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Question #44383

Difficulty: 8/10 | 🟢 Free

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 SE2S_E2 mechanism (electrophilic attack of H+H^+ at the ipso-carbon). Let NN 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 MM be the total number of oxygen atoms in the most reactive acid among the given list. Let PP be the number of π\pi electrons delocalized in the arenium ion (Wheland intermediate) formed during the rate-determining step of this decarboxylation. Calculate the value of (N×P)+M(N \times P) + M.

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 π\pi electrons, avoiding the trap of assuming a fully aromatic 6 π\pi electron transition state.

Quick Facts About This Question

Question Information

SubjectChemistry
ChapterAlcohols, Phenols, and Ethers - Class 12
ConceptPhenol Acidity vs. Substituents
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree