Activated complex solvation

It has soon been found that solvent effects are particularly large for reactions in which charge is either developed or localized or vice versa, that is, disappearance of charge or spreading out of charge. In the framework of electrostatic considerations, which have been around since Berzelius, these observations led to the concept of solvation. Weak electrostatic interactions simply created a loose solvation shell around a solute molecule. It was in this climate of opinion that Hughes and Ingold presented the first satisfactory qualitative account of solvent effects on reactivity by the concept of activated complex solvation. 

Structures with a cyclic character (70° a 110°) are less solvated than open cation structures (a < 70° a > 110°) due to a larger charge delocalization in the former. Thus, the alterations of the potential energy surface described above are plausible. There are two possible structures for activated complexes in solution. They... 

It was possible to formulate a rule describing how the copolymerization parameters depend on the polarity of the solvent used. This rule is a result of contemplation about the connection between the copolymerization parameters and propagation rate constants during the cationic polymerization as well as about the changes of solvation of educts and activated complexes of the crossed propagation steps in solvents with varied polarity 14 U7). The rule is as follows ... 

For Mn(CO)5Br in nitrobenzene, k1 = 3.6x 107 exp(—30,900/RT) sec-1. The variation in k1 with solvent and the unusually high pre-exponential factors reflect the decrease in solvation on formation of the activated complex. 

The degree of solvation of the reactants and activated complex affect the rate of reaction. When the activated complex is solvated to a greater extent in comparison to reactants, the rate of reaction will be greater than that in a non solvating solvent. This is because the activity coefficient of the complex is smaller than it is in a solvent that does not solvate it. This lowers the potential energy of activated complex or causes a decrease in the activation energy of the reaction. 

On the other hand, if one or both the reactants are solvated, while the activated complex is not, the necessary energy of activation is increased and consequently the rate of reaction is diminished. In case the reactant and activated complex are both solvated, the overall effect on both the activation energy and rate of reaction may be small. 

The pseudothermodynamic analysis of solvent elfects in 1-PrOH-water mixtures over the whole composition range (shown in Figure 7.3) depicts a combination of thermodynamic transfer parameters for diene and dienophile with isobaric activation parameters that allows for a distinction between solvent elfects on reactants (initial state) and on the activated complex. The results clearly indicate that the aqueous rate accelerations are heavily dominated by initial-state solvation effects. It can be concluded that for Diels-Alder reactions in water the causes of the acceleration involve stabilization of the activated complex by enforced hydrophobic interactions and by hydrogen bonding to water (Table 7.1, Figure 7.4). °... 

Those solvatization effects that reflect the ongoing conversion of the hydroformylation are supposed to be similar for catalytically active complexes with molecular similarity. This knowledge opens up the perspective of a pressure-induced catalyst separation in a catalyzed reaction in a desired conversion or a desired operation range of the reactor pressure, where no dissolved metal complex remains in the CO2 phase a defined point (Sect. 5.5). 

The barrier that the reaction must overcome in order to proceed is determined by the difference in the solvation of the activated complex and the reactants. The activated complex itself is generally considered to be a transitory moiety, which cannot be isolated for its solvation properties to be studied, but in rare cases it is a reactive intermediate of a finite lifetime. The solvation properties of the activated complex must generally be inferred from its postulated chemical composition and conformation, whereas those of the reactants can be studied independently of the reaction. For organic nucleophilic substitution reactions, the Hughes-lngold rales permit qualitative predictions on the behavior of the rate when the polarity increases in a series of solvents, as is shown in Reichardt (Reichardt, 1988). 

It is assumed in the Hughes-lngold rules that the entropy of activation is small relative to the enthalpy of activation, that is, AG AH, and that the temperatnre effect on the rate follows with an assumed temperature independent valne of AH. If the nnmber of solvent molecules solvating the activated complex is very different from that solvating the reactants, then this assnmption is no longer valid. This is the case in the solvolysis of t-butyl chloride in water compared to, say, ethanol. 

Apparently, solvation of ion-pairs is not very much different from solvation of activated complex. 

Note that the region where solvent is least well equilibrated to the solute is expected to be in the vicinity of the activated complex, since it has so short a lifetime. Since non-equilibrium solvation is less favorable than equilibrium solvation, the non-equilibrium free energy of the activated complex is higher than the equilibrium free energy, and the non-equilibrium lag in solvent response thus slows the reaction. This effect is sometimes referred to as solvent friction and can be accounted for by inclusion in the transmission factor a. 

In the LDK approach, AC/ is used instead of AG to describe changes in solvation energy from reactants to products. The difference is negligible if entropic terms associated with the change in spacing of levels between the activated complex and the reactants in unimportant [3]. 

The mathematical derivation of the theoretical expression for k t for solvated electron transfers has been given elsewhere (8). The following assumptions were the principal ones made, of which (a) to (c) are standard in activated complex theory ... 

Comparison of the entropies of activation of the forward reactions, AS, with AaS ° indicates that the activated complex is solvated more than the neutral reactants but less than the product ions (Pearson, 1948). The similarity of ASt and A S ° for hydroxide-ion catalysis suggests that here the transition state closely resembles the products. 

Polar aprotic solvents (solvents that cannot form hydrogen bonds in solution) do not solvate the nucleophile but rather surround the accompanying cation, thereby raising the ground state energy of the nucleophile. Because the energy of the activated complex is a fixed value, the energy of activation becomes less and, therefore, the rate of reaction increases. 

The experimental slope can be compared with the theoretical value. If it is assumed that the theory holds, then a comparison should give values of ry, which can then be used to infer something about the structure and solvation pattern of the activated complex. 

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