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Friedel-Crafts Reactions: Alkylation & Acylation

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Benzene is extremely stable, as such it is hard to force a reaction to occur involving benzene. Friedel-Crafts reaction is a way to make a new carbon-carbon bond on a benzene ring. This can occur through alkylation or acylation.

Reacting with Benzene

Benzene is extremely stable, and it does not want to break the aromaticity of the ring. So how does a compound such as benzene ever react? There are many different substituted benzene rings, how did these rings form? The mechanism by which a new R group is added to an aromatic ring was discovered by Charles Friedel and James Crafts, and named after them. These Friedel-Crafts reactions use an alkyl halide or another carbocation to add the R group onto the benzene ring.

The general Friedel Crafts reaction involves a new carbon to carbon bond forming
General Alkylation

There are two types of Friedel-Crafts reactions, alkylation and acylation. Alkylation reaction add a simple carbon chain to the benzene ring. Acylation adds an acyl group, creating a ketone or aldehyde.

Friedel Crafts reactions can also have a new ketone through acylation
General Acylation

General Reaction

The Friedel-Crafts reaction starts with a benzene and an alkyl chloride. The alkyl chloride reacts with AlCl3 (aluminum chloride) or FeCl3 (Ferric chloride), which removes the chorine from the alkyl chloride, making a positive charge on the carbon. This positive charge now needs electrons. So even though the benzene doesn't like to break the aromaticity the positive charge on the carbon allows it to break the aromaticity in the benzene ring.

The aromaticity isn't broken for long, because the aluminum chloride (or ferric chloride) can take the hydrogen off the benzene ring making it aromatic again.

Friedel
Mechanism

When using Ferric chloride, the reaction will often keep adding more R groups to the benzene ring until the entire ring is filled up with R groups, so you need to be careful using Ferric chloride as the catalyst.

Alkylation Examples

In general, the alkylation reactions occur by first making a carbocation. We can either use aluminum chloride or ferric chloride to remove the chlorine from the alkyl chloride. Now the R group has a positive charge on it. The electron rich benzene can give some electrons to the electron deficient R group, forming a new bond. But this then creates a positive charge on the benzene ring. The ferric chloride (or aluminum chloride) has an extra chlorine attached, so this chlorine can take the hydrogen, forming hydrochloric acid. Left behind we have the benzene ring with a new R group attached and the catalyst has been re-formed.

When a secondary or tertiary carbon reacts a full carbocation forms
Secondary R group reaction

This is the way the reaction occurs when the alkyl chloride is either a secondary or a tertiary, because the carbon can hold a positive charge.

When a primary carbon reactions a full carbocation never forms
Primary carbon reaction

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