JEE Advanced 2025 Paper 1 Q07 Chemistry Organic Reactions & Mechanisms Named Reactions & Reagents Medium

JEE Advanced 2025 Paper 1 · Q07 · Named Reactions & Reagents

For the reaction sequence shown above (cyclohexene $\xrightarrow{\text{HBr}}$ \textbf{P} $\xrightarrow{\text{Finkelstein}}$ \textbf{Q}; \textbf{P} $\xrightarrow{\text{Swarts}}$ \textbf{R}), the correct statement(s) is(are): (In the options, X is any atom other than carbon and hydrogen, and it is different in \textbf{P}, \textbf{Q} and \textbf{R}.)

(A) C–X bond length in \textbf{P}, \textbf{Q} and \textbf{R} follows the order $\textbf{Q}>\textbf{R}>\textbf{P}$. (B) C–X bond enthalpy in \textbf{P}, \textbf{Q} and \textbf{R} follows the order $\textbf{R}>\textbf{P}>\textbf{Q}$. (C) Relative reactivity toward S$_N$2 in \textbf{P}, \textbf{Q} and \textbf{R} follows the order $\textbf{P}>\textbf{R}>\textbf{Q}$. (D) p$K_a$ value of the conjugate acids of the leaving groups in \textbf{P}, \textbf{Q} and \textbf{R} follows the order $\textbf{R}>\textbf{Q}>\textbf{P}$.

  1. A. Statement (A)
  2. B. Statement (B)
  3. C. Statement (C)
  4. D. Statement (D)
Reveal answer + step-by-step solution

Correct answer:B

Solution

Cyclohexene + HBr (Markovnikov) gives bromocyclohexane → \textbf{P} = C$_6$H$_{11}$\textbf{Br}. Finkelstein (NaI/acetone) on \textbf{P} swaps Br for I → \textbf{Q} = C$_6$H$_{11}$\textbf{I}. Swarts reaction (AgF or SbF$_3$) on \textbf{P} swaps Br for F → \textbf{R} = C$_6$H$_{11}$\textbf{F}.

So X is F in \textbf{R}, Br in \textbf{P}, I in \textbf{Q}.

(A) Bond length: C–F < C–Br < C–I, i.e. $\textbf{Q}>\textbf{P}>\textbf{R}$ — not $\textbf{Q}>\textbf{R}>\textbf{P}$. FALSE. (B) Bond enthalpy: C–F (∼485) > C–Br (∼285) > C–I (∼213) kJ/mol, i.e. $\textbf{R}>\textbf{P}>\textbf{Q}$. TRUE. (C) S$_N$2 reactivity: best leaving group is I$^-$, then Br$^-$, then F$^-$, so $\textbf{Q}>\textbf{P}>\textbf{R}$, not $\textbf{P}>\textbf{R}>\textbf{Q}$. FALSE. (D) p$K_a$ of HX: HF (3.2) > HBr (∓9) > HI (∓10), giving $\textbf{R}>\textbf{P}>\textbf{Q}$, not $\textbf{R}>\textbf{Q}>\textbf{P}$. FALSE.

Only (B) is correct.

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