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Mechanism Of Free Radical Substitution Of Alkanes

Halogenation reactions occur via a mechanism called free radical substitution. There are three main steps in the free radical mechanism. They are:
  1. initiation
  2. propagation
  3. termination.
The overall reaction of chlorination of methane.

CH4  +  Cl2 ______> CH3Cl +  HCl

  1. Initiation
    This step involves the dissociation of the halogen molecule (e.g., chlorine molecule) into two chlorine atoms. Even though the total reaction is exothermic, initially energy should be supplied for the reaction to proceed.

image\25440 ch 15.png

  1. Propagation steps
    During the propagation step, the hydrogen atom is abstracted from methane by a chlorine atom. This is followed by the reaction between the methyl radical and the chlorine molecule.

image\25450 ch 15.png

image\25461 ch 15.png

  1. Termination
    The termination steps involve the combination of the radicals.

image\25476 ch 15.png

image\25485 ch 15.png

image\25493 ch 15.png

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