation attacks the benzene ring, resulting in the formation of a sigma complex. This intermediate is stabilized by resonance, as the positive charge is delocalized over the aromatic ring.

The sigma complex then undergoes deprotonation by a Lewis base, typically a chloride ion from the aluminum chloride catalyst. This step regenerates the aromaticity of the benzene ring and forms the final alkylbenzene product.

3. Role of Anhydrous Aluminum Chloride
Anhydrous aluminum chloride (AlCl3) acts as a Lewis acid catalyst in the Friedel-Crafts alkylation reaction. It facilitates the formation of the electrophilic alkyl cation by accepting a pair of electrons from the alkyl halide. The aluminum chloride also helps in the deprotonation step by providing a chloride ion to regenerate the aromaticity of the benzene ring.

4. Comparison with Other Reactions
- Halogenation: Halogenation involves the substitution of a hydrogen atom on an aromatic ring with a halogen atom, such as chlorine or bromine. This reaction is not applicable in the given scenario, as the benzene is not being treated with a halogen.
- Friedel-Crafts Acylation Reaction: The Friedel-Crafts acylation reaction is another type of electrophilic aromatic substitution reaction. It involves the introduction of an acyl group (R-C=O) onto an aromatic ring. In this reaction, an acyl chloride or an acid anhydride is used along with a Lewis acid catalyst. This reaction is not applicable in the given scenario, as the benzene is being treated with an alkyl halide, not an acyl halide or an acid anhydride.
- Sulphonation: Sulphonation involves the introduction of a sulfonic acid group (-SO3H) onto an aromatic ring. This reaction is not applicable in the given scenario, as the benzene is not being treated with a sulfonic acid or a sulfonic acid derivative.

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