The unexpected product that results from the Diels-Alder reaction of (E,E)-2,4-hexadien-1-ol with maleic anhydride provides undergraduate organic students with an intriguing puzzle that they can solve using careful spectral analysis and mechanistic reasoning. (See McDaniel, K.F. and Weekly, R.M. J. Chem. Educ., 1997, 74, 1465-1467.) We have extended this experiment by having students perform a detailed molecular modeling analysis in order to determine the major conformer of the product. Examination of physical models show two potential conformers, corresponding to a half-chair flip in the cyclohexene ring of the product; but because of the complexity of the system it is not obvious which conformer will predominate. Students perform geometry optimizations of the two conformers using the density functional EDF2/6-31G* method and compare their relative energies. They calculate IR vibrational frequencies and NMR shifts (both carbon and proton) at the EDF2/6-31G* level for each conformer and compare these computational results to the actual spectral data. In addition, they use their two EDF2-optimized conformer structures to measure dihedral angles between hydrogens involved in 3-bond H-H couplings and then use a Karplus equation to predict the expected coupling constants in each conformer. They also measure distances between appropriate hydrogens in their model structures and determine which conformer has distances that are most consistent with H-NMR NOE data. Students learn the synergistic power of combining computational and experimental analyses to provide convincing evidence for the 3-dimensional structure of the predominant conformer.

Synergy between computation and experiment: Determining the major conformer of a Diels-Alder reaction product

251st National Meeting (2016)

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