Friedel-Crafts Reactions
Compounds containing aromatic groups are widespread in nature, and for this reason chemists who aim to synthesize naturally occurring compounds in the laboratory often need to introduce substituents to aromatic rings. In the organic synthesis laboratory, electrophilic aromatic substitutions which result in the formation of new carbon-carbon bonds are called ‘Friedel-Crafts’ alkylations and acylations, named for Charles Friedel of France and James Crafts of the United States, who together developed the procedures in 1877. The Friedel-Crafts reactions are an important part of a synthetic chemist's toolbox to this day.
Friedel Crafts reactions, like their biochemical counterparts, require reactive electrophiles with significant carbocation character. One of the most common ways to alkylate an aromatic ring is to use an alkyl chloride electrophile that is activated by the addition of aluminum or iron trichloride. The metal chloride serves as a Lewis acid, accepting electron density from the alkyl chloride. This has the effect of magnifying the carbon-chlorine dipole, making the carbon end more electropositive - and thus more electrophilic – even to the point where the bond breaks and an ion-pair is formed. 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
Friedel Crafts reactions, like their biochemical counterparts, require reactive electrophiles with significant carbocation character. One of the most common ways to alkylate an aromatic ring is to use an alkyl chloride electrophile that is activated by the addition of aluminum or iron trichloride. The metal chloride serves as a Lewis acid, accepting electron density from the alkyl chloride. This has the effect of magnifying the carbon-chlorine dipole, making the carbon end more electropositive - and thus more electrophilic – even to the point where the bond breaks and an ion-pair is formed. 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