CD 12.4 Reactions of Arenes: Friedel-Crafts Reactions

Finally, I've reached the end of the chapter! The final electrophilic substitution reactions to look at are the Friedel-Crafts reactions. These are used to introduce side chains to the benzene ring.

Alkylation is the addition of an alkyl group (think of it as an alkane) and Acylation is the addition of an acyl group (R-CO-) The video above explains alkylation only, but don't start worrying! Acylation isn't that different.



The example we're given in the book for alkylation is with chloromethane, but the reaction can be done with any halogen containing organic molecule.

The chloromethane and benzene are warmed with an aluminium chloride catalyst:

As with the other reactions, when the chloromethane approaches the benzene, it becomes polarised. The aluminium chloride polarises it further:

It becomes polarised so much that the molecule splits up:

The chlorine molecule binds to the catalyst, aluminium chloride, leaving a carbocation, CH3+. The carbocation is an electrophile (loves those little negative electrons!) It attacks the benzene as benzene is electron rich. In doing so, the benzene releases a hydrogen ion to remain stable and form methylbenzene.

The hydrogen ion, as seen in the other reactions, forms HCl by reacting with the AlCl4-, this regenerates the aluminium chloride catalyst.



Acylation is the same process, but instead of using chloromethane, an acyl chloride is used such as ethanoyl chloride.

Ethanoic anhydride can also be used:

Ionic liquids can be used as solvents/catalysts. They are liquids at room temperature and contain only ions. They are seen s being more 'green' because of the following reasons:

1. They have low volatility (doesn't evaporate easily at room temperature) so reduce emissions.
2. Low flammability and toxicity so are safer to use.
3. Reactions can be carries out at lower temperatures with the ionic liquid than by conventional methods.
4. The ionic liquid can be recycled.

Friedel-Crafts come up quite a lot in the exams so make sure you've got them up to scratch. Howdy doo dah, I'm glad this chapter is over, my paint skills were really being stretched.

Answers to the exam questions are here.