Radical Halogenation of Alkanes

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Free Radical Halogenation

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Key Questions

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  • In organic chemistry, halogenation is the reaction of a halogen with another substance in which a halogen atom ends up as part of that substance.

    There are two types of halogenation.

    Halogen Addition

    An example is the addition of bromine to ethene.

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    Halogen substitution

    Halogens react with alkanes under the influence of heat or light to form alkyl halides.

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    The halogen atom replaces a hydrogen atom in the alkane, so this is a substitution reaction.

    Aromatic compounds undergo halogen substitution reactions in the presence of Lewis acids.

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    Here's a video on the halogenation of alkanes.

  • Answer:

    Free radical halogenation involves initiation, propagation, and termination steps.

    Explanation:

    Consider the free radical chlorination of methane.

    Step 1. Initiation

    The initiation step involves the homolytic cleavage of a #"Cl-Cl"# bond to form two #"Cl"# atoms.

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    Step 2. Propagation

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    • A #"C"# atom removes an #"H"# from methane, producing #"HCl"# and a methyl radical.

    • The newly-formed methyl radical abstracts a #"Cl"# from a chlorine molecule, producing chloromethane and re-forming a #"Cl"# atom.

    Step 3. Termination

    In the termination steps, the radicals combine in all possible combinations.

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    The termination products are chlorine, chloromethane, and ethane.

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