Chlorination of alkenes with Cl2 to give vicinal dichlorides

Description: Treatment of alkenes with chlorine (Cl2) leads to formation of vicinal dichlorides (1,2-dichlorides) 

Notes: Note that the chlorines add to opposite faces of the double bond (“anti-addition”)

Examples:

Notes: Again note that the stereochemistry is trans.

Mechanism: Attack of the alkene on chlorine (Step 1, arrows A and B) gives the chloronium ion, which is then attacked by chloride at carbon to give the 1,2-dichloride (vicinal dichloride). Note that this should occur at the more substituted carbon of the chloronium ion where appropriate (“Markovnikoff” addition).

Notes: Note that only one enantiomer is shown here: chlorine has an equal chance of adding to either face of the alkene, and thus a 1:1 mixture of enantiomers will be formed.