Alkene identification via HBr addition mass increase

Steps:

• Compute mass difference: 164 - 84 = 80, atomic mass of HBr (H=1, Br=79).
• Deduce reactant formula: M=84 suggests C6H12, general alkene (CnH2n for n=6).
• Predict product: Alkene + HBr → alkyl bromide C6H13Br (M=84 + 80 = 164) via electrophilic addition.
• Select option matching C6H12 alkene structure.

Why D is correct:

• D is cyclohexene (C6H12), undergoes Markovnikov addition with HBr to form bromocyclohexane (C6H11Br, M=164).

Why the others are wrong:

• A (1,3-cyclohexadiene, C6H8) has M=80, not 84.
• B (methylcyclopentane, C6H12) is saturated cycloalkane, does not add HBr.
• C (hexane, C6H14) has M=86, not 84.

Final answer: D