Final answer:
An imine group forms in an acid-catalyzed reaction between a secondary amine and a ketone, resulting from a nucleophilic addition followed by dehydration.
Step-by-step explanation:
The functional group that forms in an acid-catalyzed reaction between a secondary amine and a ketone is an imine. Imines are compounds that have a nitrogen atom bonded to one side of a carbonyl group. In the case of a reaction between an amine and a ketone under acidic conditions, the ketone's oxygen is replaced by the nitrogen from the secondary amine, resulting in the formation of an imine. This reaction is one way in which amines can engage in a nucleophilic addition to the carbonyl group, followed by a dehydration to form the imine linkage, -C=N-.