Reaction rates solvent effects

A dissection, therefore, of the reaction rate solvent effects into initial and transition state contributions would lead to more direct information concerning the nature of the solvent involvement [453, 467]. 

The Marcus theory, as described above, is a transition state theory (TST, see Section 14.3) by which the rate of an electron transfer process (in both the adiabatic and nonadiabatic limits) is assumed to be determined by the probability to reach a subset of solvent configurations defined by a certain value of the reaction coordinate. The rate expressions (16.50) for adiabatic, and (16.59) or (16.51) for nonadiabatic electron transfer were obtained by making the TST assumptions that (1) the probability to reach transition state configuration(s) is thermal, and (2) once the reaction coordinate reaches its transition state value, the electron transfer reaction proceeds to completion. Both assumptions rely on the supposition that the overall reaction is slow relative to the thermal relaxation of the nuclear environment. We have seen in Sections 14.4.2 and 14.4.4 that the breakdown of this picture leads to dynamic solvent effects, that in the Markovian limit can be characterized by a friction coefficient y The rate is proportional to y in the low friction, y 0, limit where assumption (1) breaks down, and varies like y when y oo and assumption (2) does. What stands in common to these situations is that in these opposing limits the solvent affects dynamically the reaction rate. Solvent effects in TST appear only through its effect on the free energy surface of the reactant subspace. 

Like reaction rates, the effect of solvent polarity on equilibria may be rationalized by consideration of the relative polarities of the species on each side of the equilibrium. A polar solvent will therefore favour polar species. A good example is the keto-enol tautomerization of ethyl acetoacetate, in which the 1,3-dicarbonyl, or keto, form is more polar than the enol form, which is stabilized by an intramolecular H-bond. The equilibrium is shown in Scheme 1.3. In cyclohexane, the enol form is slightly more abundant. Increasing the polarity of the solvent moves the equilibrium towards the keto form [28], In this example, H-bonding solvents will compete with the intramolecular H-bond, destabilizing the enol form of the compound. 

The higher reactivity of the PVMI-Co(III) complex is attributed to the electrostatic domain of the polymer complex, as in the above PVP system. When the PVMI chain contracts, the charge density in the polymer domain increases and the reaction rate also increases. On the other hand, when the polymer chain expands, the electrostatic domain is weakened, which produces a fall in reactivity. These results confirm that the conformation of the polymer complex is closely related to the strength of its electrostatic domain and to the reaction rate. The effects of the polymer chain on reactivity are to be understood not only in terms of static chemical environment but also as dynamic effects which vary with the solution conditions, e.g. pH, ionic strength, solvent composition, temperature, and so on. 

Reactants Activated complex Reaction type Solvent effect on rate... 

House, J. E. (2007). Principles of Chemical Kinetics (2nd ed.). Amsterdam Academic Press/Elsevier. Discussions of many topics on rates of reactions including solvent effects and reactions in solids. 

The interactions between organic molecules and the pore walls of similar size are very strong (Type I adsorption isotherms) and zeolites may be considered as solid solvents (11, 15, 45). The reactants concentration in zeolite micropores is therefore considerably higher than in the gas phase with a significant positive effect on the reaction rates. This effect is all the more pronounced as the reaction order is greater, favoring more the bimolecular over the monomolecular reactions. 

Two types of solvent effects have been determined for prolyl isomerization and amide rotation (1) the effect of solvent deuterium on reaction rate and (2) the effect of organic solvents on reaction rate. Solvent deuterium isotope effects are useful tools in probing the role of proton transfer... 

In the case of one-step cycloaddition reactions involving an activated complex with a different dipolarity than the reactants, an increase in solvent polarity should enhance the reaction rate (c/ Fig. 5-6a). However, since two-step cycloadditions are consecutive reactions, the solvent effect depends on the relative size of AGf and AGf or of AGfi and AG cf. Fig. 5-6b). If the formation of the zwitterionic intermediate is irreversible, and AG > AG, then the first step is rate-determining in all solvents. Consequently, there is a rate acceleration with increasing solvent polarity. When AG < AG, this behaviour is reversed. If ever AG x AG, then only relatively... 

Further examples of Diels-Alder cycloaddition reactions with small or negligible rate solvent effects can be found in the literature [531-535], The thermolysis of 7-oxabicyclo[2.2.1]hept-5-ene derivatives is an example of a solvent-independent retro-Diels-Alder reaction [537]. For some theoretical treatments of the solvent influence on Diels-Alder cycloaddition reactions, which, in general, confirm their small solvent-dependence, see references [536, 797-799]. 

Product analysis and reaction-rate medium effects reveal such a change in mechanism on passing from nonpolar to polar solvents [207, 209]. The decomposition of this... 

Dielectric constants cannot explain, quantitatively, most physicochemical properties and laws of solutions, and we shall soon see that they can become unimportant. The molecules of more polar solvents, which tend to cluster around the ions and dipole ions, produce a preferential or selective solvation that is reflected in measurements of such properties as solubility, acid—base equilibria, and reaction rates. Nonelectrostatic effects, such as the basicity of some solvents, their hydrogen-bonding, and the internal cohesion and the viscosity of mixtures, probably interfere with the electrostatic effects and thus reduce their actual influence. On the other hand, mixtures of water and nonaqueous solvents are enormously complicated systems, and their effective microscopic properties may be vastly different from their macroscopic properties, varying with the solute because of selective attraction of one of the solvents for the solute. 

The LFER method was used in the heterogeneous catalysis mainly in the investigation of the effect of the structure of reactants on the reaction rate the effect of solvents on the hydrogenation process was interpreted for a long time on the qualitative level only. Such an approach, as well as the correlation of kinetic data with various physical characteristics of solvents, was of course unsatisfactory, because it was rather far from any potential generalization. 

The Knoevenagel condensation reaction of benzaldehyde with ethylcyanoacetate (Scheme 1) was first studied on CsNaY 7Cs in order to check the better conditions to control the different reaction parameters (solvent effect on the rate and on the selectivity of the uncatalyzed and catalyzed reactions, mass effect of the catalyst, concentration effect of both reactants) [21],... 

In summary ionic liquids have a proven ability to enhance current industrial chemistry. Within current industrial processes using conventional solvents, selectivities, TOF and reaction rates are effectively uncontrolled however by using ionic liquid media for the eataly-sis, it is possible to have a profound influence on all these faetors. Tailoring of the ionic liquid by a combination of subtle (i.e., changing cation substitution patterns) and gross (anion type) modifications can permit very precise tuning of reactions. 

Numerous examinations have shown the effects of solvents in various reactions of the Grignard type [Cu 65, No 63, Za 63, Za 64, Za 65]. Most of these indicate that strongly solvating solvents increase the rates of nucleophilic substitution reactions. However, a number of data point to the reaction rate-decreasing effect of solvation [As 64, Be 63]. For instance, the Grignard reagent reacts with nitriles more slowly in polar solvents than in inert solvents [Be 63]. 

In order to establish the mechanism of the reaction, the solvent effect on the reaction of pyridinyl radical with dibromomethane was investigated. As the results listed in Table 1 show, there is no solvent effect on the rate of the reaction. How could one reconcile the formation of a salt with the lack of solvent polarity effect on the rate Since the initial state (Py + RX) is not very polar (pyridinyl radical with a 1-ethyl group is soluble in n-hex ine, BrCH2Br has a dipole moment of ca. 1 Debye), the lack of... 

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