JEE Advanced 2025 · Paper 1 · Chemistry Q7

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ChemistryMultiple correct+4 / -2Question 39 of 48

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

$\text{cyclohexene} \xrightarrow{\ce{HBr}} \mathbf{P},\qquad \mathbf{P} \xrightarrow{\text{Finkelstein reaction}} \mathbf{Q},\qquad \mathbf{P} \xrightarrow{\text{Swarts reaction}} \mathbf{R}$

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

Cyclohexene reacts with HBr to give P. P gives Q by the Finkelstein reaction (horizontal arrow) and R by the Swarts reaction (downward arrow).

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

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

Relative reactivity toward

\mathrm{S_N2}

reaction in P, Q and R follows the order P > R > Q.

K_a

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

Answer and worked solution

Official answer: B

The compounds.

P = bromocyclohexane (Markovnikov addition of HBr).
Q = iodocyclohexane (Finkelstein: NaI in acetone).
R = fluorocyclohexane (Swarts: AgF or $\ce{Hg2F2}$ ).

Options.

(A) False. C-X length follows the halogen size, I > Br > F, so Q > P > R.
(B) True. C-X bond enthalpy is C-F > C-Br > C-I, so R > P > Q.
(C) False. $\mathrm{S_N2}$ reactivity follows leaving-group ability, I > Br > F, so Q > P > R.
(D) False. The pKa of HX is HF (about 3) > HBr > HI, so R > P > Q, not R > Q > P.

Topics:SN1 and SN2 Alkenes Acidity and basicity

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JEE Advanced 2025 · Paper 1, Chemistry question 7