Hydrogen halides (HCl, HBr, HI)  adds to alkenes to yield alkyl halides.  This addition is regioselective in accordance with Markovnikov's rule, "the proton from the hydrogen halide attaches itself to the less substituted carbon.

Mechanism:

Step 1. Proton addition acordin to Markovnikov's rule to form a carbocation

 

 

Step 2. Nucleophilic addition by halide ion

Addition of HCl to 2-Metylbut-2-ene