Solvent effects, reaction coordinates, and

Solvent effects, reaction coordinates, and reorganization energies on nucleophilic substitution reactions in aqueous solution, 38, 161... 

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,... 

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, 5,173 Solvent-induced changes in the selectivity of solvolyses in aqueous alcohols and related mixtures, 27, 239... 

Dealkoxycarbonylation of activated esters occurs classically under drastic thermal conditions [90]. It constitutes a typical example of a very slow-reacting system (with a late TS along the reaction coordinates) and is therefore prone to a microwave effect. The rate determining step involves a nucleophilic attack by halide anion and requires anionic activation, which can be provided by solvent-free PTC conditions under the action of microwave irradiation [91]. The above results illustrate the difficult example of cyclic /1-ketoesters with a quaternary carbon atom in the a position relative to each carbonyl group (Eq. 36). 

Reactions in solution proceed in a similar manner, by elementary steps, to those in the gas phase. Many of the concepts, such as reaction coordinates and energy barriers, are the same. The two theories for elementary reactions have also been extended to liquid-phase reactions. The TST naturally extends to the liquid phase, since the transition state is treated as a thermodynamic entity. Features not present in gas-phase reactions, such as solvent effects and activity coefficients of ionic species in polar media, are treated as for stable species. Molecules in a liquid are in an almost constant state of collision so that the collision-based rate theories require modification to be used quantitatively. The energy distributions in the jostling motion in a liquid are similar to those in gas-phase collisions, but any reaction trajectory is modified by interaction with neighboring molecules. Furthermore, the frequency with which reaction partners approach each other is governed by diffusion rather than by random collisions, and, once together, multiple encounters between a reactant pair occur in this molecular traffic jam. This can modify the rate constants for individual reaction steps significantly. Thus, several aspects of reaction in a condensed phase differ from those in the gas phase ... 

Equation (35) is an example of a multiparametric equation17 18 64 for describing solvent effects on equilibria and is similar to the multiparametric equations used to describe solvent effects on rates. The use of these equations in describing solvent effects on equilibria involving coordination compounds is, like their application to reaction rates of coordination compounds, not common. Equilibrium (36), where L, U and X are the same as in reaction (12),... 

While the model employed in the present work provides a reasonable picture of a unimolecular reaction involving a large molecule in solution, other ingredients not considered here may play a role in some systems. The possible role played by intramolecular friction (nonlinear coupling between the reaction coordinate and other nonreactive modes near the barrier) has been discussed in Section IV. Also, the dependence of the molecular potential surface, in particular the activation barrier on the molecule-solvent interaction, may dominate in some cases the observed solvent effect on the rate. Such may be the case (see Section VIII) in a polar solvent when the reaction involves a change in the molecular dipole moment (such as a charge transfer reaction). 

As shown in Scheme 34, a rather profound solvent effect on dienolate alkylation diastereoselectivity has been noted for the steroidal enone (71). Such large solvent effects have not been documented for other systems. Possible explanations based upon the position of the transition state along the reaction coordinate and/or specific solvation of the dienolate have been advanced to account for preferential axial alkylation in benzene and equatorial alkylation in t-butyl alcohol.However, in view of the fact that the degree of aggregation of the dienolate as well as the structure of the aggregates may be modified considerably in going from one solvent to the other, rationalization of the results is difficult. 

The reaction presumably involves a pre-equilibrium complex of 48 and R MgX, and this makes the subsequent carbomagnesation step intramolecular in nature. The importance of this pre-equilibrium complex was further supported by the observation of dramatic solvent effects - weakly coordinating solvents such as EtgO favor this reaction whereas strongly coordinating solvents such as THF suppress it. These results may be attributed to inhibition of the formation of the pre-equilibrium complex by the coordinating solvent. 

Dynamic medium effects in solution kinetics were first recognized by Kramers [41], He treated the problem on the basis of the Langevin equation [42] according to which the velocity of the reactants along the reaction coordinate and the friction of the surrounding medium play a role. Details of Kramers theory are not given here but an introduction to this subject can be found elsewhere [G3], The parameters involved in quantitatively assessing the dynamic solvent effect are the frequency associated with the shape of the barrier of the transition state and a friction parameter which is related to solvent viscosity. 

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