Chapter 4: Alcohols and Alkyl Halides

Radical Halogenation of Alkanes

Summary:

• When treated with Br₂ or Cl₂, radical substitution of R-H generates the alkyl halide and HX.
• Alkane R-H relative reactivity order : tertiary > secondary > primary > methyl.
• Halogen reactivity F₂ > Cl₂ > Br₂ > I₂
• Only chlorination and bromination are useful in the laboratory.
• Bromination is selective for the R-H that gives the most stable radical.
• Chlorination is less selective
• Reaction proceeds via an radical chain mechanism which involves radical intermediates.
• The termination steps are of low probability due to the low concentration of the radical species meaning that the chances of them colliding is very low.

• Halogenation of allylic systems (see Chapter 10)
  
• Halogenation of benzylic systems (see Chapter 11)

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