Advices For Studying Organic Chemistry

through a radical mechanism)

An

Ionic Mechanism for the Reaction

Addition of Bromine to an Alkene
Step 1

CC CC

Br

+ Br −

Br

Br

δ+

δ−

Bromonium ion Bromide ion

+

A bromine molecule becomes polarized as it approaches the alkene. The polarized

bromine molecule transfers a positive bromine atom (with six electrons in its

valence shell) to the alkene resulting in the formation of a bromonium ion.

Step 2

CC

Br

Br

vic-Dibromide

CC

Br

+ Br −

Bromonium ion Bromideion

+

A bromide ion attacks at the back side of one carbon (or the other) of the

bromonium ion in an SN2 reacion causing the ring to open and resulting in the

formation of a vic-dibromide.

(^)

1. The bromine molecule becomes polarized as the π electrons of the alkene
   approaches the bromine molecule.


2. The electrons of the Br–Br bond drift in the direction of the bromine atom more
   distant from the approaching alkene ⇒ the more distant bromine develops a
   omes partially positive.
   ⇒
   partial negative charge; the nearer bromine bec


3. Polarization weakens the Br–Br bond, causing it to break heterolytically a
   bromide ion departs, and a bromonium ion forms.