Chemical shift prediction programs are used increasingly in teaching NMR spectral interpretation. However, these have standard errors of 1 – 3 ppm for 13C and 0.2-0.5 ppm for 1H spectra and therefore are unreliable for assigning singlets for carbons and hydrogens which have no coupling information. The chalcone synthesis, prepared by condensing benzaldehyde derivatives with substituted acetophenones, is widely used in general organic chemistry lab courses. One approach to identify the chalcone formed is by NMR spectroscopy. Utilizing methyl substituted acetophenones forces the students to analyze closely spaced singlet NMR signals to correctly assign the peaks of the starting materials and products. NMR prediction programs are unreliable for closely spaced signals and thus students must rely on alternative NMR data to correctly assign the peaks. For example, for the methyl substituted acetophenones, the height of the ring methyl group is less than that of methyl of the acetyl group. In this presentation, we will discuss the use of the 1H coupled 13C and CH COSY experiments to correctly assign the signals of closely spaced singlet NMR signals for use in the chalcone experiment as well as other general organic chemistry laboratory experiments. Using these approaches students will gain an understanding on how to correctly determine the chemical shift of closely spaced signals.

Utilizing NMR experiments to correctly assign closely spaced singlets in NMR spectra: A method to introduce 2D NMR spectroscopy in general organic chemistry laboratory courses

253rd National Meeting (2017)

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