Question

I'm studying organic chemistry, more specifically the imines and enamines. I've read about the mechanism of formation of enamine from a generic ketone and a question popped up in my mind. The mechanism proposed by my book (and everywhere else) goes from the ketone, to the iminium ion when the water molecule goes out, and then to the enamine by the the breaking of the C=N double bond. Now, I've studied in the past the reactions of alcohols, and I remembered that a tertiary alchol can undergo by dehydration as an elimination reaction (E1 or E2), forming as a consequence a C=C bond. Now going back to the enamine formation where the water molecule is present, can't the water go out with the direct formation of the C=C bond forming the enamine by conseguence? Is the choice of the mechanism determinated by some kind of kinetics or spectroscopic study that confirm the presence of the iminium ion?

Answer 1

The formation of enamines from ketones typically involves an iminium ion intermediate, as this pathway is likely supported by kinetic and spectroscopic studies indicating the transient existence of the iminium ion. Direct elimination to form the enamine is less favored, and the stability of intermediates such as tertiary carbocations significantly influences the pathway of a reaction.

Step-by-step explanation:

The formation of an enamine from a ketone involves the formation of an iminium ion intermediate, rather than the direct formation of the enamine by elimination of water. This pathway is favored because the iminium ion is a highly reactive species, which facilitates the subsequent nucleophilic attack by the amine's lone pair to form the C=N double bond, completing the enamine formation.

The preference for this mechanism over a direct elimination akin to the dehydration of tertiary alcohols is likely determined through kinetic and spectroscopic studies which support the transient existence of the iminium ion.

Moreover, the carbocation stability and the likelihood of forming stable intermediates play a critical role in the mechanism that a reaction follows, with tertiary carbocations being stable enough to facilitate elimination reactions like E1 and E2.