Organic Chemistry: Friedel-Crafts Alkylation and Acylation Reactions

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Friedel Crafts Alkylation Reaction, image by Benjah-bmm27

Two of the most powerful tools of the organic chemist are the Friedel-Crafts alkylation reaction and the Friedel-Crafts acylation reaction.

Here, Decoded Science presents two sample syntheses for clarity.

What is Organic Chemistry

A simplistic definition of organic chemistry is “the chemistry of carbon.”

There are hundreds of thousands of organic chemical structures, since carbon can link to form to itself to form chains and rings. Entire sections or chunks of molecules can 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.

Why Friedel-Crafts Reactions are Important

If you’re going to build a house, the proper tools are important. To the artist, the correct brush is essential. To the poet, it’s just the right word. To the chemist, the process is similar.

Friedel-Crafts reactions are a powerful tool in the hands of the chemist; the correct chemical reaction type produces the correct result.

Friedel-Crafts Alkylations

An alkyl group can be attached to a ring structure such as a benzene ring.

This reaction is called a substitution reaction, because a hydrogen atom attached to the benzene ring is replaced by an alkyl group, symbolized by -R.

The reactant, an alkyl halide – often a chloride – is written R-X (or for the chloride) R-Cl.

This reaction generally requires a catalyst, often aluminum chloride, AlCl3.

A Simple Alkylation Example

If benzene is reacted with an ethyl chloride (an alkane, ethane – C2H– 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.

You can also write ethylbenzene 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 ketone plus one molecule of hydrogen chloride gas. Ketones are very important compounds that offer a variety of reaction paths for additional synthesis.

Friedel-Crafts: Significant Reactions

Every craft has its tools – and organic chemistry is no exception. The Friedel-Crafts acylation and Alkylation reactions provide insight and information, and as such are extremely useful for chemists.

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