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Alkanes and radicals 3: Selectivity & reactivity principle

Alkanes and radicals

Selectivity & reactivity principle
Radical bromation is more selective than radical chlorination. Have a look on following illustrations to get more insight in this process:


Why is it that the Bromation radical is more selective? This can be explained using the Hammand postulate. Bromation is an endothermic process, while chlorination is exothermic. The postulate states the following:
  • Endothermic process: a rather productish (radical) transition state
  • Exothermic: a rather reagentic (alkane) transition state
Thus the Bromine atom can see the difference between the 1°, 2° and 3° hydrogens more clearly. 

The more reactive a particle is, the less likely it will react selective. (More reactivity = less selectivity, vice versa). Alkanes undergo Bromation and Chlorination, but no Fluoration or Iodisation. Iodisation simply does not occur, and Fluoration is a way to heavy reaction to be useful.

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