Question

Why is halogenation anti addition?

Answer 1

First, we need to know about the halogenation addition reaction. As the name indicates that it is for the addition of halogens. Now, the question arises on the term ‘anti’ which indicates the opposite sides.

Complete answer:
Let’s understand the answer in complete detail.
We know that the halogen addition reaction is a simple organic reaction when a halogen molecule is added to the carbon-carbon double bond of an alkene functional group. In this reaction we get a vicinal dihalides which means two halogens are attached on the adjacent carbon atoms.
The following is the reaction of halogenation addition reaction:
$C = C + {X_2} \to X - C - C - X$
In the above reaction, $X$ denotes the halogen atom. It can be $Cl$ , $Br$ , $F$ and $I$ etc.
The halogen atom on the adjacent carbon atom can be on the same side or opposite sizes. If they are on the same side, it is called syn addition whereas if they are on the opposite side, it is called anti addition.
Here in this type of reaction, stereochemical peculiarity is also observed when halogen is added to cis- $2$ - butene versus trans- $2$ - butene.
Because of the anti-addition , cis- $2$ -butene gives R,R and S,S enantiomers whereas addition to trans- $2$ -butene, it gives the optically inactive R,S and S,R meso compounds. So this is an excellent example of stereospecific reaction in which the mechanism of the reaction determines the stereochemical outcome of the reaction.

Note:
It must be noted that the halogenation addition reaction is an electrophilic reaction. Halogen acts as electrophile and it attacks the carbon-carbon double bond. The double bond has high electron density as it has pi-electrons.