Haloalkanes and Haloarenes · Question 1

For the reaction sequence given below, the correct statement(s) is(are) [figure] (In the options, X is any atom other than carbon and hydrogen, and it is different in P, Q and R)

1. C−X bond length in P, Q and R follows the order Q > R > P.
2. C−X bond enthalpy in P, Q and R follows the order R > P > Q.
3. Relative reactivity toward SN2 reaction in P, Q and R follows the order P > R > Q.
4. pKa value of the conjugate acids of the leaving groups in P, Q and R follows the order R > Q > P.

Answer

(2) C−X bond enthalpy in P, Q and R follows the order R > P > Q.

Solution

[figure] HBr [figure] (P) — electrophilic addition of HBr.

P NaI, acetone [figure] (Q) — Finkelstein: Br replaced by I.

P AgF [figure] (R) — Swarts: Br replaced by F.

(A) C–X bond length: C–I > C–Br > C–F, i.e. Q > P > R. False.

(B) C–X bond enthalpy: C–F > C–Br > C–I, i.e. R > P > Q. True.

(C) SN2 reactivity: R–I > R–Br > R–F, i.e. Q > P > R. False.

(D) pKa of HX: HF (≈ 3.2) > HBr (≈ −9) > HI (≈ −10), i.e. R > P > Q. False.

Answer: (B)