PROTON NMR-STUDIES OF SYMMETRICALLY SUBSTITUTED N,N-DIALKYLTRIFLUOROACETAMIDES - MEDIUM EFFECTS

Activation parameters characterizing the internal rotation about the C-N partial double bond in a series of symmetrically N,N-disubstituted trifluoroacetamides (CF3CON(R)2; R = ethyl, isopropyl, and isobutyl) have been determined from exchange-broadened 1H NMR spectra. Gibbs energies of activation, ΔG‡298 (kcal mol-1), are for the gaseous trifluoroacetamides and their 1% CCl4 solutions as follows: diethyl, 16.1 (0.1)/17.8 (0.1); diisopropyl, 15.8 (0.1)/16.3 (0.1); diisobutyl, 16.4 (0.1)/17.3 (0.1). In both the gas and liquid phases rotational barriers decrease with increasing N-alkyl substituent bulk. The phase dependence of these parameters is compatible with expected solvent internal pressure effects. Lower gas-phase activation energies are consistent with a process proceeding via a transition state that has greater steric requirements than the ground-state configuration. © 1990 American Chemical Society.