Lewis Acid in the Friedel-Crafts Reaction

The Friedel crafts reaction is a very important method for introducing alkyl substituents on an aromatic ring. It involves generation of a carbocation or related electrophilic species. Reaction between an alkyl halide and a Lewis acid is the most prevalent technique of producing these electrophiles.

The Friedel crafts reaction is one that is catalyzed by a Lewis acid. The Lewis acid is a stoichiometric reagent rather than a catalyst since it cannot be recovered or reused at the end of the reaction and indeed in many instances an excess of the Lewis acid must be used.

Commonly used Lewis Acid for Friedel Crafts Reaction

  • Acidic metal halides of which AlCl3 and AlBr3 are the most frequently used, Comprise a large number of Lewis acid catalysts for Friedel Crafts Reactions.
  • Other frequently used active metal halide catalysis include: BeCl2, CdCl2, ZnCl2, ZrCl4, SnCl4, BiCl3 and FeCl3
  • Such Lewis acids have a key atom with an electron deficiency that can accept electrons from fundamental substances.
  • AlCl3 and BF3 are very important Freidel crafts catalyst and their applications cover a wide area of Freidel crafts type reactions including alkylation’s, different types of cyclization’s, isomerization’s, polymerizations and many other reactions.

A number of other acidic metal halides could also be employed however these were less reactive. The strength of coordinating power of different Lewis acids can vary widely against different Lewis bases. Hence it is extremely difficult to establish a scale of strength of Lewis acids in a manner analogous to that used for Bronstead acids.

Drawbacks of Friedel Crafts Reaction

Freidel crafts reaction suffer from a several drawbacks. These include limited substrate scope, being restricted to electron rich arenes, harsh reaction conditions and the need for stoichiometric amounts of Lewis acid. The last of these arises from the strong complication of the aryl ketone product to the Lewis acid. In addition, the complex is destroyed after hydrolytic work and the Lewis acid is lost, resulting in large volumes of corrosive waste.

As a result of the drawbacks associated with Freidel crafts reactions, great efforts have been made to find Lewis acids which allow electron deficient arenes to undergo acylation improve the eco-compatibility of these reactions, make them catalytic in Lewis acid and reduce or eliminate the generation of corrosive waste products.

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