Chlorine (Cl2) in the presence of alkenes reacts to form dichlorides:

Chlorine reacts with the alkene molecule to form a vicinal dichloride (1,2-dichloride) where chlorines are added to the opposite side of each other in anti addition, giving 2 products that are enantiomers:

Anti Addition

Partial anti Addition

No stereocenters

The reaction mechanism is depicted below:

In the first step, electrons from the alkene bond react with the first chlorine to make a bond, and electrons from the first chlorine atom react with the other carbon to form another bond, forming a ring structure as an intermediate.

In the second step, the free, negatively charged chlorine , attacks the backside of the intermediate structure, causing one chlorine to finish with one type of stereochemistry and the other chlorine to finish with the opposite type of stereochemistry, resulting in an anti addition.

The reagent used for this reaction is Cl2.