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Question #44648

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The Question

Consider the nucleophilic aromatic substitution ( $\text{S}_\text{N}\text{Ar}$ ) of 1-halo-2,4-dinitrobenzenes with sodium methoxide in methanol at 298 K. Let the relative rate constants for the halogens be $k_F$ , $k_{Cl}$ , $k_{Br}$ , and $k_I$ . Separately, consider the acid dissociation constants ( $K_a$ ) of ortho-halobenzoic acids in aqueous solution at 298 K, denoted as $K_a(F)$ , $K_a(Cl)$ , $K_a(Br)$ , and $K_a(I)$ .

Which of the following correctly represents the trends for both the $\text{S}_\text{N}\text{Ar}$ reaction rates and the acid strengths?

Options

A.
$k_F > k_{Cl} > k_{Br} > k_I$ and $K_a(F) > K_a(Cl) > K_a(Br) > K_a(I)$
B.
$k_F > k_{Cl} > k_{Br} > k_I$ and $K_a(Br) \approx K_a(I) > K_a(Cl) > K_a(F)$
C.
$k_I > k_{Br} > k_{Cl} > k_F$ and $K_a(F) > K_a(Cl) > K_a(Br) > K_a(I)$
D.
$k_I > k_{Br} > k_{Cl} > k_F$ and $K_a(Br) \approx K_a(I) > K_a(Cl) > K_a(F)$

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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

Aryl Halide Resonance|Nucleophilic Aromatic Substitution (SNAr)|Carboxylic Acid Strength|Inductive Effect Trends

Why This Question Is Tricky

Common Mistakes

The question tests two critical exceptions to standard periodic trends in organic chemistry.

First, in $\text{S}_\text{N}\text{Ar}$ , students often incorrectly assume the reaction rate is governed by leaving group ability ( $\ce{I^-} > \ce{F^-}$ ), failing to recognize that the formation of the Meisenheimer complex is the rate-determining step, which is accelerated by the strong $-I$ effect of fluorine.

Second, in the acidity of ortho-halobenzoic acids, students typically rely solely on the $-I$ effect, incorrectly predicting o-fluorobenzoic acid to be the strongest.

They overlook the 'Ortho Effect' driven by Steric Inhibition of Resonance (SIR), where larger halogens force the carboxylate group out of planarity, preventing destabilizing $+R$ cross-conjugation from the aromatic ring and making the bulkier halogen-substituted acids stronger.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Haloalkanes and Haloarenes - Class 12
Concept	Aryl Halide Resonance
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free