As an organic chemistry student you will learn many reaction mechanisms. Molecules will follow different patterns of reactivity depending on the kind of reaction taking place. In this article I will help you understand why the alkene halogenation reaction undergoes anti-addition
Alkene Halogenation is the reaction in which a carbon to carbon double bond attacks a neutral halogen molecule and winds up forming a vicinal dihalide. The two halogen atoms add to opposite faces of the pi bond which is why this reaction is considered to be an anti-addition.
The reason the halogens add anti to each other has to do with the structure and function of the mechanism intermediate. Let’s use the example of bromine adding to a generic alkene molecule
The reaction begins when the nucleophilic pi bond reaches out for a bromine molecule. Halogens are highly electronegative and do not like being attacked. When the bromine atom is attacked it retaliates or attacks right back using one of its 3 lone pairs of electrons.
This results in the bromine atom bound to 2 carbon atoms, the very two atoms that used to keep up a pi bond between them. The second bromine molecule grabs the electrons that used to bind the bromines, and breaks away into solution as a negative nucleophilic halide.
The Crucial Intermediate
The halogen atom which attacked and attacked back, is now bound to both carbon atoms forming a 3-atom ring. This is called a bromonium given that bromine has a positive charge.
If this reaction took place with chlorine the resulting bridge would be called a Chloronium bridge.
The bromonium bridge is very unstable due to the fact that it has just 2 lone pairs, 2 bonds to carbon, and a positive charge. To compensate for this charge, bromine pulls on the electrons binding it to carbon. This makes the attached carbon atoms slightly positive and attracts the second negative bromine from solution.
Bromine in solution approaches the molecule with the intention of attacking one of the two slightly positive carbon atoms. However, if it attempts to attack from the same confront of the molecule as its style, its progress will be confined. The bromonium bridge sits high on the molecule, with its bonds to carbon hindering the progressive of any nucleophile in solution.
This forces the attacking bromide ion to attack from the opposite confront of the molecule, away from the bromonium bridge and associated steric hindrance.