Reaction mechanisms

Radical substitution, Electrophillic addition, Nucleophillic substitution

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Alkanes-Radical substitution

  • Alkanes take part in radical substitution with Halogens
  • eg. the reaction between methane and Bromine (UV radiation)
  • CH4 + Br2 ----------> CH3Br + HBr

 

  • Initiation:  UV radiation breaks the covalent bond between the bromine molecules Br--Br, this is homolytic fission. Radicals are formed.
  • Br2--------> 2Br   (radicals)
  • Propagation: The products of the reaction are made.
  •  Br (radical) + CH4-------->CH3  (radical) + HBr
  •  CH3  (radical) + Br2 ----------> Ch3Br + Br  (radical)
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termination

In propagation, a Bromine radical is used in the first step and then a Bromine radical is regenerated in the second step. Bromine radicals catalyse the propagation stage. They are not used up in the overall reaction.

Termination: Any two radicals can combine.

CH3  (radical) + Br  (radical) -------> CH3Br

Br  (radical) + Br  (radical) -------> Br2

CH3  (radical) + CH3  (radical) -------> C2H6

Further substitution reactions can take place as the bromoalkane product can react further with bromine radicals. A mixture of products is processed.

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Comments

Catherine

Thanks, this is useful :)

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