Electrophilic Substitution and the Friedel-Crafts Acylation of Benzene

This page gives you the facts and a simple, uncluttered mechanism for the electrophilic substitution reaction between benzene and ethanoyl chloride in the presence of an aluminium chloride catalyst. If you want the Friedel-Crafts acylation mechanism explained to you in detail, there is a link at the bottom of the page.

The Electrophilic Substitution Reaction Between Benzene and Ethanoyl chloride

What is Acylation?

An acyl group is an alkyl group attached to a carbon-oxygen double bond. If "R" represents any alkyl group, then an acyl group has the formula RCO-. Acylation means substituting an acyl group into something – in this case, into a benzene ring.

The most commonly used acyl group is CH₃CO-. This is called the ethanoyl group. In the example which follows we are substituting a CH₃CO- group into the ring, but you could equally well use any other alkyl group instead of the CH₃.

The Facts

The most reactive substance containing an acyl group is an acyl chloride (also known as an acid chloride). These have the general formula RCOCl.

Benzene is treated with a mixture of ethanoyl chloride, CH₃COCl, and aluminium chloride as the catalyst. A ketone called phenylethanone is formed.

Note: Ketones: A family of compounds containing a carbon-oxygen double bond with a hydrocarbon group either side of it. In this case there is a methyl group on one side and a benzene ring on the other.

Don't worry too much about the name "phenylethanone" – all that matters is that you can draw the structure.

\text{C}_6\text{H}_6 + \text{CH}_3\text{COCl} \longrightarrow \text{C}_6\text{H}_5\text{COCH}_3 + \text{HCl}

or better:

The aluminium chloride isn't written into these equations because it is acting as a catalyst. If you wanted to include it, you could write AlCl₃ over the top of the arrow.

The Formation of the Electrophile

The electrophile is CH₃CO⁺. It is formed by reaction between the ethanoyl chloride and the aluminium chloride catalyst.

\text{CH}_3\text{COCl} + \text{AlCl}_3 \longrightarrow \text{CH}_3\text{CO}^+ + {\text{AlCl}_4}^-

The Electrophilic Substitution Mechanism

Stage one

Stage two

The hydrogen is removed by the AlCl₄^- ion which was formed at the same time as the CH₃CO⁺ electrophile. The aluminium chloride catalyst is re-generated in this second stage.

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