Hydrogen halide addition:

 Alkenes react along with hydrogen halides (HCl, HBr, and HI) to provide an alkyl halide. The hydrogen halide molecule is split and the hydrogen atom adds to one end of the double bond whereas the halogen atom adds to another. The reaction of HBr with 2, 3-dimethyl-2-butene is an instance of this reaction. In this reaction, the alkene works as a nucleophile.

Figure: Electrophilic additions.

Figure: (a) Symmetrical alkenes; (b) unsymmetrical alkenes.