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Question #44381

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The Question

Consider the isomeric substrates 1-fluoro-2-nitrobenzene ( $\textbf{I}$ ) and 1-fluoro-4-nitrobenzene ( $\textbf{II}$ ).\n\n\textbf{Reaction A:} $\textbf{I}$ and $\textbf{II}$ undergo $S_NAr$ with \ce{NaOCH3} in \ce{CH3OH} at 298 K with second-order rate constants $k_{ortho}$ and $k_{para}$ , respectively.\n\n\textbf{Reaction B:} $\textbf{I}$ and $\textbf{II}$ are hydrolyzed using aqueous \ce{NaOH} at 400 K, followed by acidification, yielding phenols $\textbf{P}$ and $\textbf{Q}$ , respectively, with acid dissociation constants $K_{a,ortho}$ and $K_{a,para}$ in water at 298 K.\n\nGiven that the Mesomeric ( $-M$ ) effect of the \ce{-NO2} group is approximately equal at the ortho and para positions, select the correct relationship between the kinetic and thermodynamic parameters along with the primary governing factors.

Options

A.
$k_{ortho} > k_{para}$ because the stronger $-I$ effect of the ortho \ce{-NO2} group provides superior stabilization to the anionic Meisenheimer transition state; $K_{a,para} > K_{a,ortho}$ because intramolecular hydrogen bonding in $\textbf{P}$ thermodynamically stabilizes the undissociated phenol.
B.
$k_{para} > k_{ortho}$ because steric hindrance by the ortho \ce{-NO2} group raises the activation energy for nucleophilic attack; $K_{a,para} > K_{a,ortho}$ because the $-M$ effect is more effective at stabilizing the phenoxide in $\textbf{Q}$ due to the lack of steric inhibition of resonance.
C.
$k_{ortho} > k_{para}$ because the ortho \ce{-NO2} group can participate in an intramolecular hydrogen bond with the incoming methoxide, lowering the transition state energy; $K_{a,ortho} > K_{a,para}$ because the proximity of the \ce{-NO2} group maximizes the $-I$ stabilization of the conjugate base.
D.
$k_{para} > k_{ortho}$ because the para Meisenheimer complex has a more extended conjugated system; $K_{a,ortho} > K_{a,para}$ because the ortho-effect inherently increases the acidity of ortho-substituted benzenes regardless of the substituent nature.

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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

Phenol Acidity vs. Substituents|Nucleophilic Aromatic Substitution (SNAr)|Inductive Effect Trends

Why This Question Is Tricky

Common Mistakes

The question tests the dichotomy between kinetic reactivity and thermodynamic stability, specifically targeting two common student traps.

First, students often assume that steric hindrance will make the ortho-isomer react slower in $S_NAr$ ( $k_{para} > k_{ortho}$ ), ignoring that for small nucleophiles, the proximity-dependent $-I$ effect dominates transition state stabilization.

Second, students frequently misapply the 'ortho-effect' from benzoic acids to phenols, incorrectly assuming $o$ -nitrophenol is more acidic than $p$ -nitrophenol.

The trap exposes the hidden variable of intramolecular hydrogen bonding, which stabilizes the reactant (undissociated phenol) rather than the intermediate/product, thereby lowering the thermodynamic acidity despite favorable inductive effects.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Alcohols, Phenols, and Ethers - Class 12
Concept	Phenol Acidity vs. Substituents
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free