Halogenation

$$\text{R--CH=CH}_2 + \text{X}_2 \xrightarrow{\text{CCl}_4} \text{R--CHX--CH}_2\text{X}$$

Concept: Addition of halogens ($$\text{Br}_2$$ or $$\text{Cl}_2$$) across an alkene double bond via a cyclic halonium ion intermediate, resulting in a vicinal dihalide. Proceeds via anti-addition through a cyclic halonium ion intermediate ($$\text{cyclic bromonium ion}$$). No carbocation rearrangement occurs.

Unsaturation Test: Decolorization of the reddish-brown color of bromine water or $$\text{Br}_2 / \text{CCl}_4$$ serves as a qualitative laboratory test for unsaturation (presence of double or triple bonds).