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Question #43995

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The Question

Glycine ( $\\ce{H2N-CH2-COOH}$ ) and Propanoic acid ( $\\ce{CH3-CH2-COOH}$ ) are independently subjected to Fischer esterification using absolute ethanol and excess dry $\\ce{HCl}$ gas at $298\\text{ K}$ . The rate of esterification of glycine is observed to be substantially lower than that of propanoic acid. Which of the following statements correctly explains this kinetic observation?

Options

A.
The strong $-I$ effect of the adjacent $-\\ce{NH3+}$ group decreases the basicity of the carbonyl oxygen, drastically reducing the equilibrium concentration of the reactive oxonium intermediate.
B.
The $-\\ce{NH3+}$ group exerts a strong $-I$ effect, which decreases the electrophilicity of the carbonyl carbon, thereby preventing the nucleophilic attack by ethanol.
C.
Glycine exists exclusively as a zwitterion in strongly acidic media, trapping the carboxylate group in a resonance-stabilized form that cannot undergo nucleophilic addition.
D.
The $-\\ce{NH3+}$ group increases the $pK_a$ of the carboxylic acid, shifting the pre-equilibrium towards the unprotonated carboxylate, which is unreactive towards ethanol.

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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

Esterification Reversibility|Amino Acid Zwitterion pH Sensitivity

Why This Question Is Tricky

Common Mistakes

This question tests the 'Hidden Variable' of pre-equilibrium intermediate concentration in acid-catalyzed reactions.

Students often apply the heuristic that 'Electron Withdrawing Groups (EWGs) increase the rate of nucleophilic attack by increasing electrophilicity'.

However, they overlook that in Fischer esterification, the reaction strictly requires the protonation of the carbonyl oxygen first.

The powerful $-I$ effect of the $-\\ce{NH3+}$ group drastically lowers the basicity of this oxygen.

The resulting negligible concentration of the protonated oxonium intermediate kinetically starves the reaction, overriding the theoretical increase in electrophilicity of the carbon center.

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