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 <i>meso

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.