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Question #44935

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The Question

Consider the ammonolysis of methyl iodide in an aqueous solvent system at $298\text{ K}$ , which sequentially generates primary ( $1^\circ$ ), secondary ( $2^\circ$ ), and tertiary ( $3^\circ$ ) methylamines. The thermodynamic basicity of these amines in the aqueous phase exhibits a stark deviation from their intrinsic gas-phase proton affinities. Let $pK_{b1}$ , $pK_{b2}$ , and $pK_{b3}$ denote the base dissociation constants (expressed as $pK_b$ ) for the $1^\circ$ , $2^\circ$ , and $3^\circ$ amines, respectively. Which of the following options correctly establishes the sequence of $pK_b$ values and accurately identifies the dominant thermodynamic factor responsible for the anomalous position of the $3^\circ$ amine?

Options

A.
$pK_{b2} < pK_{b1} < pK_{b3}$ ; The diminished exothermic hydration enthalpy ( $\Delta H_{hyd}^\circ$ ) of the trimethylammonium cation, due to fewer N-H hydrogen-bond donor sites and steric crowding, outweighs the stabilization gained from the +I effect of the methyl groups.
B.
$pK_{b2} < pK_{b3} < pK_{b1}$ ; The cumulative +I effect of three methyl groups in the $3^\circ$ amine overrides the attenuation in solvation energy, rendering it more basic than the $1^\circ$ amine.
C.
$pK_{b1} < pK_{b2} < pK_{b3}$ ; The primary amine exhibits the lowest $pK_b$ because the entropic penalty ( $\Delta S_{hyd}^\circ$ ) of organizing water molecules around the hydrophobic methyl groups is minimized.
D.
$pK_{b3} < pK_{b2} < pK_{b1}$ ; Intrinsic gas-phase basicity dictates the aqueous trend because the high dielectric constant of water completely shields the alkyl groups from steric interactions during protonation.

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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

Ammonolysis Complexity|Basicity-Solvation Competition|Inductive Effect Trends

Why This Question Is Tricky

Common Mistakes

This question tests the 'Solvent Effect in Gas vs Aqueous Phase' trap.

Students frequently apply purely inductive reasoning (predicting $3^\circ > 2^\circ > 1^\circ$ ) or memorize the ethylamine aqueous trend ( $2^\circ > 3^\circ > 1^\circ$ ) and misapply it to methylamines.

The hidden variable is the differential solvation enthalpy of the conjugate acids.

For methylamines, the steric inhibition of solvation and the lack of H-bond donors in the $3^\circ$ conjugate acid are so severe that its basicity drops below that of the $1^\circ$ amine, a classic exception to the inductive rule.

Quick Facts About This Question

Question Information

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