A teaching module for the undergraduate organic chemistry lab course at FAU recapitulates recent research in the Lepore group to elucidate a novel cation−π interaction between tetraalkylammonium cations and propargyl hydrazines. This novel noncovalent interaction plays a key role in explaining an ammonium-catalyzed intramolecular cycloaddition of hydrazine nitrogen to the terminal alkyne carbon of a propargyl substrate. In this teaching experiment, our current mechanistic conclusion featuring this ammonium-alkyne interaction is presented as an hypothesis along with a rival hypothesis that assumes ammonium acts only as phase transfer agent. In this presentation, we discuss how the lab module includes reactions and control experiments to help students decide between the two competing hypotheses. Students set up cyclization reactions promoted by a soluble base with and without tetrabutylammonium bromide under homogeneous conditions. They observe that the added ammonium salt accelerates the reaction and analyze whether this finding rules out either of the mechanistic hypotheses based on the logical implications of each. IR spectroscopy is then employed to observe directly a dose-dependent shift of the alkyne-stretching mode due to a cation−π interaction. This module also compares and contrasts approaches based on falsification where the mechanism is interrogated indirectly through rate experiments relative to experiments involving direct corroboration with spectroscopic techniques. Student learning outcomes and feedback of this lab module will also be presented.

Teaching experiment to illustrate experimental design in the investigation of a mechanism involving a novel ammonium-alkyne interaction

2019 Spring National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.