What type of reaction occurs when bromine adds to an alkene in this test?

• A. Electrophilic addition
• B. Nucleophilic substitution
• C. Elimination
• D. Hydration

Answer: (A)

When a bromine molecule encounters an alkene, the double bond’s high electron density attracts an electrophile. The Br2 molecule is induced to polarize, giving a temporary δ+ on one bromine. That bromine atom then attacks the carbon–carbon double bond, forming a bond with one carbon and breaking the Br–Br bond, creating a bromonium-like intermediate. The bromide ion that is produced then attacks to open the intermediate, yielding a vic-dibromide. In short, the reaction proceeds via the addition of an electrophile to the alkene, so it is electrophilic addition. This is different from substitution (no leaving group replaced), elimination (no loss to form a new double bond), or hydration (addition of water across the double bond).