Effects of solute–solvent interactions and acid catalysis on the rotational barriers in polarized ethylenes. The significance of large entropies of activation as measured by dynamic NMR

Abstract

Complete ¹H NMR bandshape analysis has been used to study the rotational barrier around the carbon–carbon double bond in two types of polarized ethylenes; on the one hand for planar ketene mercaptals (A), for which the hindered rotation over the 90 degrees twisted transition state is measured and, on the other hand, for sterically hindered ketene aminals (B and C) which are twisted in the ground state, and have measurable barriers over a planar, sterically crowded transition state. For compounds A activation entropies of the order of −45 to −76 J mol⁻¹ K⁻¹ (−11 to −8 cal mol⁻¹ K⁻¹), and for compounds B and C +28 to +42 J mol⁻¹ K⁻¹ (+7 to +10 cal mol⁻¹ K⁻¹) were obtained. This important difference was interpreted in terms of opposite charge development during the course of the rotation in the two cases, with concomitant reversed changes in the orientations of the solvent. The effects of solvent polarity and the presence of acid are shown to be reversed for the two types of compounds. A more polar solvent, and, in particular, addition of small amounts of acid decrease the barrier in system A but increase the barrier in systems B and C. A large negative entropy of activation was found for the acid‐catalyzed process, ΔS^≠ = −107 J mol⁻¹ K⁻¹ (−25.5 cal mol⁻¹ K⁻¹). This low value is briefly discussed in relation to the mechanism of the acid‐catalyzed rotation.