Hi. I understand your confusion, and it is one that I had too. *For those of you who read my original post, there was an error, I found the answer on wikipedia.
http://en.wikipedia.org/wiki/Hydrohalogenation
In the presence of
peroxides (chemicals containing the general structure ROOR'), HBr adds to a given
alkene in an
anti-Markovnikov fashion. This is because the reaction proceeds through the most stable carbon
radical intermediate (relative stability: 3o>
2o>1o>methyl) instead of a
carbocation. The mechanism for this reaction is similar to a
chain reaction such as
free radical halogenation in which the peroxide promote the formation of the
Bromide radical (
radicals are indicated in
bold):
Initiation: RO:OR →
2RO. (proceeds in the presence of light or heat) RO. + HBr → ROH +
Br. Propagation: CH3-CH=CH2 +
Br. → CH3-
ĊH-CH2Br CH3-
ĊH-CH2Br + HBr → CH3-CH2-CH2Br +
Br. Termination Occurs when two
radicals form a covalent bond.
[edit] Why only HBr reacts in this way
No other
hydrogen halide behaves in the manner described above, this can be explained by a survey of the different halogen acids: HF (
hydrogen fluoride), HCl (
hydrogen chloride -- more commonly known by the aqueous species
hydrochloric acid), and HI (
hydrogen iodide).
HF
The hydrogen-
fluorine bond is simply too strong and therefore no
fluorine radicals can be generated in the propagation step.
HCl
Hydrogen chloride will react in a manner that is so slow that it is essentially synthetically useless. This is because the hydrogen-
chlorine bond is strong and thus the second step of the reaction would be extremely slow due to the heat required (it is an
endothermic reaction).