Organic Chemistry: Friedel-Crafts Alkylation and Acylation Reactions

A simplistic definition of organic chemistry would be it is "the chemistry of carbon."¹ Hundreds of thousands of organic chemical structures come under the purview of organic chemistry, since carbon can link to form to itself to form chains and rings. Entire sections or chunks of molecules can even be added to each other; such is the case with the reaction known as the Friedel-Crafts alkylation reaction and the Friedel-Crafts acylation reaction. We will briefly consider a couple of simple examples of such reactions here.

First: Why are Friedel-Crafts Reactions Important?

If one builds a house, having the right tool is important. To the artist, the correct brush may be essential. It is the same to the organic chemist. A large assortment of reaction types that produce precisely the correct result is essential. Friedel-Crafts reactions are a most powerful in the hands of the chemist or technician desirous of achieving a specific result-a unique chemical.

Friedel-Crafts Alkylations

An alkyl group² may be attached to a ring structure such as a benzene ring. The reaction is called a substitution reaction, because a bonded hydrogen attached to the benzene ring is replaced by an alkyl group, -R. The reactant is an alkyl halide, often a chloride, R-Cl. A catalyst is generally required, often aluminum chloride, AlCl₃.

A Simple Alkylation Example

If benzene is reacted with an ethyl chloride (an alkane, ethane-C₂H₆-with a bonded hydrogen replaced by a chlorine atom) in the presence of aluminum chloride, the reaction is,

C₆H₆ + C₂H₅Cl + catalyst --> C₆H₅-C₂H₅ + HCl

This reaction sequence says that one molecule of benzene plus one molecule of ethyl chloride (in the presence of some aluminum chloride catalyst) produces one molecule of ethyl benzene plus one molecule of hydrogen chloride gas. Ethylbenzene can also be written in its simplest elemental form, C₈H₁₀.

The correct placement of alkyl chains on aromatic rings is very important to the organic chemist. In this endeavor, the Friedel-Crafts alkylation reaction is an important tool.

Friedel-Crafts Acylations

The term "acyl" refers to part of an organic acid group, symbolized in general R-COOH, in which R is an alkyl group, and -COOH is a carboxyl group, which makes it an acid. Thus R-COOH is called a carboxylic acid. In general, the portion R-CO- can be attached to the species to be acylated. Generally, another form, such as an acyl chloride is used, rather than the acid itself.

A Simple Acylation Example

Acetic acid or CH₃COOH (vinegar is 5 percent acetic acid in water) in the form of its chloride, is acetyl chloride, with the chemical structure CH₃-COCl. Acylating benzene with this substance in the presence of aluminum chloride catalyst gives the reaction,

C₆H₆ + CH₃-COCl + catalyst --> C₆H₅-CO-CH₃ + HCl

The above reaction sequence says that one molecule of benzene plus one molecule of acetyl chloride (in the presence of some aluminum chloride catalyst) produces one molecule of methylphenyl ketone³ plus one molecule of hydrogen chloride gas.

Ketones are very important compounds that offer a variety of reaction paths for additional synthesis. This makes the Friedel-Crafts acylation reaction one of great significance.

¹ Princeton's WebNet Search defines organic chemistry as "the chemistry of compounds containing carbon (originally defined as the chemistry of substances produced by living organisms but now extended to substances synthesized artificially)."
² An alkyl group is any non-aromatic hydrocarbon structure derived from an alkane. Examples include the simple methyl (-CH₃) and ethyl (-C₂H₅) groups.
³ A substituted benzene ring is sometimes called a phenyl group, while the CO group with adjacent carbon atoms is called a ketone.

References and Resources:

Friedel-Crafts Alkylation of Benzene - Organic Chemistry, Ch. 12, Francis A. Carey.

Friedel-Crafts Acylation of Benzene - Organic Chemistry, Ch. 17, Francis A. Carey.

Organic Chemistry Portal - Friedel-Crafts Alkylation

Organic Chemistry Portal - Friedel-Crafts Acylation