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Question #45107

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The Question

Consider the Fischer esterification of the following cyclic carboxylic acids with methanol in the presence of catalytic \ce{H2SO4} at 298 K: (I) Cyclopropanecarboxylic acid (II) Cyclopentanecarboxylic acid (III) Cyclohexanecarboxylic acid (IV) 1-Methylcyclohexanecarboxylic acid

Let $k_1, k_2, k_3,$ and $k_4$ be their respective rate constants. Which of the following correctly represents the order of their reaction rates, and what is the primary mechanistic reason for the anomalously low rate of (I) compared to (II) and (III)?

Options

A.
$k_2 > k_3 > k_1 > k_4$ ; The rate of (I) is retarded because the $sp^2 \rightarrow sp^3$ hybridization change at the carbonyl carbon disrupts the stabilizing pseudo-\pi conjugation with the ring and increases I-strain in the transition state.
B.
$k_3 > k_2 > k_1 > k_4$ ; The rate of (I) is retarded because the cyclopropane ring exerts a strong $+I$ effect, which decreases the electrophilicity of the carbonyl carbon.
C.
$k_1 > k_2 > k_3 > k_4$ ; The rate of (I) is the highest because the high angle strain of the cyclopropane ring is relieved when the carbonyl carbon transitions to the $sp^3$ hybridized tetrahedral intermediate.
D.
$k_2 > k_1 > k_3 > k_4$ ; The rate of (I) is moderately high, but lower than (II) due to the formation of a non-classical carbocation intermediate which undergoes ring expansion.

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

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Concepts Behind This Question

Bond Angle and Strain|Esterification Steric Hindrance

Why This Question Is Tricky

Common Mistakes

The question tests the 'Chemical Exception' of cyclopropanecarboxylic acid in esterification.

Students often assume that smaller rings have less steric bulk and should react faster (predicting $k_1 > k_2 > k_3$ ).

However, they overlook two 'Hidden Variables': 1) The pseudo-\pi conjugation of the cyclopropane ring with the carbonyl group, which stabilizes the ground state and is lost in the $sp^3$ transition state. 2) The concept of I-strain, where the rigid geometry of the small ring strongly resists the bond angle compression required to form the tetrahedral intermediate.

This dual effect makes cyclopropanecarboxylic acid react exceptionally slowly.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Organic Chemistry – Some Basic Principles and Techniques - Class 11 Part 2
Concept	Bond Angle and Strain
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free