Distinguishing Vinylic and Aromatic ~1H NMR Signals Using Selectively Deuterated Chalcones

摘要

~1H NMR spectroscopy is a molecular characterization technique that is ubiquitously used in research and teaching laboratories alike. Strategic deuteration of molecules is a commonly employed technique in the deconvolution of ~1H NMR signals. Here this approach is applied in a second-year chemistry course in which students synthesize 4-methoxychalcone using the Claisen? Schmidt variant of the foundational aldol reaction. The ~1H NMR spectrum is complicated by a crowded aromatic and alkene region due in part to extensive conjugation through the two phenyl groups surrounding an α,β-unsaturated ketone. This report demonstrates that students gain competence and confidence in making ~1H NMR assignments when presented with a curated set of selectively deuterated chalcones rather than the corresponding spectrum of the fully protiated chalcone. While deuteration does not resolve all of the challenges associated with interpreting this spectrum, it is an invaluable tool to complement chemical shifts (including the predictive nature of resonance structures), integration, and spin?spin splitting. A proposed lecture is presented that is timed to correlate with the performance of the chalcone synthesis lab. Assessment data collected before and after the lecture and then again on the final exam point to the effectiveness of this approach.