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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.