When an alkene is heated with Cl2 at high temperature, hydrogen atom of allylic carbon is substituted with halogen atom giving allyl halide. CH2=CH−CH3+Cl2→CH2=
CH−CH2Cl
3−Chloropropene
+HCl Alkenes form addition product, vicinal dihalide, with chlorine or bromine usually in inert solvent like CCl4 at room temperature.