Electrophilic addition of bromine to butene forms a vicinal dibromide

Steps:

• Butene (C4H8) is an alkene with a C=C double bond that undergoes electrophilic addition with Br2.
• Br2 approaches the double bond, forming a bromonium ion intermediate.
• A bromide ion then attacks the intermediate, adding Br atoms across the double bond.
• The product is 1,2-dibromobutane (for 1-butene) or 2,3-dibromobutane (for 2-butene), a vicinal dihalide.

Why B is correct:

• B represents the vicinal dibromide (e.g., CH3CH2CHBrCH2Br), matching the anti-addition product from the reaction mechanism.

Why the others are wrong:

• A is a substitution product, but alkenes undergo addition, not substitution with Br2.
• C is a geminal dibromide, which requires different reagents like NBS, not Br2 addition.
• D is an unsaturated compound, but the reaction saturates the double bond.

Final answer: B