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Question #44748

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The Question

An aromatic primary amine $X$ (molecular formula $\ce{C7H9N}$ ) yields toluene upon diazotization at $273 \text{ K}$ followed by reduction with $\ce{H3PO2}$ . A $0.1 \text{ M}$ aqueous solution of $X$ at $298 \text{ K}$ exhibits a pH of $8.7$ . Another isomer $Y$ of $X$ , which is also a primary aromatic amine, has a $pK_b$ of $9.0$ . The $pK_b$ of aniline is $9.4$ . Let $\alpha_X$ , $\alpha_Y$ , and $\alpha_{\text{aniline}}$ be the degrees of dissociation of $X$ , $Y$ , and aniline respectively, in their $0.1 \text{ M}$ aqueous solutions. Which of the following correctly identifies $X$ , the relationship between the degrees of dissociation, and the $pK_b$ of $X$ ?

Options

A.
2-Methylaniline, $\alpha_Y > \alpha_{\text{aniline}} > \alpha_X$ , $pK_b = 9.6$
B.
3-Methylaniline, $\alpha_X > \alpha_{\text{aniline}} > \alpha_Y$ , $pK_b = 9.6$
C.
4-Methylaniline, $\alpha_Y > \alpha_X > \alpha_{\text{aniline}}$ , $pK_b = 8.7$
D.
2-Methylaniline, $\alpha_X > \alpha_Y > \alpha_{\text{aniline}}$ , $pK_b = 5.3$

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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

Degree of Dissociation (Alpha)|Ortho-Effect in Anilines|Diazotization Temperature Sensitivity

Why This Question Is Tricky

Common Mistakes

The problem tests the intersection of ionic equilibrium and organic electronic effects.

Students often assume that any electron-donating group (like $-\ce{CH3}$ ) will increase the basicity of aniline, leading them to incorrectly reject 2-methylaniline as a weaker base than aniline.

The 'hidden variable' is the Ortho-Effect (steric inhibition of solvation), which anomalously decreases the $K_b$ of ortho-substituted anilines.

This forces a conceptual conflict where the calculated $pK_b$ from physical data must be reconciled with an exception in organic chemistry to identify the correct isomer and its dissociation behavior.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Electrochemistry - Class 12
Concept	Degree of Dissociation (Alpha)
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free