Alkene Reactions: Hydrohalogenation using Hydrohalic Acids

Using a hydrohalic acid, such as HBr or HCl, with an alkene will result in the formation of an an alkyl halide in a reaction known as hydrohalogenation:

The reaction takes place on the most substituted carbon, following Markovnikoff selectivity. Additionally, hydride and/or methyl rearrangement can occur, causing the halogen group to be added to a more highly substituted carbon adjacent to the alkene bond:

Hydride Shift

Methyl Shift

The reaction mechanism is depicted below using HBr, but any HX would follow the same mechanism:

In the first step, the alkene is protonated by the acid, removing the double bond and causing 1 carbon to carry a positive charge and the halogen (Br in this example) to carry a negative charge.

In the second step, the negatively charged bromine reacts with the positively charged carbon to form the alkyl halide. It is at this second step where a rearrangement could occur if the carbon atoms adjacent to the positively charged carbon atom are tertiary or quaternary.

This is the mechanism for all hydrohalogens (HX).

Practice this reaction using our Reaction Solver!