Question 7

**Identifying P, Q, R** - Cyclohexene + HBr (electrophilic addition) → **P = bromocyclohexane** (X = Br) - Finkelstein reaction (NaI in dry acetone) → **Q = iodocyclohexane** (X = I) - Swarts reaction (AgF / Hg₂F₂ etc.) → **R = fluorocyclohexane** (X = F) **Evaluating the options** (A) C–X bond length increases with size of halogen: C–I > C–Br > C–F, i.e., Q > P > R. The stated order Q > R > P is **incorrect**. (B) C–X bond enthalpy: C–F > C–Br > C–I, i.e., R > P > Q. **Correct**. (C) S_N2 reactivity depends on leaving-group ability: I⁻ > Br⁻ > F⁻, so Q > P > R. The stated order P > R > Q is **incorrect**. (D) pKa of conjugate acids of leaving groups: HF (≈3.2) > HBr (≈ −9) > HI (≈ −10), i.e., R > P > Q. The stated order R > Q > P is **incorrect**. Hence only (B) is correct.

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