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Question #43953

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The Question

Consider the formation of Schiff bases from benzaldehyde with two different aromatic amines at $298\text{ K}$ in an aqueous ethanolic buffer:

Reaction I: $\ce{Ph-CHO} + \ce{Ph-NH2} \rightleftharpoons \text{Imine 1} + \ce{H2O}$ Reaction II: $\ce{Ph-CHO} + \ce{2,6-(CH3)2C6H3NH2} \rightleftharpoons \text{Imine 2} + \ce{H2O}$

For Reaction I, the optimal pH is approximately $4.5$ , and the rate-determining step (RDS) is typically the acid-catalyzed dehydration of the hemiaminal intermediate. Due to the 'Ortho-Effect' and steric constraints in Reaction II, how do the optimal pH profile and the RDS change compared to Reaction I?

Options

A.
Optimal pH shifts to $< 4.5$ ; RDS is the nucleophilic attack of the amine.
B.
Optimal pH shifts to $> 4.5$ ; RDS is the dehydration of the hemiaminal.
C.
Optimal pH shifts to $< 4.5$ ; RDS is the dehydration of the hemiaminal.
D.
Optimal pH shifts to $> 4.5$ ; RDS is the nucleophilic attack of the amine.

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Exception_Exposure

Learn to identify edge cases and boundary exceptions that JEE examiners hide in math problems.

Concepts Behind This Question

Schiff Base pH Dependence|Ortho-Effect in Anilines

Why This Question Is Tricky

Common Mistakes

This question tests the intersection of kinetics, pH-dependence, and stereoelectronic effects (the Ortho-Effect).

Students typically memorize that imine formation has an optimal pH of 4.5 and that dehydration is the RDS.

The trap lies in failing to recognize that ortho-substitution alters both the basicity of the amine (shifting the optimal pH to a more acidic region because the amine's conjugate acid pKa is lower) and the steric profile of the transition state (shifting the RDS to the nucleophilic attack due to the massive steric penalty of forming the tetrahedral intermediate).

Quick Facts About This Question

Question Information

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