Using 1H NMR Spectroscopy to Investigate the Diastereoselectivity of Benzaldehyde Pinacol Coupling Mediated by Al-KOH in Aqueous Media: An Undergraduate Lab Experiment Involving a Green Carbon–Carbon Bond-Forming Reaction Affording meso- and dl-Hydrobenzoins

Abstract

In this second-semester exploratory lab experiment, undergraduate students are introduced to the more than a century-old reductive pinacol coupling reaction of aldehydes and ketones as a powerful carbon–carbon bond forming reaction. Students learn that this reaction is often mediated by an active electron donor metal, such as Na, Mg, or Al, which produces a radical anion known as a ketyl that dimerizes via a carbon–carbon bond to yield a 1,2-diol (also known as a vicinal diol). Students perform this reaction on benzaldehyde using Al-KOH in an aqueous medium as a “green” route to the vicinal diol 1,2-diphenylethane-1,2-diol (also known as hydrobenzoin), which exists in meso- and dl-diastereomeric forms. These diols are widely used as ligands, chiral auxiliaries, and synthetic intermediates and have received particular attention owing to their diverse applications. Students then use ¹H NMR spectral analysis to experimentally establish the diastereoselectivity (dl:meso ratio) associated with this reaction by obtaining the ¹H NMR spectrum of their product and comparing it with the NMR spectra of authentic meso-hydrobenzoin and authentic dl-hydrobenzoin. Specifically, they analyze the NMR spectra of individual samples of their pinacol coupling product spiked with authentic meso-hydrobenzoin as well as authentic dl-hydrobenzoin and benzyl alcohol (the latter being a possible byproduct) and look for changes in the intensity of the benzylic proton resonances that appear in the NMR spectra.