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Friedel-Crafts alkylation and the ring's interaction with substituents

  • 3 things to know
  • Component 2

Overview

How aluminium chloride is regenerated in a Friedel-Crafts alkylation and general principle about arenes that the behaviour of chlorobenzene illustrates.

What you need to know

Answer each one before you reveal it.

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Key term

Why does a Friedel-Crafts alkylation often yield a mixture of products?

Key term

State the reagents and conditions for the Friedel-Crafts alkylation of benzene with chloromethane.

Key term

Name the electrophile in the Friedel-Crafts alkylation of benzene by chloromethane and give the equation for its formation.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    State the reagents and conditions for the Friedel-Crafts alkylation of benzene with chloromethane.

    Model answer

    Chloromethane and anhydrous aluminium chloride, heated under reflux at about 50 °C with all the apparatus dry.

  2. 2

    Name the electrophile in the Friedel-Crafts alkylation of benzene by chloromethane and give the equation for its formation.

    Model answer

    The electrophile is the methyl carbocation, CH₃⁺, formed by CH₃Cl + AlCl₃ → CH₃⁺ + AlCl₄⁻.

  3. 3

    Give the overall equation for the Friedel-Crafts alkylation of benzene with chloromethane and name the product.

    Model answer

    C₆H₆ + CH₃Cl → C₆H₅CH₃ + HCl, and the product is methylbenzene.

  4. 4

    Explain how aluminium chloride is regenerated in a Friedel-Crafts alkylation.

    Model answer

    The proton lost from the ring is taken up by the tetrachloroaluminate ion, AlCl₄⁻ + H⁺ → AlCl₃ + HCl, so only a small amount of aluminium chloride is needed.

  5. 5

    Why must a Friedel-Crafts alkylation be carried out in dry apparatus?

    Model answer

    Aluminium chloride is rapidly hydrolysed by water, which destroys the halide acceptor so that no carbocation can be generated and the reaction stops.

  6. 6

    Why is Friedel-Crafts alkylation valuable to a synthetic chemist?

    Model answer

    It forms a new carbon–carbon bond, so it lengthens the carbon skeleton attached to the ring. Ethylbenzene made this way is the feedstock for phenylethene and hence for poly(phenylethene).

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