Which of the following, on heating with pyrrolidine and a trace of acid with water removed, can give an enamine?

Abenzaldehyde
Bcyclohexanone
Cpropanal
D2,2-dimethylpropanal

Answer: B, C

An enamine needs an alpha C-H that can be lost from the iminium ion. Cyclohexanone and propanal have alpha hydrogens. Benzaldehyde and 2,2-dimethylpropanal have no alpha hydrogen, so only an iminium ion would form.

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

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