1877

Friedel-Crafts Reaction

Charles Friedel (1832–1899), James Mason Crafts (1839–1917)

Reactions involving aromatic ring compounds (like benzene) have marked a number of historic steps in the field of organic chemistry. One of the most famous, the Friedel-Crafts reaction, provided countless industrial uses—including the catalytic cracking of crude oil—along with insights into some deep principles of organic synthesis, such as the behavior of aromatic ring compounds.

Chemists Charles Friedel, a Frenchman, and James Mason Crafts, an American, were studying the reactions of organochlorine compounds and found that the addition of aluminum metal had a dramatic (and unexpected) effect. Very little happened at first, but when the reactions were heated up, they became so vigorous that they generated large amounts of heat on their own and produced a variety of products. This behavior suggested that some new species was being produced—slowly at first—that accelerated the reaction as time went on. It turned out, as Friedel and Crafts suspected, that aluminum chloride was being produced, and it was this compound that accelerated the reaction because, when it was added at the beginning, the reactions started immediately. In 1877, they published the results of their work, which quickly became known as the Friedel-Crafts reaction.

More study showed that positively charged carbons (carbocations) must be intermediates in the reaction, because the starting materials that formed these most easily were by far the most reactive. These carbocations readily attacked aromatic rings (see Benzene and Aromaticity), especially electron-rich ones, to produce new substituted ring systems. The positions where the new groups attached were mostly predictable, and we now know that the carbocations attack the positions with the most electron density (positive charges going toward negative ones).

A wide range of compound classes generate carbocation species for the reaction, and variations of the Friedel-Crafts reaction quickly became the preferred routes into many aromatic derivatives. The reaction is still widely used, from the research bench all the way up to the industrial scale. Aluminum chloride is still the classic recipe, but many other Lewis acids (see Acids and Bases) are also useful.

Charles Friedel.

SEE ALSO Benzene and Aromaticity (1865), Acids and Bases (1923), Reaction Mechanisms (1937), Nonclassical Ion Controversy (1949)

Aluminum reacts with hydrochloric acid to form aluminum chloride.