The reaction of an aldehyde or ketone with a secondary amine forms an enamine (double bond between two carbons, one of which is bonded to nitrogen). As with reaction with a primary amine, the reaction of an aldehyde or ketone with a secondary amine begins with the formation of a tetrahedral carbinolamine through nucleophilic addition of the amine to the carbonyl group of the aldehyde or ketone. The carbinolamine is not stable because of the unequal tug of war across the central carbon between the two electronegative elements oxygen and nitrogen. The central carbon is like an electron deficient rope. In the reaction involving a primary amine, the bonding electrons shift to the central carbon from nitrogen, which loses a proton, with oxygen then departing; the resulting double bond between nitrogen and carbon characterizes an imine. However, when the addition initially involved a secondary amine, nitrogen has no hydrogen substituents after addition. In this case the extreme electron deficiency of the central carbon is satisfied when a hydrogen departs from an adjacent carbon forming a double bond between the two carbons, an enamine form, with oxygen departing as a result. The second step is similar to E2 elimination with alkyl halides. In imine formation the new bonding electrons arise from a N-H bond, while with enamine formation their source is a C-H bond. Further reduction of an imine with H2 leads to forming an amine (the overall conversion of an aldehyde or ketone to an amine is called reductive amination).
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