For the reaction of a ketone with a primary amine, the rate is measured as a function of pH. Above about pH 6, the rate falls as pH rises. The step that is rate determining in this region is

Anucleophilic attack of the amine on the carbonyl carbon
Bprotonation of the amine nitrogen
Cacid-catalysed dehydration of the carbinolamine
Dhydrolysis of the imine product

Answer: C

At higher pH the amine is free and attacks the carbonyl rapidly, so addition is not limiting. The loss of water from the carbinolamine needs protonation of its OH, so it slows as

[H X^{+}]

falls and becomes rate limiting. Below pH 4 the amine protonation makes the first step limiting instead.

Practice Organic Reaction Mechanisms I: Nucleophiles and Carbonyl Addition More on Formation of acetals, imines and enamines

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For the reaction of a ketone with a primary amine, the rate is measured as a function of pH. Above about pH 6, the rate falls as pH rises. The step that is rate determining in this region is

Answer: C

More questions on Formation of acetals, imines and enamines