Question

Imine formation using an aldehyde/ketone and primary amine is acid-catalyzed, yet the rate drops below $\text{pH =}4-5$ . Why does the rate drop below this $\text{pH}$ ?

• A. The carbinolamine intermediate is stable at low pH
• B. The imine product is hydrolyzed at low pH
• C. Protonation of the amine decreases its nucleophilicity
• D. The amine is hydrolyzed at low pH

Answer 1

Answer: (C)

Imine formation:

The reaction is possible at pH $4\text{to}5$ only. Hence, the reaction is pH dependent.
At very low pH (high conc. of $H^{+}$ ): ammonia derivative will form their respective salts due to their basic nature and at the same time will lose their nucleophilic nature.
At very high pH (high conc. of $O{H}^{-}$ ): carbonyl group will not be able to undergo sufficient protonation.