DETERMINATION OF THE ACIDITY OF LEWIS-ACIDS

The chemical shift (H-3) of crotonaldehyde complexed by a Lewis acid provides a measure of its strength as a catalyst, according to R. F. Childs, D. L. Mulholland, and A. Nixon, (Can. J. Chem. 1982, 60, 801-808). A theoretical justification was highly desirable and is provided here: the experimental method works because of the linear dependence of the H-3 chemical shift on the energy of the lowest π⋆ MO in the complex. While the experimental determination has some drawbacks (problems of precise concentration, solubility, impurities, and availability), MO calculations appear trustworthy and have some advantages for the estimation of the catalytic activity of a Lewis acid. © 1990, American Chemical Society. All rights reserved.