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Question #44916

Difficulty: 8/10 | 🟢 Free

The Question

Consider the following aromatic amines in aqueous medium at 298 K: (I) Aniline (II) 2-Methylaniline (III) $N,N$ -Dimethylaniline (IV) $N,N,2$ -Trimethylaniline

Let $\Delta pK_{b1} = pK_b(\text{I}) - pK_b(\text{II})$ and $\Delta pK_{b2} = pK_b(\text{III}) - pK_b(\text{IV})$ .

Furthermore, compounds III and IV are independently treated with Benzenediazonium chloride (\ce{PhN2+Cl-}) at pH 6 and 273 K. Compound III yields a highly colored azo dye, whereas compound IV yields product Y.

Which of the following correctly identifies the signs of $\Delta pK_{b1}$ and $\Delta pK_{b2}$ , and the identity of product Y?

Options

A.
$\Delta pK_{b1} < 0$ , $\Delta pK_{b2} > 0$ , Y is No reaction
B.
$\Delta pK_{b1} > 0$ , $\Delta pK_{b2} < 0$ , Y is 4-(phenyldiazenyl)- $N,N,2$ -trimethylaniline
C.
$\Delta pK_{b1} < 0$ , $\Delta pK_{b2} < 0$ , Y is 5-(phenyldiazenyl)- $N,N,2$ -trimethylaniline
D.
$\Delta pK_{b1} < 0$ , $\Delta pK_{b2} > 0$ , Y is 4-(phenyldiazenyl)- $N,N,2$ -trimethylaniline

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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

Electrophilic Substitution Control|Inductive Effect Trends|Ortho-Effect in Anilines

Why This Question Is Tricky

Common Mistakes

Students often blindly apply the 'Ortho Effect' as a universal rule that ortho-substituted anilines are always weaker bases.

This question exposes the 'Hidden Variable' of nitrogen substitution.

While primary anilines suffer from Steric Inhibition of Solvation (SIS) leading to decreased basicity, tertiary anilines experience Steric Inhibition of Resonance (SIR), which traps the lone pair on nitrogen and actually increases basicity.

Furthermore, it tests the kinetic consequence of SIR: the loss of the $+M$ effect deactivates the ring towards weak electrophiles, breaking the assumption that all dialkyl anilines undergo diazonium coupling.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Amines - Class 12
Concept	Electrophilic Substitution Control
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free