Br2 addition to alkenes forming chiral centers in allylic bromides

Steps:

• Cold, dark Br2 performs electrophilic anti-addition to C=C; vinylic Br deactivates the bond, preventing reaction, while allylic Br allows it.
• Eliminate A and B as they contain vinylic Br (Br directly on sp2 carbon), so no addition product forms.
• For C (allylic): addition gives BrCH2-CHBr-CH2Br; central carbon bonds to H, Br, CH2Br, CH2Br (two identical groups), no chiral center.
• For D (allylic): addition gives CH3-CHBr-CHBr-CH2Br; new central carbon bonds to H, Br, CH2Br, CHBrCH3 (all different), forming chiral center.

Why D is correct:

• Per electrophilic addition mechanism, Br+ forms bromonium ion on terminal =CH2, Br- attacks internal =CH-, yielding a carbon with four unique substituents (stereocenter definition).

Why the others are wrong:

• A: Vinylic Br blocks addition; hypothetical product BrCH2-CHBr2 has gem-dibromide carbon with two Br (not chiral).
• B: Vinylic Br blocks addition; hypothetical product CH3-CHBr-CHBr2 unreactive under conditions.
• C: Addition occurs but product symmetric (two -CH2Br), no stereocenter.

Final answer: D