Before You Begin

Question #44109

Difficulty: 8/10 | 🟢 Free

The Question

Maleic acid (\ce{cis-HOOC-CH=CH-COOH}, denoted as $M$ ) and Fumaric acid (\ce{trans-HOOC-CH=CH-COOH}, denoted as $F$ ) are dissolved in water at $298\text{ K}$ and $1\text{ atm}$ . The transition from their respective mono-anions to di-anions is governed by a delicate balance of electrostatic repulsion, intramolecular hydrogen bonding, and steric strain (B-strain). Assuming ideal dilute solutions, which of the following represents the correct strictly increasing order of their acid dissociation constants expressed as $pK_a$ values?

Options

A.
$pK_{a1}(M) < pK_{a1}(F) < pK_{a2}(F) < pK_{a2}(M)$
B.
$pK_{a1}(F) < pK_{a1}(M) < pK_{a2}(M) < pK_{a2}(F)$
C.
$pK_{a1}(M) < pK_{a1}(F) < pK_{a2}(M) < pK_{a2}(F)$
D.
$pK_{a1}(M) < pK_{a2}(M) < pK_{a1}(F) < pK_{a2}(F)$

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

Polyprotic Acid Dissociation|Carboxylic Acid Strength|Steric Hindrance (B-Strain)|Resonance Energy and Stability

Why This Question Is Tricky

Common Mistakes

The problem tests the 'Hidden Variable' of steric inhibition of resonance (B-strain) coupled with electrostatic repulsion in polyprotic acid dissociation.

Students typically memorize that maleic acid has a lower $pK_{a1}$ due to intramolecular hydrogen bonding.

However, they often incorrectly extrapolate this stability to the second dissociation or assume statistical probability governs $pK_{a2}$ .

The trap is failing to recognize the 'Lethal Collision': the formation of the cis-dianion forces the bulky, negatively charged carboxylate groups out of the alkene's plane to relieve steric and electrostatic strain.

This destroys the resonance stabilization of the conjugate base, making $pK_{a2}$ of maleic acid anomalously high compared to its trans-isomer.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Equilibrium - Class 11 Part 1
Concept	Polyprotic Acid Dissociation
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free