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 α‐alkyl‐ and α‐halo‐substituted N ‐theyl‐ N ‐methylamides were determined by 1 H NMR spectroscopy at coalescence temperature. Plots of Gibbs free energy (Δ G °) and Gibbs energy of activation (Δ G ‡) values against Charton's electronic and steric parameters of the α‐carbonyl groups allows the conclusion that both the isomeric preference and the barrier to rotation about the C(O)N bond of the N ‐ethyl‐ N ‐methylamides depend mainly on the steric effect of the α‐carbonyl group.