Push-pull ethylenes activation entropies

One should expect the activation entropy (AS ) to C=C rotation in Case 1 push-pull ethylenes to be negative, since the increase in polarity in the transition state should increase the order in the solvated structure. The effect should increase with increasing difference in polarity between ground and transition states, and also with increasing solvent polarity. These expectations have been completely borne out by experiments (78,140,143), as Table 22 shows. Contrary to what is generally found for conformational processes (144), AS values -20 e.u. are frequently found for C=C rotation in push-pull systems. 

Activation Enthalpies (A//, kcal/mol) and Entropies (AS, cal/mol K) for Rotations Through Perpendicular (it) and Planar (S) Transition states (TS) in Case 1 and Case 2 Push-Pull Ethylenes... 

Table XIII gives typical examples of 7t-barriers for planar heterocycles comparing the electron-attracting or electron-donating moiety to push-pull ethylenes. Steric effects destabilize the ground state and thus greatly reduce the 71-barriers to rotation. In these push-pull ethylenes the large entropy of activation and solvent effects hampered easy comparison of the barriers without high-quality determination of the activation parameters.

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