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Question #44308

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The Question

Consider the second-order rate constants for the nucleophilic substitution reactions of the following pairs of substrates with sodium methoxide ( $\ce{CH3ONa}$ ) in methanol at $298\text{ K}$ :

\textbf{Pair I} ( $S_N\text{Ar}$ ): 1-fluoro-2,4-dinitrobenzene ( $k_1$ ) vs. 1-chloro-2,4-dinitrobenzene ( $k_2$ ) \textbf{Pair II} (Acyl Substitution): Acetyl fluoride ( $k_3$ ) vs. Acetyl chloride ( $k_4$ ) \textbf{Pair III} ( $S_N2$ ): Benzyl fluoride ( $k_5$ ) vs. Benzyl chloride ( $k_6$ )

Which of the following correctly represents the relative rate constants and identifies the dominant electronic factor responsible for the reversed halogen reactivity trend between Pair I and Pair II?

Options

A.
$k_1 > k_2$ , $k_4 > k_3$ , $k_6 > k_5$ ; In Pair I, the strong $-I$ effect of F stabilizes the Meisenheimer complex. In Pair II, strong $+M$ effect ( $2p\pi-2p\pi$ overlap) of F stabilizes the ground state, reducing carbonyl electrophilicity.
B.
$k_2 > k_1$ , $k_4 > k_3$ , $k_6 > k_5$ ; In Pair I, the weaker $\ce{C-Cl}$ bond lowers the activation energy of the rate-determining step. In Pair II, Cl is a better leaving group than F.
C.
$k_1 > k_2$ , $k_3 > k_4$ , $k_5 > k_6$ ; The strong $-I$ effect of F increases the electrophilicity of the attacked carbon in all three pairs, accelerating the nucleophilic attack.
D.
$k_1 > k_2$ , $k_4 > k_3$ , $k_5 > k_6$ ; In Pair I, F is a better leaving group due to its high electronegativity. In Pair III, the $-I$ effect of F stabilizes the $S_N2$ transition state.

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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

Electronegativity Scales|Aryl Halide Resonance|Nucleophilic Acyl Substitution

Why This Question Is Tricky

Common Mistakes

The question tests the student's ability to distinguish between the kinetic roles of halogens across different substitution mechanisms.

A common trap is assuming that high electronegativity always increases electrophilicity and reaction rate (as correctly seen in $S_N\text{Ar}$ ).

However, in acyl halides, the $2p\pi-2p\pi$ orbital overlap (resonance) between fluorine and the carbonyl carbon masks its electronegativity, stabilizing the ground state and drastically reducing reactivity.

The student must recognize that the $+M$ effect dominates in the acyl system's ground state, while the $-I$ effect dominates the transition state stabilization in the aryl system.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Classification of Elements and Periodicity in Properties - Class 11 Part 1
Concept	Electronegativity Scales
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free