Hydrohalogenation

Carbocation stability helps determine which carbon–carbon bond arrangement forms during protonation. The π bond can accept a proton in more than one orientation, but the pathway that generates the more stable carbocation is favored. The halide ion then reacts with that intermediate, so the final haloalkane commonly follows Markovnikov orientation.

Protonation converts the alkene’s π bond into a carbocation-containing intermediate. The carbon–carbon multiple bond is therefore replaced by a structure containing a positively charged carbon and a newly attached hydrogen. This intermediate is short-lived in the reaction sequence because a halide ion then reacts with the carbocation to form the haloalkane.

The halide ion provides the second part of the added hydrogen halide. After protonation creates the carbocation, the halide reacts with the positively charged carbon and establishes the carbon–halogen bond. Using HCl, HBr, or HI therefore determines whether the resulting haloalkane contains chlorine, bromine, or iodine.

First identify the two carbons joined by the multiple bond and consider where protonation would produce the more stable carbocation. Hydrogen generally adds to the carbon that already bears more hydrogen atoms, while the halide becomes attached to the other carbon. This predicts the commonly observed Markovnikov product.

A useful sequence begins by selecting an alkene and identifying the hydrogen halide being added, such as HCl, HBr, or HI. The π bond attacks a proton, the favored carbocation forms, and the halide ion reacts with that intermediate. The outcome is an alkyl halide suitable for subsequent synthesis.

The products provide versatile synthetic intermediates rather than being final compounds only. Their carbon–halogen functionality can support substitution and elimination reactions, and it can also participate in carbon–carbon bond-forming sequences. Consequently, converting an alkene into an alkyl halide creates a practical connection between a multiple-bond starting material and several downstream transformations.