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Question #44441

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The Question

2,2,2-Trifluoroacetaldehyde exists predominantly as its gem-diol hydrate, $\ce{CF3CH(OH)2}$ , in aqueous solution at 298 K ( $K_{hyd} \approx 2.8 \times 10^4$ ). The synthesis of its Schiff base via reaction with aniline ( $\ce{PhNH2}$ , $pK_a$ of $\ce{PhNH3+}$ = 4.6) exhibits a bell-shaped rate-pH profile, with the maximum rate of imine formation occurring at pH $\approx$ 4.5.

Which of the following statements correctly identifies the dominant arrested species and the kinetic bottleneck at the extreme pH conditions of 1.0 and 7.0?

Options

A.
At pH = 1.0: The system arrests as $\ce{CF3CH(OH)2}$ and $\ce{PhNH3+}$ ; the bottleneck is the absence of a free nucleophile. At pH = 7.0: The system arrests at the carbinolamine, $\ce{CF3CH(OH)(NHPh)}$ ; the bottleneck is its acid-catalyzed dehydration.
B.
At pH = 1.0: The system arrests at the protonated hydrate, $\ce{CF3CH(OH)(OH2+)}$ ; the bottleneck is its dehydration to the free aldehyde. At pH = 7.0: The system arrests as $\ce{CF3CH(OH)2}$ and $\ce{PhNH2}$ ; the bottleneck is the direct nucleophilic displacement of $\ce{OH-}$ from the gem-diol.
C.
At pH = 1.0: The system arrests at the protonated carbinolamine, $\ce{CF3CH(OH2+)(NHPh)}$ ; the bottleneck is the elimination of water due to the electron-withdrawing $\ce{CF3}$ group. At pH = 7.0: The system arrests as the zwitterion $\ce{CF3CH(O^{-})(NH2+Ph)}$ ; the bottleneck is the intramolecular proton transfer.
D.
At pH = 1.0: The system arrests as free $\ce{CF3CHO}$ and $\ce{PhNH3+}$ ; the bottleneck is the deprotonation of the anilinium ion. At pH = 7.0: The system arrests at the imine $\ce{CF3CH=NPh}$ ; the bottleneck is the thermodynamic instability of the Schiff base in neutral water.

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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

Hydrate Stability vs. Carbonyl|Schiff Base pH Dependence

Why This Question Is Tricky

Common Mistakes

This question tests the student's understanding of the pH dependence of Schiff base formation, specifically the transition of the rate-determining step (RDS) across different pH regimes.

A common trap is assuming that neutral pH is optimal because the nucleophile is fully deprotonated, ignoring the necessity of acid catalysis for the dehydration of the carbinolamine intermediate.

Another trap is misunderstanding the state of the electrophile at low pH; students might assume the hydrate dehydrates to the free aldehyde and reacts, forgetting that the amine nucleophile is completely quenched via protonation.

The inclusion of the strongly electron-withdrawing CF3 group further emphasizes the stability of the hydrate and the necessity of acid to force the dehydration of the carbinolamine.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Aldehydes, Ketones, and Carboxylic Acids - Class 12
Concept	Hydrate Stability vs. Carbonyl
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free