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E. An Industrial Alkylation of Benzene

This page gives you the facts and simple, uncluttered mechanisms for the electrophilic substitution reaction between benzene and alkenes in the presence of a mixture of aluminium chloride and hydrogen chloride as the catalyst.

 

The electrophilic substitution reaction between benzene and ethene

 

The facts

Industrially, alkyl groups can be substituted into a benzene ring using a variant on Friedel-Crafts alkylation. One possibility is that instead of using a chloroalkane with an aluminium chloride catalyst, they use an alkene and a mixture of aluminium chloride and hydrogen chloride as the catalyst.

This is a cheaper method because it saves having to make the chloroalkane first.

 

To put an ethyl group on the ring (to make ethylbenzene), benzene is treated with a mixture of ethene, HCl and aluminium chloride.

 

 

or better:

 

 

The aluminium chloride and HCl aren't written into these equations because they are acting as catalysts. If you wanted to include them, you could write AlCl3 and HCl over the top of the arrow.

 

The formation of the electrophile

The electrophile is CH3CH2+. It is formed by reaction between the ethene and the HCl - exactly as if you were beginning to add the HCl to the ethene.

 

 

The chloride ion is immediately picked up by the aluminium chloride to form an AlCl4- ion. That prevents the chloride ion from reacting with the CH3CH2+ ion to form chloroethane.

 

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 CH3CH2+ electrophile. The aluminium chloride and hydrogen chloride catalysts are re-generated in this second stage.

 

The electrophilic substitution reaction between benzene and propene

 

The facts

The problem with more complicated alkenes like propene is that you have to be careful about the structure of the product. In each case, you can only really be sure of that structure if you work through the mechanism first.

For example, the propyl group becomes attached to the ring via its middle carbon atom - and not its end one.

 

 

You still need a mixture of aluminium chloride and hydrogen chloride as catalysts.

 

The formation of the electrophile

When the propene reacts with the HCl, the hydrogen becomes attached to the end carbon atom. A secondary carbocation (carbonium ion) is formed because it is more stable than the primary one which would have been formed if the addition was the other way round.

 

 

Because the positive charge is on the centre carbon atom, that is the one which will become attached to the ring.

 

The electrophilic substitution mechanism

 

Stage one

 

 

Stage two

 

 

Again, the hydrogen is removed by the AlCl4- ion. The aluminium chloride and hydrogen chloride catalysts are re-generated in this second stage.