Using Experimental Spectroscopic Data to Guide and Validate Mechanisms in Catalyzed Aldol Reactions

Abstract

Organic chemistry students often struggle with reaction mechanisms, particularly in how they are proposed and justified. In this activity targeting second year organic undergraduates, students used infrared spectroscopy (IR) to track the reaction progress of two distinct aldol reactions and used polarimetry to analyze the stereoselectivity of aldol catalysts. Students worked in two pairs, with one focusing on the traditional hydroxide-catalyzed aldol reaction (two units of propionaldehyde combining via an enolate intermediate) and the other focusing on the enantioselective L-proline-catalyzed aldol reaction (propionaldehyde catalyzed by Lproline, showing an iminium intermediate). During the course of the lab period, students used IR spectra showing kinetic data and guided questions to propose and validate the reaction mechanisms. After the pairs of students analyzed their individual reactions, they formed groups of four to further analyze and compare the two mechanisms. This comparison of IR and polarimetry data allowed students to discuss both pathways and consider why chemists use different reaction conditions to reach the same product. The focus of this experiment is to improve the understanding of reaction mechanisms and the process by which scientists propose and justify mechanisms, while giving students practical experience with IR spectroscopy, polarimetry, and intermediate analysis.

Reduction of Vanillin Isobutyrate: An Infrared Spectroscopy Structure Determination Experiment

“…1 Many fewer laboratory experiments rely solely on infrared spectroscopy for structure determination. 2,3 In our year-long organic chemistry sequence, infrared spectroscopy is taught in the first semester while mass spectrometry and NMR spectroscopy are introduced in the second semester. We usually include an IR-based structure determination experiment in the first semester laboratory.…”