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Friedel Crafts alkylation reaction

When benzene is treated with an alkyl halide in the presence of a Lewis acid such as anhydrous aluminium chloride, alkyl benzene is formed. This reaction is popularly known as Friedel Crafts alkylation reaction.

Friedel Crafts Alkylation and Acylation | Chemistry for JEE Main & Advanced
Friedel Crafts acylation reaction:

When benzene is treated with an acyl halide in the presence of Lewis acids such as anhydrous aluminium chloride, acyl benzene is formed. This reaction is popularly known as Friedel Crafts acylation reaction.

Friedel Crafts Alkylation and Acylation | Chemistry for JEE Main & Advanced
The mechanism for Friedel Crafts alkylation and acylation reactions of benzene:

Friedel Crafts alkylation and acylation reactions involve a three-step mechanism:

  1. Generation of electrophile:

    Due to the presence of a Lewis acid, generation of electrophile takes place. As Lewis acid accepts the electron pair from the attacking reagent.

  2. Formation of arenium ion:

    The electrophiles generated attacks on the benzene ring to form positively charged cyclohexadienyl cation better called arenium ion containing one sp3 hybridized carbon atom. The positive charge is effectively distributed over three carbon atoms due to resonance which makes it partially stable.

    As delocalization of electrons stops at sp3 hybridized carbon atom, the arenium ion is not aromatic in nature.

  3. Removal of positive charge from the carbo-cation intermediate:

    The arenium ion finally loses its proton from sphybridized carbon to a Lewis base restoring the aromaticity.

A general mechanism for Friedel Craft’s acylation is depicted below:

Friedel Crafts Alkylation and Acylation | Chemistry for JEE Main & Advanced

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FAQs on Friedel Crafts Alkylation and Acylation - Chemistry for JEE Main & Advanced

1. What is Friedel Crafts alkylation and how does it work?
Ans. Friedel Crafts alkylation is a chemical reaction that involves the introduction of an alkyl group onto an aromatic compound. It is achieved by using a Lewis acid catalyst, typically aluminum chloride (AlCl3), and an alkyl halide. The Lewis acid catalyst facilitates the formation of a carbocation intermediate, which then reacts with the aromatic compound to form the alkylated product.
2. What is the purpose of using a Lewis acid catalyst in Friedel Crafts alkylation?
Ans. A Lewis acid catalyst, such as aluminum chloride, is used in Friedel Crafts alkylation to facilitate the formation of a carbocation intermediate. The Lewis acid catalyst coordinates with the alkyl halide, making it more electrophilic and enabling it to easily react with the aromatic compound. Additionally, the Lewis acid catalyst helps in stabilizing the negative charge generated during the reaction.
3. What are the limitations of Friedel Crafts alkylation?
Ans. Friedel Crafts alkylation has several limitations. One limitation is the possibility of polyalkylation, where multiple alkyl groups are introduced onto the aromatic compound. This can lead to undesired side reactions and a loss of selectivity. Another limitation is the formation of carbocation rearrangements, which can result in the formation of different alkylated products. Additionally, Friedel Crafts alkylation is not suitable for aromatic compounds that have strong electron-withdrawing groups or that are deactivated due to the presence of other functional groups.
4. How is Friedel Crafts acylation different from Friedel Crafts alkylation?
Ans. Friedel Crafts acylation is a chemical reaction that involves the introduction of an acyl group (R-C=O) onto an aromatic compound. Unlike alkylation, acylation does not require the use of a Lewis acid catalyst. Instead, an acyl chloride or an acid anhydride is directly reacted with the aromatic compound. The reaction proceeds through the formation of an intermediate complex between the Lewis acid catalyst and the acylating agent, which then reacts with the aromatic compound to form the acylated product.
5. What are some common applications of Friedel Crafts alkylation and acylation reactions?
Ans. Friedel Crafts alkylation and acylation reactions are widely used in organic synthesis for the introduction of alkyl and acyl groups onto aromatic compounds. They are particularly useful in the synthesis of pharmaceuticals, agrochemicals, and fragrances. These reactions allow chemists to selectively modify aromatic compounds and introduce specific functional groups, which is crucial for the development of new drugs and other important organic molecules.
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