Electrophilic addition of hydrogen halides (H-X) to alkenes

Hydrogen halides such as H-Br and H-Cl are suitable electrophiles for a simple addition to an alkene.  Since the halogen is much more electronegative than the hydrogen, the H-X bond is quite polarized, with the H carrying a partial positive charge (δ+) and serving as the electrophilic atom.  In this example, HCl adds to but-1-ene to form racemic 2-chlorobutane:

The mechanism begins with an electrophilic addition elementary step, where the alkene forms a new sigma bond to the electrophilic H, and breaks the H-Cl bond displacing Cl-.  The second elementary step is a coordination step, where the chloride ion attacks the carbocation to form a second sigma bond.  Note that overall we have lost one pi bond, but gained a sigma bond.  Notice that Markovnikov’s Rule is followed here, since the secondary carbocation formed is more stable than the alternative, which would have been primary.