Entropy dipolar effect

A decrease in the activation energy AG is certainly a major effect. Because of the contribution of enthalpy and entropy to the value of AG (= AH -TAS ), it might be predicted that the magnitude of the -TAS term would increase in a microwave-induced reaction, because organization is greater than with classical heating, as a consequence of dipolar polarization. 

Short-lived organic radicals, electron spin resonance studies of, 5, 53 Small-ring hydrocarbons, gas-phase pyrolysis of, 4, 147 Solid state, tautomerism in the, 32, 129 Solid-state chemistry, topochemical phenomena in, 15, 63 Solids, organic, electrical conduction in, 16, 159 Solutions, reactions in, entropies of activation and mechanisms, 1, 1 Solvation and protonation in strong aqueous acids, 13, 83 Solvent effects, reaction coordinates, and reorganization energies on nucleophilic substitution reactions in aqueous solution, 38, 161 Solvent, protic and dipolar aprotic, rates of bimolecular substitution-reactions in,... 

The effects of the solvent and finite temperature (entropy) on the Wittig reaction are studied by using density functional theory in combination with molecular dynamics and a continuum solvation model.21 The introduction of the solvent dimethyl sulfoxide causes a change in the structure of the intermediate from the oxaphosphetane structure to the dipolar betaine structure. 

The effect of transfer from dipolar aprotic to protic solvent on the enthalpy and entropy of activation of Su2 reactions is a topic which is still in its infancy. Up till now, the discussion in this article has been in terms of free energies, but much more can be learnt about solvent effects on rate, if changes in the entropy and enthalpy of solvation of reactants and transition states are known and understood. Arnett and McKelvey (1966) have reported some preliminary measurements of enthalpies of transfer from water to DMSO for some ions and molecules, but as yet neither these, nor measurements of entropies and enthalpies of activation, are sufficiently extensive for any calculation of entropies and enthalpies of transfer for 8 2 transition states. 

This mechanism accounts for the observed primary isotope effect, the slow rate of exchange, and the large negative entropies of activation. It receives additional support from the observation that, for potassium rer -butoxide catalyzed-isomerization of 1-butene in dipolar aprotic solvents, log/cj is proportional to the reciprocal of the dielectric constant of the solvent. ... 

The weakest interaction is that occurring between a permanent dipolar molecule and a dipole induced by the electric field. The London forces, part of the van der Waals interactions, are the weakest intermolecular forces. Being the weakest forces does not mean that they have no importance and/or no significant role to play in molecular behavior these forces are, for example, responsible for the hydrophobic effect that is responsible for a great part of reversed-phase liquid chromatography (RPLC) compound separations and for entropy-driven forces causing oil to separate from water. 

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