Monday, January 28, 2008

IIT JEE Revision Alkanes Halogenation

Halogenation of alkanes

This involves substitution o fhydrogen atom by halogen atom. The order of reactivity is F2>Cl2>Br2(>I2).

The mechanism of chlorination and bromination involves free radicals.

Step 1 (Initiation)

Heat or uv light cause the weak halogen bond to undergo homolytic cleavage to generate two bromine radicals and starting the chain process.

Step 2 (Propagation)

(a) A bromine radical abstracts a hydrogen to form HBr and a methyl radical, then
(b) The methyl radical abstracts a bromine atom from another molecule of Br2 to form the methyl bromide product and another bromine radical, which can then itself undergo reaction 2(a) creating a cycle that can repeat.

Step 3 (Termination)
Various reactions between the possible pairs of radicals allow for the formation of ethane, Br2 or the product, methyl bromide. These reactions remove radicals and do not perpetuate the cycle. There will be equilibrium and reaction terminates.

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