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Question #44624

Difficulty: 8/10 | 🟢 Free

The Question

Consider the following substrates undergoing nucleophilic substitution with $\ce{NaOCH3}$ in $\ce{CH3OH}$ at 298 K:

(P) 1-chloro-2,4-dinitrobenzene (Q) 1-chloro-3,5-dimethyl-2,4-dinitrobenzene (U) 1-chloro-4-nitrobenzene (R) 1-bromo-2,2-dimethylpropane (S) 1-bromobutane

Let $E_{a,P}$ , $E_{a,Q}$ , and $E_{a,U}$ be the activation energies for the $\text{S}_\text{N}\text{Ar}$ reactions of P, Q, and U respectively. Let $E_{a,R}$ and $E_{a,S}$ be the activation energies for the $\text{S}_\text{N}2$ reactions of R and S respectively.

Which of the following correctly orders the activation energies, reflecting the dominant transition state electronic and steric effects?

Options

A.
$E_{a,P} < E_{a,U} < E_{a,Q}$ and $E_{a,S} < E_{a,R}$
B.
$E_{a,P} < E_{a,Q} < E_{a,U}$ and $E_{a,S} < E_{a,R}$
C.
$E_{a,Q} < E_{a,P} < E_{a,U}$ and $E_{a,R} < E_{a,S}$
D.
$E_{a,U} < E_{a,P} < E_{a,Q}$ and $E_{a,S} < E_{a,R}$

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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

Steric Hindrance in SN2|Nucleophilic Aromatic Substitution (SNAr)

Why This Question Is Tricky

Common Mistakes

The question tests the student's ability to distinguish between two distinct types of steric effects in nucleophilic substitutions.

In $\text{S}_\text{N}2$ reactions (aliphatic), steric bulk directly blocks the kinetic trajectory of the incoming nucleophile, drastically increasing $E_a$ (as seen in neopentyl systems).

In $\text{S}_\text{N}\text{Ar}$ reactions (aromatic), the nucleophile attacks perpendicular to the ring, so trajectory blocking is minimal.

However, bulky groups ortho to an electron-withdrawing group (like $-\text{NO}_2$ ) force it out of coplanarity with the ring.

This 'Steric Inhibition of Resonance' (SIR) prevents the $-\text{NO}_2$ group from stabilizing the Meisenheimer complex via the $-\text{M}$ effect.

Students often fall into the trap of counting the number of $-\text{NO}_2$ groups (assuming Q > U) without accounting for their 3D spatial orientation and the resulting loss of electronic activation.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Haloalkanes and Haloarenes - Class 12
Concept	Steric Hindrance in SN2
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free