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Question #43996

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The Question

Consider the acid-catalyzed Fischer esterification of acetic acid ( $\ce{CH3COOH}$ ) with ethanol ( $\ce{C2H5OH}$ ) using anhydrous $\ce{HCl}$ gas as a catalyst at 298 K. The reaction kinetics are studied in two different solvent systems: System I (neat $\ce{C2H5OH}$ , dielectric constant $\epsilon = 24.5$ ) and System II (cyclohexane, $\epsilon = 2.02$ ). The initial rate of esterification in System II is found to be negligible compared to System I. What is the primary mechanistic reason for this kinetic bottleneck in System II?

Options

A.
The higher basicity of ethanol relative to the carboxylic acid carbonyl leads to the formation of a tight $[ \ce{C2H5OH2+} \cdots \ce{Cl-} ]$ ion pair. The low-dielectric solvent cannot solvate the highly polar transition state needed to transfer the proton to the carbonyl oxygen, halting the catalytic cycle.
B.
In cyclohexane, $\ce{CH3COOH}$ exists exclusively as a hydrogen-bonded cyclic dimer. This dimerization lowers the ground state energy of the acid so significantly that the activation energy for protonation by $\ce{HCl}$ becomes insurmountable at 298 K.
C.
The non-polar solvent destabilizes the neutral tetrahedral intermediate, causing it to immediately revert to the reactants rather than undergoing the rate-determining proton transfer required for water elimination.
D.
Anhydrous $\ce{HCl}$ in a low-dielectric medium undergoes homolytic cleavage instead of heterolytic cleavage, initiating a free-radical pathway that fails to activate the carbonyl carbon for nucleophilic attack.

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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

Esterification Reversibility|Basicity-Solvation Competition

Why This Question Is Tricky

Common Mistakes

This question tests the 'Basicity-Solvation Competition' trap in acid catalysis.

Students typically assume that adding an acid catalyst ( $H^+$ ) automatically activates the carbonyl group, ignoring the relative basicity of the reactants.

The hidden variable is the solvent's dielectric constant.

In a non-polar medium, the acid protonates the most basic species (the alcohol nucleophile) to form an unreactive ion pair.

Without a polar solvent to stabilize the highly charged transition state required to protonate the less basic carbonyl oxygen, the reaction is kinetically trapped.

This challenges the rote assumption that $H^+$ always implies a high reaction rate, emphasizing the critical role of solvent-mediated transition state stabilization.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Aldehydes, Ketones, and Carboxylic Acids - Class 12
Concept	Esterification Reversibility
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free