Mechanism of electrophilic addition of HBr:

This structure is termed as a reaction intermediate. It is a reactive species and will not survive extremely long with the bromide ion in the vicinity. The carbocation is an electrophile because it is positively charged. The bromide ion is a nucleophile because it is negatively charged. Hence, the bromide ion uses one of its lone pairs of electrons to make a new σ bond to the carbocation and the final product is created.

The mechanism includes the addition of HBr to the alkene. It is an electrophilic addition because the first step of the mechanism includes the addition of the electrophilic hydrogen to the alkene.

Note: The second step includes a nucleophilic addition of the bromide ion to the carbocation intermediate, but it is the first step that defines this reaction.

In the mechanism displayed, the π electrons of the alkene give the electrons for a new bond among the right hand carbon and hydrogen. They could evenly well have been employed to form a bond among the left hand carbon and hydrogen.

 Figure: Mechanism of electrophilic addition of HBr to 2,3-dimethyl-2-butene.