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Question #43990
The Question
Consider the following carboxylic acids: I. II. III. IV.
Which of the following options correctly identifies the decreasing order of their thermodynamic acidity () in aqueous solution at 298 K, and the decreasing order of their kinetic rate of thermal decarboxylation?
Options
- A.
Acidity: I > IV > II > III ; Decarboxylation Rate: III > IV > II > I
- B.
Acidity: I > IV > II > III ; Decarboxylation Rate: IV > III > II > I
- C.
Acidity: I > IV > II > III ; Decarboxylation Rate: I > IV > II > III
- D.
Acidity: IV > I > II > III ; Decarboxylation Rate: III > IV > II > 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
Carboxylic Acid Strength|Decarboxylation Mechanisms|Alpha-Hydrogen Acidity
Why This Question Is Tricky
Common Mistakes
The problem tests the 'Lethal Collision' between thermodynamic acidity and kinetic decarboxylation rate.
Students often conflate the two, assuming a stronger acid will decarboxylate faster.
It exposes two specific exceptions: 1) The pericyclic 6-membered cyclic transition state of -keto acids bypasses high-energy charged intermediates, making the weakest acid (III) the fastest to decarboxylate. 2) The dichotomy of Halogen electronic effects: Fluorine's superior electronegativity makes a stronger acid than (charge on Oxygen), but Chlorine's d-orbitals stabilize the carbanion transition state during decarboxylation, making decarboxylate faster than (charge on Carbon).
Quick Facts About This Question
Question Information
| Subject | Chemistry |
| Chapter | Aldehydes, Ketones, and Carboxylic Acids - Class 12 |
| Concept | Carboxylic Acid Strength |
| Difficulty | 8 |
| Cognitive Load | 7 |
| Target Solve Time | 160 sec |
| Access | Free |