Lecture Notes on Electrophilic Addition to Alkenes. Part I.

ELECTROPHILIC ADDITION TO ALKENES

Both of the carbons of the double bond become saturated when a fourth sigma bond is added to them.
ADDITION OF HYDROGEN HALIDES TO ALKENES

 
 
The pi bond of the alkene is an electrophile which attacks the electrophilic hydrogen of the HX.

Both electron from the pi bond are used in forming the new carbon-H bond generating a carbocationic site at the other position of the original C=C.
MARKOVNIKOV'S RULE

The hydrogen of the HX is added to the carbon which has the greater number of hydrogens and the halogen is added to the carbon which has the lesser number of hydrogens.
 
 
 
 
When HBr is added to alkenes in the presence of peroxides, the addition is Anti Markovnikov