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Question #44285

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The Question

Consider the electrophilic monobromination ( $\ce{Br_2}$ in glacial $\ce{CH_3COOH}$ , $298\text{ K}$ ) of three structurally related nitrogenous systems:

I. 2-Hydroxypyridine II. 2-Methoxypyridine III. 1-Methylpyridin-2(1H)-one

Let $k_I$ , $k_{II}$ , and $k_{III}$ be their respective initial rate constants. Which of the following correctly represents the order of reactivity and the primary electronic rationale governing the reactivity difference between the locked tautomeric forms (II and III)?

Options

A.
$k_{III} > k_I > k_{II}$ ; The N-atom in III has a lower $Z_{eff}$ than the O-atom in II, making it a superior +M donor and highly activating the ring via an enamine-like resonance.
B.
$k_{II} > k_I > k_{III}$ ; The O-atom in II has a higher $Z_{eff}$ , leading to a more effective 2p-2p orbital overlap with the aromatic ring compared to the N-atom in III.
C.
$k_I > k_{III} > k_{II}$ ; Rapid keto-enol tautomerization in I dynamically lowers the activation energy of the Wheland intermediate via a concerted proton transfer.
D.
$k_{III} > k_{II} > k_I$ ; The $\ce{>C=O}$ group in III acts as a strong +M director, whereas the $\ce{-OCH_3}$ group in II deactivates the ring via a strong -I effect.

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.

Concepts Behind This Question

Effective Nuclear Charge (Zeff)|Electrophilic Substitution Activation/Deactivation|Nitrogenous Base Keto-Enol Tautomerism

Why This Question Is Tricky

Common Mistakes

The trap tests the misconception that '-OH and -OR are strongly activating, while >C=O is deactivating' in EAS.

Students often erroneously assume the lactim form (II) is more reactive than the lactam form (III).

The hidden variable is that tautomerism shifts the primary electron donor from Oxygen to Nitrogen.

Because Nitrogen has a lower Z_eff, it is a far superior +M donor, making the lactam form highly reactive (enamine-like) despite the presence of a carbonyl group.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Classification of Elements and Periodicity in Properties - Class 11 Part 1
Concept	Effective Nuclear Charge (Zeff)
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free