Both give the same product (alkene) on reaction with alcoholic KOH.
Gem dihalide
When the two halogen atoms are attached to the same carbon atom and are prepared by the addition reaction of vinyl halide, when vinyl halide undergoes an additional reaction with hydrogen chloride, then the formation of geminal dihalide takes place.
Vicinal dihalide
When the two halogen atoms are on adjacent carbon atoms and are prepared by the reaction between a halogen and an alkene.
- Gem dihalide or vicinal dihalide gives alkene on treating with alcoholic KOH.
