EFFECT OF CARBONYL SUBSTITUENTS ON THE BARRIER TO ROTATION IN N-ETHYL-N-METHYLAMIDES

The barriers to rotation about the C(O)-N bond for 13 alpha-alkyl- and alpha-halo-substituted N-theyl-N-methylamides were determined by H-1 NMR spectroscopy at coalescence temperature. Plots of Gibbs free energ y (DELTAG-degrees) and Gibbs energy of activation (DELTAG(double dagge r)) values against Charton's electronic and steric parameters of the a lpha-carbonyl groups allows the conclusion that both the isomeric pref erence and the barrier to rotation about the C(O)-N bond of the N-ethy l-N-methylamides depend mainly on the steric effect of the alpha-carbo nyl group.