Both Cl2 and Br2 are reactive enough to add to
alkenes. In practice, we prefer to use dibromine because dibromine is a
liquid @RT(room temperature) and Cl2 is a gas. Gases must be stored
in cylinders and present many dangers if the fumes were to escape into the air.
Do you remember why Cl2 is a gas?
Even though Br2 is nonpolar, because it is so big the electrons
are highly polarizable and a form of Br2 can exist as a resonance
hybrid like Br+ Br-. The Br+ is thought
to be the electrophile.
Let's look at the reaction of Br2 with cyclopentene.
Step 1 starts as before but the lone pair on the electrophile combines
with C+ resulting in a bridged intermediate called the bromonium
ion:
Step 2 is made complicated by the ability of the bromide anion to
combine with the bridged intermediate from either the right or the left
side. Both products are always observed in exactly 50:50 mixtures.
