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Question #44841

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The Question

A kinetic study of the electrophilic monobromination of a series of substituted $N,N$ -dimethylanilines ( $\ce{XC6H4NMe2}$ ) is conducted in glacial acetic acid at 298 K. The relative rate constants ( $k_X$ ) are measured, and a Hammett plot of $\log(k_X/k_H)$ versus the Brown-Okamoto substituent constants ( $\sigma^+$ ) is constructed. The \textit{meta} and \textit{para} substituted derivatives exhibit an excellent linear correlation with a slope of $\rho$ . Which of the following accurately describes the sign of $\rho$ and the graphical position of the \textit{ortho}-methyl derivative ( $\ce{X} = o-\ce{CH3}$ ) relative to this linear fit?

Options

A.
$\rho < 0$ ; the $o-\ce{CH3}$ point lies significantly below the line due to Steric Inhibition of Resonance.
B.
$\rho < 0$ ; the $o-\ce{CH3}$ point lies significantly above the line due to the enhanced $+I$ proximity effect.
C.
$\rho > 0$ ; the $o-\ce{CH3}$ point lies significantly below the line due to Steric Inhibition of Protonation.
D.
$\rho > 0$ ; the $o-\ce{CH3}$ point lies on the line because $\sigma^+$ inherently accounts for all electronic effects.

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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

Graphical Slopes Analysis|Ortho-Effect in Anilines|Electrophilic Substitution Control

Why This Question Is Tricky

Common Mistakes

The problem tests the boundary conditions of the Hammett equation and the physical reality of the 'Ortho-Effect'.

Students often blindly apply the linear equation $\log(k/k_0) = \rho\sigma^+$ to all isomers, expecting a uniform slope.

The hidden variable here is Steric Inhibition of Resonance (SIR).

While a methyl group is electronically activating, placing it \textit{ortho} to a bulky $-\ce{NMe2}$ group forces the nitrogen lone pair out of conjugation with the aromatic $\pi$ -system.

This decoupling of the electronic $+M$ effect drastically lowers the reaction rate, causing a severe downward deviation in the kinetic plot.

It forces the student to differentiate between pure electronic parameters and stereoelectronic destabilization.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Chemical Kinetics - Class 12
Concept	Graphical Slopes Analysis
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free