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.