|Chapter 4: Alcohols and Alkyl Halides|
Radical Halogenation of Alkanes
RADICAL CHAIN MECHANISM FOR REACTION OF METHANE WITH Br2
|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.
More highly brominated by-products are possible if methyl bromide reacts with a bromine radical in the same fashion as methane does. Can you draw the cycle that leads to the formation of dibromomethane ?
|© Dr. Ian Hunt, Department of Chemistry|