The end result will be that one of the two bonds opens up. #H# adds to one side, and #X# to the other. This is called mono-halogenation.
(#X# stands for #F#, #Cl#, #Br# or #I#)
This is an example of an addition reaction, because the whole #HX# molecule is added tot the alkene. There are no other reaction products.
The mechanism is as follows:
#H-X# is always very polar, the #X#-side drawing the electrons more to its side of the molecule. The extra electron pair in the double bond matches up with the (positive) #H# and then the #X# has 'no other choice' but matching up with the other side of the now open bond.
Example :
Ethene + hydrogen bromide
#(H_2C=CH_2)+ (H-Br)->(H_3C-CH_2Br)#
Extra :
Contrary to mono-halogenation of alkAnes, this reaction occurs under rather normal conditions.
