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Question #44026

Difficulty: 8/10 | 🟢 Free

The Question

Consider the following aromatic amines in an aqueous solution at $298\text{ K}$ :

I. Benzenamine II. 2-Methylbenzenamine III. $N,N$ -Dimethylbenzenamine IV. $N,N,2$ -Trimethylbenzenamine

Which of the following represents the correct decreasing order of their basic strength ( $K_b$ )?

Options

A.
$\text{IV} > \text{III} > \text{I} > \text{II}$
B.
$\text{III} > \text{I} > \text{IV} > \text{II}$
C.
$\text{II} > \text{I} > \text{IV} > \text{III}$
D.
$\text{IV} > \text{II} > \text{III} > \text{I}$

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

Basicity-Solvation Competition|Ortho-Effect in Anilines|Inductive Effect Trends

Why This Question Is Tricky

Common Mistakes

This problem tests the dual, opposing consequences of steric hindrance in aromatic amines.

Students often apply a blanket rule, assuming ortho-substitution always decreases basicity (Ortho-Effect) or always increases it (+I effect).

The hidden variable is the size of the amine group.

In primary amines (II), the $-NH_2$ group remains planar, allowing resonance, but the ortho-group hinders conjugate acid solvation, decreasing basicity.

In tertiary amines (IV), the bulky $-NMe_2$ group is forced out of plane (Steric Inhibition of Resonance), shutting down lone-pair delocalization and drastically increasing basicity.

The trap is failing to recognize when SIR overrides the Ortho-Effect.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Amines - Class 12
Concept	Basicity-Solvation Competition
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free