Before You Begin

Question #44926

Difficulty: 8/10 | 🟢 Free

The Question

Consider the azo coupling reactions of $N,N$ -dimethylaniline (\textbf{P}) and $2$ -methyl- $N,N$ -dimethylaniline (\textbf{Q}) with benzenediazonium chloride (\ce{PhN2+ Cl-}) at $273\text{ K}$ in an aqueous buffer of $\text{pH} = 4.5$ . Which of the following statements correctly describes the outcome and the underlying chemical principles governing the reactivity of \textbf{P} and \textbf{Q}?

Options

A.
\textbf{P} forms a para-coupled azo dye. \textbf{Q} fails to couple because Steric Inhibition of Resonance (SIR) localizes the nitrogen lone pair, increasing its basicity ( $pK_a$ of \ce{QH+} > \ce{PH+}). Thus, \textbf{Q} exists predominantly as the deactivated \ce{QH+} at pH 4.5, and its free base lacks resonance activation.
B.
\textbf{Q} forms a para-coupled azo dye faster than \textbf{P}. The ortho-effect in \textbf{Q} decreases its basicity ( $pK_a$ of \ce{QH+} < \ce{PH+}), ensuring a higher concentration of the reactive free base at pH 4.5, while the $+I$ effect of the methyl group further activates the ring.
C.
\textbf{P} forms a para-coupled azo dye. \textbf{Q} undergoes ortho-coupling because the ortho-methyl group sterically hinders the para position, and its lower basicity ensures it remains unprotonated and reactive at pH 4.5.
D.
Both \textbf{P} and \textbf{Q} fail to couple at pH 4.5 because the strongly activating \ce{-N(CH3)2} group is completely protonated in both compounds, rendering the aromatic rings deactivated towards electrophilic attack.

Check Your Answer

Correct Answer

Step-by-Step Solution

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

Azo Coupling Regioselectivity|Ortho-Effect in Anilines|Electrophilic Substitution Control

Why This Question Is Tricky

Common Mistakes

The question tests the 'Chemical Exception' of the Ortho-Effect in tertiary anilines.

Students typically memorize that the ortho-effect decreases the basicity of anilines (which is true for primary anilines like o-toluidine).

However, in N,N-dimethylanilines, the ortho-substituent causes Steric Inhibition of Resonance (SIR), which localizes the lone pair on nitrogen, paradoxically INCREASING basicity.

This hidden variable shifts the acid-base equilibrium at the reaction pH, heavily protonating the amine.

Concurrently, SIR prevents resonance activation of the ring, rendering the free base unreactive towards the weak diazonium electrophile.

The trap exploits the misapplication of the primary aniline ortho-effect and the assumption that activating groups always guarantee EAS.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Amines - Class 12
Concept	Azo Coupling Regioselectivity
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free