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Question #44383
The Question
Consider the following aromatic carboxylic acids (typical products of Kolbe-Schmitt type reactions): I. \ce{2-Hydroxybenzoic acid} II. \ce{2,4-Dihydroxybenzoic acid} III. \ce{2,4,6-Trihydroxybenzoic acid} IV. \ce{2,6-Dihydroxybenzoic acid} V. \ce{3-Hydroxybenzoic acid}
These acids undergo acid-catalyzed decarboxylation via an mechanism (electrophilic attack of at the ipso-carbon). Let be the rank of \ce{2,4-Dihydroxybenzoic acid} when the above acids are arranged in decreasing order of their decarboxylation rate (i.e., Rank 1 = fastest). Let be the total number of oxygen atoms in the most reactive acid among the given list. Let be the number of electrons delocalized in the arenium ion (Wheland intermediate) formed during the rate-determining step of this decarboxylation. Calculate the value of .
Options
- A.
15
- B.
17
- C.
19
- D.
21
Check Your Answer
Correct Answer
Step-by-Step Solution
Explanation
How the Examiner Thinks
Mechanism_Trap
Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.
Concepts Behind This Question
Phenol Acidity vs. Substituents|Kolbe-Schmitt Reaction Mechanism|Decarboxylation Mechanisms
Why This Question Is Tricky
Common Mistakes
The trap tests the kinetic vs thermodynamic stability of Kolbe-Schmitt products.
Students often assume para-substituents activate the ring more than ortho-substituents due to less steric hindrance, incorrectly ranking 2,4-dihydroxybenzoic acid as faster than 2,6-dihydroxybenzoic acid.
However, in acid-catalyzed decarboxylation, the two ortho-OH groups in the 2,6-isomer cause 'steric acceleration' (forcing the -COOH out of plane, raising ground-state energy) while providing strong +M stabilization to the intermediate, making it faster than the 2,4-isomer.
Additionally, students must correctly identify that the arenium ion intermediate contains exactly 4 electrons, avoiding the trap of assuming a fully aromatic 6 electron transition state.
Quick Facts About This Question
Question Information
| Subject | Chemistry |
| Chapter | Alcohols, Phenols, and Ethers - Class 12 |
| Concept | Phenol Acidity vs. Substituents |
| Difficulty | 8 |
| Cognitive Load | 7 |
| Target Solve Time | 160 sec |
| Access | Free |