Halogenation reaction of alkane: a free radical substitution reaction'

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Halogenation reaction of alkane a free radical
substitution reaction.

• Alkenes react with halogen such as chlorine and
  bromine to produce haloalkanes in the presence of
  light or temperature greater than 100 oC.
• RH X2 RX HX
• With methane, the reaction produces a mixture of
  halomethanes and a hydrogen halide.

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Examples
i. CH4 Cl2
CH3Cl CH2Cl2 CHCl3 CCl4 HCl
ii. CH3CH3 Cl2
CH3CH2Cl HCl
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CH3
CH3
iii. CH3 CCH3 Cl2
CH3 CCH2Cl HCl
CH3
CH3
(ii) (iii) 1 product only because all the
hydrogen atoms are identical
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CH3CH2CH2Br ( minor) CH3CHCH3
Br ( major) HBr
iv. CH3CH2CH3 Br2
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Reaction mechanism

• CH4 Br2 CH3Br HBr
• Chain initiation step

Note illustrate electrons movement by using
half-headed curve arrow
VIDEO1
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• Chain propagation steps

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• Chain termination step.

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Chain propagation step can be repeated to form
CH3Cl , CH2Cl2 , CHCl3 and CCl4 .
CH3 Cl ?? CH2Cl HCl
CH2Cl Cl2 ?? CH2Cl2 Cl
CH2Cl2 Cl ?? CHCl2 HCl
CHCl2 Cl2 ?? CHCl3 Cl
CHCl3 Cl ?? CCl3 HCl
CH3 Cl2 ?? CCl4 Cl
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Example

• The bromination of 2-methylbutane(neopentane)
  yields a mixture of isomers.

Increasing yield
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Exercise

• Chlorination reaction of certain alkanes can be
  used for laboratory preparations, for example in
  the preparation of chlorocyclopentane from
  cyclopentane. Give the mechanism for the reaction.