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Friedel Crafts Alkylation-reactivity, Limitations, Friedel Crafts Acylation

Friedel-Crafts alkylation involves changing an aromatic proton with an alkyl group.

Freidel Crafts Alkylation

Friedel-Crafts alkylation involves changing an aromatic proton with an alkyl group. Through the use of carbohydrate, an electrophilic attack is achieved on the aromatic rings. Alkyl halides are used as reactants in this Friedel-Crafts alkylation reaction to produce alkylbenzenes.

Benzene goes through the Friedel-Crafts alkylation reaction as shown below.



FeCl3 and AlCl3 are Lewis acid catalysts, which form a carbocation in the presence of halides. An alkylation reaction requires the rearrangement of the resulting carbocation before continuing.

In Friedel-Crafts alkylation, three steps are required.

Step 1

Electrophilic carbocations are formed when Lewis acid catalyst (AlCl3) reacts with alkyl halide.

Step 2

An intermediate formed by the carbocation is the cyclohexadienyl cation. As a result of the breakage of the carbon-carbon double bond, the aromatic property of the arene temporarily disappears.

Step 3

Through the deprotonation of the intermediate, the double bond between carbon and carbon is reformatted, restoring the compound's aromaticity. Hydrochloric acid is formed from this proton, which regenerates the catalyst.



Limitations

Listed below are some of the main drawbacks of Friedel-Crafts alkylation.
  • In this reaction, aryl and vinyl carbocations are not applicable since they are extremely unstable.
  • If an aromatic ring is covered with a deactivating group (such as an NH2), the catalyst can be rendered inactive because complexes are formed.
  • Polyalkylation (the addition of multiple alkyl groups to an aromatic compound) must be prevented by using an excess of the aromatic compound in these reactions.
  • An aromatic compound having a lower reactivity than mono-halobenzenes is not included in Friedel-Crafts alkylation reactions.
Reaction involving carbocations is likely to result in a rearrangement of the carbocation.



Freidel Crafts Acylation

By adding an acyl group to a ring of an aromatic compound, Friedel-Crafts acylation occurs. It is accomplished by combining a Lewis acid catalyst (AlCl3) with R-(C=O)-Cl. Friedrich-Crafts acylation leads to the ketone being formed from the aromatic ring. Here is an illustration of how benzene reacts with an acyl chloride under these conditions.



A Friedel-Crafts acylation takes place in four steps.

Step 1

An acidic Lewis catalyst (AlCl3) reacts with the halide acyl. An acyl halide loses an ion, and the acylium ion becomes resonance stabilized.



Step 2

An electrophilic attack is then performed on the aromatic ring by the acylium ion (RCO+). A complex is formed that temporarily reduces the aromaticity of the ring.


Step 3

By deprotonating the ring, the intermediate complex regains its aromaticity. During this reaction, the proton forms a complex with a chloride ion (from the Lewis acid), resulting in the generation of HCl. HCl is now generated from the AlCl3 catalyst.



Step 4

Catalysts that have been regenerated now attack carbonyl oxygen. In step 3, the products formed from step 2 must be diluted with water to release their ketone product. A simplified illustration of this step is shown below.



As a result, the Friedel-Crafts acylation is used to obtain the desired acyl benzene product.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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