Determination of the Rotational Barrier for Kinetically Stable Conformational Isomers via NMR and 2D TLC
An Introductory Organic Chemistry Experiment
Gregory T. Rushton, William G. Burns, Judi M. Lavin, Yong S. Chong, Perry Pellechia, and Ken D. Shimizu
Department of Chemistry and Biochemistry, University of South Carolina, Columbia, SC 29208
An experiment to determine the rotational barrier about a Caryl–Nimide single bond that is suitable for first-semester organic chemistry students is presented. The investigation begins with the one-step synthesis of a N,N′-diaryl naphthalene diimide, which exists as two room temperature-stable atropisomers (syn and anti). The students then estimate the rotational barrier, ΔG‡, by following the reequilibration of the two isomers using two-dimensional thin-layer chromatography. A more accurate determination of the rotational barrier is conducted by a 1H NMR time study. The synthesis, purification, and kinetics experiments can be carried out within two, three-hour laboratory sessions using commercially available reagents and instrumentation found in most undergraduate laboratories.
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Instructions for the students including pre- and postlab assessment and notes for the instructor are available.
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