Halogenation of an alkene is the electrophilic addition of a halogen molecule (X₂) across a carbon–carbon double bond (C=C) to give a 1,2-dihalide. It matters as a standard test for unsaturation and a route to vicinal dihalides. The reaction proceeds through a bridged halonium ion, which forces the second halide to attack from the opposite face, so the two halogens add anti (to opposite sides of the former double bond).

烯烃的卤化是卤素分子(X₂)对碳碳双键(C=C)的亲电加成,生成 1,2-二卤化物的反应。它之所以重要,是因为它是检验不饱和度的标准方法,也是制备邻二卤化物的途径。该反应经桥连卤鎓离子进行,迫使第二个卤负离子从相反一侧进攻,因此两个卤素以反式加成(加到原双键的相反两侧)。