JEE Advanced · 2025 · 18 May 2025 · Paper 1 · Haloalkanes and Haloarenes

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 $\mathbf{P},\mathbf{Q}$ and $\mathbf{R}$ follows the order $\mathbf{Q}>\mathbf{R}>\mathbf{P}$.
2. (B)C-X bond enthalpy in $\mathbf{P},\mathbf{Q}$ and $\mathbf{R}$ follows the order $\mathbf{R}>\mathbf{P}>\mathbf{Q}$.✓ Correct
3. (C)Relative reactivity toward ${\mathrm{S}}_{\mathrm{N}}2$ reaction in $\mathbf{P},\mathbf{Q}$ and $\mathbf{R}$ follows the order $\mathbf{P}>\mathbf{R}>\mathbf{Q}$.
4. (D)$\mathrm{p}{K}_{\mathrm{a}}$ value of the conjugate acids of the leaving groups in $\mathbf{P}$, $\mathbf{Q}$ and $\mathbf{R}$ follows the order $\mathbf{R}>\mathbf{Q}>\mathbf{P}$.

Answer: (B)

Solution

From the reaction sequence, P, Q, and R are benzyl halides with X = F, Br, and I respectively.

As we go down the halogen group (F → Br → I):

• Atomic size increases: F < Br < I
• Bond length increases: C-F < C-Br < C-I, so Q > P > R (not Q > R > P)
• Bond enthalpy decreases: C-F > C-Br > C-I, so R > P > Q ✓
• ${\mathrm{S}}_{N}2$ reactivity increases with better leaving group: I⁻ > Br⁻ > F⁻, so Q > P > R (not P > R > Q)
• ${\mathrm{pK}}_a$ of conjugate acids: HF > HBr > HI, so R > P > Q (not R > Q > P)

Only statement (B) is correct.

JEE Advanced · 2025 · 18 May 2025 · Paper 1 · One or more correct options