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Question #44357

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The Question

The heterocyclic compound 4-chloro-2-hydroxypyridine ( $\textbf{X}$ ) exhibits pronounced solvent-dependent reactivity.

When $\textbf{X}$ is treated with piperidine in anhydrous toluene at $80^\circ\text{C}$ , it rapidly undergoes a substitution reaction to form product $\textbf{P}$ .

Conversely, when $\textbf{X}$ is treated with piperidine in water at $80^\circ\text{C}$ , the substitution is exceptionally slow. However, treating this aqueous solution of $\textbf{X}$ with $\ce{Br_2}$ (1 equivalent) at $25^\circ\text{C}$ results in the rapid formation of a mono-brominated major product $\textbf{Q}$ .

Which of the following best describes the dominant tautomer in each solvent, its effect on reactivity, and the structure of $\textbf{Q}$ ?

Options

A.
Lactim in toluene (activates $S_NAr$ ); Lactam in water (activates EAS $\rightarrow$ 5-bromo-4-chloro-2-pyridone)
B.
Lactam in toluene (activates $S_NAr$ ); Lactim in water (activates EAS $\rightarrow$ 3-bromo-4-chloro-2-hydroxypyridine)
C.
Lactim in both; water H-bonding blocks $S_NAr$ (activates EAS $\rightarrow$ 5-bromo-4-chloro-2-hydroxypyridine)
D.
Lactim in toluene ( $S_N2$ at C4); Lactam in water (carbonyl directs EAS $\rightarrow$ 6-bromo-4-chloro-2-pyridone)

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Correct Answer

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Explanation

How the Examiner Thinks

Exception_Exposure

Learn to identify edge cases and boundary exceptions that JEE examiners hide in math problems.

Concepts Behind This Question

Nucleophilic Aromatic Substitution (SNAr)|Electrophilic Substitution Control|Nitrogenous Base Keto-Enol Tautomerism

Why This Question Is Tricky

Common Mistakes

This problem tests the 'Chemical Exception' where a molecule's reactivity profile completely inverts based on solvent-induced tautomerism.

Students often apply static resonance models, forgetting that 2-hydroxypyridines exist as lactams (pyridones) in polar solvents.

The trap is failing to recognize that the nitrogen atom switches from an electron-withdrawing imine (activating $S_NAr$ ) to an electron-donating amide (deactivating $S_NAr$ but activating EAS).

Furthermore, it tests the regioselectivity of EAS on the pyridone ring, requiring steric analysis to distinguish between the C3 and C5 positions.

Distractor D also tests the classic trap of proposing an impossible $S_N2$ mechanism on an $sp^2$ hybridized carbon.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Haloalkanes and Haloarenes - Class 12
Concept	Nucleophilic Aromatic Substitution (SNAr)
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free