Hydrohalogenation

Basic Hydrohalogenation
Hydrohalogenation is the electrophilic addition of halogen acids like hydrogen chloride or hydrogen bromide to alkenes to yield the corresponding haloalkanes.


 * CH3-CH=CH2 + HBr &rarr; CH3-CHBr-CH3

If the two carbon atoms at the double bond are linked to a different number of hydrogen atoms, the halogen is found preferentially at the carbon with less hydrogen substituents, an observation known as Markovnikov's rule. This is due to an intermediate which is produced by the abstraction of a hydrogen atom by the alkene from the acid (HBr) to form the most stable carbocation(relative stability: 3o>2o>1o>methyl), as well as generating a Br anion:


 * CH3-CH=CH2 + HBr &rarr; CH3-C+H-CH3 + Br-

The subsequent reaction proceeds by an Sn1 mechanism due to the presence of the electrophilic carbocation and a nucleophilic halogen ion, thus resulting in the final product CH3-CHBr-CH3

HBr addition with peroxides
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 &rarr; 2RO. (proceeds in the presence of light or heat)
 * RO. + HBr &rarr; ROH + Br.


 * Propagation:
 * CH3-CH=CH2 + Br. &rarr; CH3-ĊH-CH2Br
 * CH3-ĊH-CH2Br + HBr &rarr; CH3-CH2-CH2Br + Br.


 * Termination
 * Occurs when two radicals form a covalent bond.

Therefore in the presence of peroxides, HBr adds so that the Bromine is added to the carbon bearing the most numerous hydrogen substituents and hydrogen atoms will add to carbons bearing less hydrogen substituents. However, this process is restricted to addition of HBr.

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).

HI
Due to the weakness of the carbon-iodine bond necessary to complete the first step of the propagation phase, insufficient heat is released to proceed through the reaction successfully.