Describe the reaction of an alkene with halogens (Br2 or Cl2).

• A. Elimination to form an alkyne.
• B. Hydration to form an alcohol.
• C. Hydrogenation to form an alkane.
• D. Electrophilic addition to form dihaloalkanes; bromine water decolorizes.

Answer: (C)

When an alkene meets a halogen like Br2 or Cl2, the double bond acts as a nucleophile and the halogen acts as an electrophile, so the reaction is an electrophilic addition across the C=C bond. A halogen–bridged intermediate forms (a bromonium or chloronium ion) and then a halide ion attacks from the opposite side, opening the intermediate. This adds one halogen to each carbon of the former double bond, producing a vicinal dihaloalkane. In bromine’s case, the solution decolorizes as Br2 is reduced to Br-, which is a handy test for unsaturation. The same kind of addition occurs with chlorine, giving a dichloroalkane. This reaction is not about removing atoms to form an alkyne, nor about adding water to make an alcohol, nor about adding hydrogen to make an alkane.