Introduction
A hydrohalogenation reaction involves the electrophilic addition of hydrogen halides, such as hydrogen chloride or hydrogen bromide, to alkenes, resulting in the formation of the corresponding haloalkanes. In instances where the two carbon atoms at the double bond are connected to varying numbers of hydrogen atoms, the halogen tends to attach preferentially to the carbon atom with fewer hydrogen substituents, a phenomenon encapsulated by Markovnikov’s rule. This occurrence can be attributed to the alkene’s abstraction of a hydrogen atom from the hydrogen halide (HX), leading to the formation of the most stable carbocation (with relative stability following the order: 3° > 2° > 1° > methyl). This process also results in the generation of a halogen anion.
Reaction
Regioselectivity: Markovnikov
Stereospecificity: syn + anti
Intermediate: carbocation
Since a carbocation intermediate is formed, the reaction is also susceptible to carbocation rearrangements, as shown below:
Mechanism
Mechanism without carbocation rearrangement:
Mechanism with a carbocation rearrangement: