Tuesday, January 22, 2013

Halogenation of Alkanes

1. Halogenation
  • Alkanes react with halogen (such as chlorine or bromine) when the mixture is exposed to ultraviolet light (symbolised as hv or uv) or when heated to high temperature (200 - 400oC).
  • The function of ultraviolet light is to provide energy for the homolytic cleavage of halogen (Cl-Cl or Br-Br).
  • The products are haloalkanes (RX) and hydrogen halides (HX).
  • The halogenation of alkanes is a free radical subsitution reaction, in which the mechanism involves initiation, propagation and termination steps.
  • The monosubstitution of alkanes refers to the substitution reaction where there is only one hydrogen atom in alkanes substituted by halogen free radical.       

2. General mechanism of free radical substitution reaction
   
   
    (a) Initiation step
  • Involves homolytic cleavage of halogen molecules by ultraviolet light or by heat to form the corresponding free-radical.
  • A half-headed curve arrow is used to illustrate the movement of one electron.

    (b) Propagation step
  • The halogen free-radical formed in the initiation step abstracts a hydrogen atom from an alkane molecule to form hydrogen halide and an alkyl free-radical.
  • The alkyl free-radical then reacts with another halogen molecule to form haloalkane and regenerates the halogen free-radical.
  • The regenerated halogen free-radical then propagates the reactions in chain, until the supply of the reactants diminishes.

    (c) Termination step
  • The chain is terminated when two free radicals combine to form molecules.
  • It helps to bring the reaction to an end by decreasing the number of free radicals available to propagate the reaction.

3. Mechanism for the monochlorination of methane (Alkanes containing equivalent type of hydrogen atoms)

         
4. Bromination (Alkanes containing non-equivalent type of hydrogen atoms)
    

  • The prediction of the formation of major product in bromination of alkanes containing non-equivalent type of hydrogen atom (such as propane, n-butane and isobutane) depends on the stability of free radicals.

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