See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Concept: The reaction shown is imine (Schiff base) formation, where an aldehyde reacts with a primary amine (R-NH2) to form an imine (R-CH=N-R) with loss of water. The mechanism involves two key steps: (1) nucleophilic addition of the amine to the carbonyl to form a carbinolamine (hemiaminal) intermediate, and (2) acid-catalyzed dehydration of the carbinolamine to give the imine product. Reasoning: The overall rate of imine formation depends on the balance between two competing requirements: - Step 1 (nucleophilic addition): Requires the amine to be in its free base (unprotonated) form to act as a nucleophile. At very low pH (strongly acidic conditions, pH 1-2), the amine is fully protonated (R-NH3+) and cannot attack the carbonyl, so this step is slow. - Step 2 (dehydration of carbinolamine): Requires acid catalysis (protonation of the hydroxyl group to facilitate its departure as water). At high pH (basic conditions, pH 10-11 or 13-14), there is insufficient acid to protonate the OH, so dehydration is slow. Optimal pH: A mildly acidic pH of 4-5 provides the best balance: there is enough free amine (not fully protonated) for nucleophilic addition, and enough acid to catalyze the dehydration step. This mild acidity maximizes the overall reaction rate and yield. Why other options fail: - pH 1-2: Amine is completely protonated, nucleophilic addition step is severely inhibited. - pH 10-11: Insufficient acid for dehydration; carbinolamine accumulates and does not convert to imine. - pH 13-14: Strongly basic, dehydration step essentially does not occur. Therefore, the correct answer is B.