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Question #44104

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The Question

In the synthesis of a Schiff base, $0.10 \text{ mol}$ of a primary amine $\ce{RNH2}$ ( $pK_b = 9.0$ ) and $0.10 \text{ mol}$ of a ketone are added to $1.0 \text{ L}$ of an aqueous solution containing $0.10 \text{ mol}$ of an amphiprotic salt $\ce{NaHA}$ at $298 \text{ K}$ . For the diprotic acid $\ce{H2A}$ , $pK_{a1} = 3.0$ and $pK_{a2} = 7.0$ .

The rate of imine formation is theoretically maximized when the pH of the reaction medium is exactly equal to the $pK_a$ of the conjugate acid of the amine. A student assumes that since the pure $\ce{NaHA}$ solution provides this exact optimal pH, the reaction will proceed at the maximum rate.

However, the amine itself interacts with the amphiprotic buffer. Assuming no volume change upon addition, what is the actual initial pH of the reaction mixture?

Options

A.
6.0
B.
5.0
C.
7.0
D.
9.0

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

Amphiprotic Ion pH|Schiff Base pH Dependence|Electrophile vs Nucleophile

Why This Question Is Tricky

Common Mistakes

The question tests the 'Lethal Collision' trap in buffer chemistry and reaction optimization.

Students often memorize that an amphiprotic salt provides a fixed pH ( $\frac{pK_{a1}+pK_{a2}}{2}$ ) and that Schiff base formation is optimal at $\text{pH} = pK_a$ of the protonated amine.

Here, both values are 5.0, leading to the false conclusion that the system is perfectly optimized.

The hidden variable is that the amphiprotic ion ( $\ce{HA-}$ ) acts as an internal reactant, undergoing an acid-base reaction with the primary amine itself.

This shifts the equilibrium, altering the buffer ratio and pushing the final pH to 6.0, thereby moving the system outside the optimal kinetic window.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Equilibrium - Class 11 Part 1
Concept	Amphiprotic Ion pH
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free