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JEE Advanced2025Paper 1CHEM-II
Q.

For the reaction sequence given below, the correct statement(s) is(are)

(In the options, X is any atom other than carbon and hydrogen, and it is different in P, Q and R)

  1. A

    C–X bond length in P, Q and R follows the order Q > R > P.

  2. B

    C–X bond enthalpy in P, Q and R follows the order R > P > Q.

  3. C

    Relative reactivity toward S2 reaction in P, Q and R follows the order P > R > Q.

  4. D

    p value of the conjugate acids of the leaving groups in P, Q and R follows the order R > Q > P.

Solution

Identifying the products: P = cyclohexyl bromide (C–Br), Q = cyclohexyl iodide (C–I) from the Finkelstein exchange, R = cyclohexyl fluoride (C–F) from the Swarts reaction.

The halogens belong to periods 4, 5 and 2 respectively, so atomic size goes I > Br > F.

(A) Bond length scales with the halogen's size, giving C–I > C–Br > C–F, i.e. Q > P > R. The proposed order Q > R > P is wrong.

(B) Bond enthalpy increases as bond length decreases (and orbital overlap with C improves): C–F > C–Br > C–I, i.e. R > P > Q. Statement is correct.

(C) S2 reactivity depends on leaving group ability; iodide is the best leaving group, fluoride the worst. Order: Q > P > R, so the given order P > R > Q is wrong.

(D) p of HX: HI < HBr < HF (HF is the weakest acid here in aqueous media), so p order is HI < HBr < HF, equivalently Q < P < R. The proposed order R > Q > P is wrong.

Only (B) is correct.

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