Studies in nuclear magnetic resonance—IX. Rotational barriers in substituted N,N‐dimethylbenzamides

Abstract

AbstractThe barriers to rotation about the CN bond in eighteen substituted N,N‐dimethylbenzamides have been determined by complete line shape analysis of the NMR spectra of the N,N‐dimethyl protons. The barriers have been correlated with the substituent constants σ and σ+. It has been shown that polar solvents increase the barrier in N,N‐dimethylbenzamide.Acid catalysis of rotation about the amide CN bond in N‐(p‐N,N‐dimethylcarboxamidobenzyl)‐pyridinium bromide has been investigated. 18O exchange studies show that catalysis is due to N‐protonation rather than the formation of a tetrahedral intermediate. The rate of rotation is a function of the Hammett acidity function, H0, and the water activity, and it is shown that proton exchange between the N‐ and O‐protonated species involves the intermediacy of a water molecule.The differences in chemical shifts for the non‐equivalent N, N‐dimethyl groups of the benzamides are also a function of the substituents. Possible explanations of this phenomenon are discussed.