Hydrohalogenation

A hydrohalogenation reaction is the electrophilic addition of hydrohalic acids like hydrogen chloride or hydrogen bromide to alkenes to yield the corresponding haloalkanes.

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 fewer hydrogen substituents, an observation known as Markovnikov's rule. This is due to the abstraction of a hydrogen atom by the alkene from the acid (HX) to form the most stable carbocation(relative stability: 3°>2°>1°>methyl), as well as generating a halogen anion.

A simple example of a hydrochlorination is that of indene with hydrochloric acid gas (no solvent):

In the presence of peroxides, HBr adds to a given alkene in an anti-Markovnikov addition fashion. This is because the reaction proceeds through the most stable carbon radical intermediate (relative stability: 3° > 2° > 1°>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 promotes the formation of the bromide radical. Therefore, in the presence of peroxides, HBr adds so that the bromine atom 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.

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