1‑Dimensional Selective Nuclear Overhauser Effect NMR Spectroscopy To Characterize Products from a Two-Step Green Chemistry Synthesis

• Published in: Journal of chemical education 2018-04, Vol.95 (4), p.641-647
• Main Authors: Hopson, Russell, Lee, Po Yin Bowie, Hess, Kathleen M
• Format: Article
• Language: English
• Subjects: Ammonium; Ammonium bromides; Bromination; Chemical synthesis; College students; Discovery Learning; Green chemistry; Laboratories; Laboratory Experiments; Laboratory tests; NMR spectroscopy; Organic Chemistry; Overhauser effect; Science Experiments; Science Laboratories; Software; Spectroscopy; Spectrum analysis; Students; Synthesis; Toluidine; Undergraduate Students
• ISSN: 0021-9584; 1938-1328
• DOI: 10.1021/acs.jchemed.7b00494

One dimensional (1-D) 1H and 13C NMR experiments are common tools used in undergraduate organic laboratories to characterize synthesized molecules. However, 1-D NMR spectra cannot always provide unambiguous structure determination. In research laboratories, advanced NMR spectroscopy methods are employed for structural assignments of synthesized molecules. This experiment is designed to introduce undergraduate students to the application of an advanced method of 1H NMR spectroscopy, the 1-D selective nuclear Overhauser effect (NOE) experiment, to fully characterize the structure of two compounds. The experiment features a two-step green chemistry synthesis to produce the two compounds. In the first step of the synthesis, 4-methylaniline (para-toluidine) is acetylated to form 4′-methylacetanilide (para-acetotoluidide). The 1-D selective NOE method is used to identify the two different methyl groups present in 4′-methylacetanilide and to assign the aromatic protons. For the second step of the synthesis, bromination of the aromatic ring of 4′-methylacetanilide is accomplished using Oxone and ammonium bromide in an aqueous solvent. The 1-D selective NOE method is applied to determine the regioselective product of the bromination reaction. The 1-D selective NOE experiment is readily accessible and easily implemented. Successful interpretation of the 1-D selective NOE results leaves students confident in making structural assignments that would otherwise be based solely on theory or software prediction.