Franco-American cooperation was in effect with the alkylation reaction as Professor Friedel, a French chemist, collaborated with Professor Crafts, an American chemist, to develop the Friedel Crafts Alkylation. This is an electrophilic aromatic substitution whereby a carbocation is generated as the electrophile. There are several ways this can be done as shown in Fig 1 below.

Once the carbocation has been generated then the reaction proceeds to the alkylated Benzene Ring (Fig 2)

The reaction mechanism for this alkylation is shown in Fig 3 below.

There are a number of problems dealing with the alkylation reaction that lead to the development of a similar reaction, Friedel-Crafts Aycylation. The limitations to this alkylation reaction include:

1. Polysubstitution- Since the alkyl group that has been placed on the Benzene ring activates the ring toward further substitution, and each subsequent alkylation increases this activation of the ring, this leads to alkylation of the ring in several positions. This is called "polysubstitution" and leads to extremely low yields of the monosubstituted product.
2. Possible rearrangement of the initial carbocation- Carbocations will undergo molecular rearrangement where a Hydrogen will move over to an adjacent carbon with the bonding electrons (hydride shift) or a methyl group will move over to the adjacent carbon with the bonding electrons(methide shift). This molecular rearrangement within a carbocation will only occur if it results in a more stable carbocation. We know that the relative stability of carbocations runs:

   C₆H₅-CH₂⁺ = CH₃-CH=CH₂⁺ > tertiary > secondary > primary > CH₃⁺

   So if a hydride or methide shift results in a primary carbocation becoming a tertiary or secondary carbocation then the molecular rearrangement will occur at least partially resulting in possible multiple products for alkylation. Molecular rearrangement will not occur if it would result in a tertiary carbocation becoming a secondary or primary carbocation.

3. Benzene derivatives where the substituents deactivate the ring give very poor yields in all Friedel-Crafts reactions
4. Halobenzenes or vinylic halides cannot be used as the alkylating agent because they do not form carbocations readily. This is true for both Friedel-Crafts reactions.

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R. H. Logan, Instructor of Chemistry, Dallas County Community College District, North Lake College.

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All textual content copyrighted (c) 1997
R.H. Logan, Instructor of Chemistry, DCCCD
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Revised: 5/20/97

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