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Question #44254

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The Question

In the idealized planar conformation of $N,N$ -dimethylaniline, the $\pi$ -HOMO possesses a planar node passing through the \textit{meta}-carbon atoms, dictating high electron density at the \textit{ortho} and \textit{para} positions. Consider the electrophilic aromatic bromination of $N,N,2,6$ -tetramethylaniline ( $\mathbf{X}$ ) with $\ce{Br_2 / FeBr_3}$ at $298\text{ K}$ to yield the major product $\mathbf{Y}$ . Assuming the nitrogen atom maintains an idealized $sp^2$ geometry, which of the following correctly describes the geometric relationship between the planar node of the nitrogen lone-pair orbital and the benzene ring in the ground state of $\mathbf{X}$ , and the regiochemistry of the newly added bromo group in $\mathbf{Y}$ ?

Options

A.
Planar node is coplanar with the ring; Bromo group at C-4 (\textit{para})
B.
Planar node is orthogonal to the ring; Bromo group at C-3 (\textit{meta})
C.
Planar node is orthogonal to the ring; Bromo group at C-4 (\textit{para})
D.
Planar node is coplanar with the ring; Bromo group at C-3 (\textit{meta})

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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

Angular Nodes and Planar Nodes|Electrophilic Substitution Activation/Deactivation|Ortho-Effect in Anilines

Why This Question Is Tricky

Common Mistakes

This question tests the concept of Steric Inhibition of Resonance (SIR) overriding standard electronic nodal analysis.

Students typically rely on the $+M$ effect of amino groups, which dictates ortho/para substitution based on the $\pi$ -HOMO electron density.

However, the 'hidden variable' is the steric clash in $N,N,2,6$ -tetramethylaniline, which forces a $90^\circ$ conformational twist.

This twist rotates the nitrogen $p$ -orbital into the ring plane (making its planar node orthogonal), completely shutting down the $+M$ effect.

The $\ce{-N(CH_3)_2}$ group defaults to a $-I$ director (meta-directing), and the ortho-methyls synergistically direct the electrophile to the meta position.

Failing to account for this geometric override leads to the incorrect prediction of para-substitution.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Structure of Atom - Class 11 Part 1
Concept	Angular Nodes and Planar Nodes
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free