Vicinal dibromide formation via electrophilic addition

Steps:

• Identify the position of the C=C double bond in the alkene.
• Br2 adds across the double bond, with one Br attaching to each sp2 carbon.
• The Br atoms end up on adjacent carbons (vicinal positions).
• Name the product using the original chain numbering for the lowest Br positions.

Why D is correct:

• But-1-ene (CH2=CHCH2CH3) has the double bond between C1 and C2; addition gives BrCH2CHBrCH2CH3, or 1,2-dibromobutane, per the mechanism of electrophilic addition.

Why the others are wrong:

• A: Ethene (CH2=CH2) adds Br to each carbon, yielding 1,2-dibromoethane (vicinal), not 1,1-dibromoethane (geminal, impossible for this reaction).
• B: Propene (CH3CH=CH2) adds Br to C1 and C2, yielding 1,2-dibromopropane, not 1,3-dibromopropane (non-adjacent Br).
• C: But-2-ene (CH3CH=CHCH3) adds Br to C2 and C3, but produces stereoisomers of 2,3-dibromobutane (meso and racemic), not a single compound as stated.

Final answer: D