The Influence of Solvent on Reaction Rates

Generally speaking, the influence of solvent on reaction rates (equilibria) is determined by the difference between the effects < n the stability of transition states (products) and reactants. According to what Leffler and Grunwald (1963) call the first approximation, the free energy of a solute molecule RX is given by the sum of internal and solvent contributions, as shown in (59). The... 

Arrhenius classical theory of reaction kinetics is based on the assumption that the starting materials (reactants) have to overcome an energy barrier, the activation energy, in order to be transformed into the products. This picture has been developed and made more explicit in the theory of absolute reaction rates [2-5, 7, 8, 11, 24, 464-466, 770, 771]. The influence of solvent on reaction rates is best treated by means of this theory -also known as transition-state theory, developed almost simultaneously in 1935 by Eyr-ing as well as Evans and Polanyi [464]. 

II. The Influence of Solvent on Reaction Rates A. Basic Principles... 

The first systematic investigation on the influence of solvent on reaction rates was reported by Menschutkin(l) as long ago as 1890. Quite soon after this study, chemists began to consider whether or not solvent influences on reaction rates were connected with the effect of solvents on the reactants (i.e. with initial-state effects). However, a careful and extensive investigation by Von Halban(2) in 1913 showed conclusively that for the reaction of trimethylamine with p-nitrobenzyl chloride, solvent effects on the reactants could not account quantitatively for the overall influence of solvent on the reaction rate constant. Little further progress was made on these lines until the advent of transition state theory, when it then became clear that in principle it was possible to dissect the influence of solvent on rate constants into initial-state and transition-state contributions(3-5). 

The influence of pH on reaction rates may be looked upon as just another concentration effect, which can be dealt with in terms of the reaction orders just discussed. It merits special attention, however, for two reasons first, because it allows us to change the concentration of the reactant H3O or OH ) over many orders of magnitude, without encountering solubility limitations at the high concentration end and mass transport limitations at the low concentration end (as long as the solution is buffered). The second reason is that, in aqueous solutions, the solvent itself can be the reactant or the product in the reaction being studied. 

Much of what is understood today about the influence of solvent on rates of oxidation reactions with hydrogen peroxide, alkyl hydroperoxides and peroxyacids can be attributed to the seminal studies by Edwards and his collaborators over thirty years ago " . They provided convincing experimental data that showed that a hydroxylic solvent (e.g. ROH) can participate in a cyclic transition state where a proton relay can in principle afford a neutral leaving group attending heterolytic 0-0 bond cleavage (equation 13). 

From these results and later experiments (Ingold and co-workers [36,37, 39]) on the influence of solvents on the nitration rate Ingold came to the conclusion that splitting off the proton in the nitration of aromatics does not effect the reaction kinetics. This conclusion differed from that of Bennett and his co-workers [87]. 

A knowledge of the physico-chemical principles of solvent effects is required for proper bench-work. Therefore, a description of the intermolecular interactions between dissolved molecules and solvent is presented first, followed by a classification of solvents derived therefrom. Then follows a detailed description of the influence of solvents on chemical equilibria, reaction rates, and spectral properties of solutes. Finally, empirical parameters of solvent polarity are given, and in an appendix guidelines to the everyday choice of solvents are given in a series of Tables and Figures. 

Two groups of reactants have been used preferentially in the study of the influence of solvents on the rate of electrode reactions for a correlation of the obtained data with the existing theories. One of these groups comprises metal ions in complexes and organometallic compounds where the first coordination sphere remains un-... 

In our earlier work [85] the literature data were analyzed under the assumption that the influence of solvents on the standard rate constant of the electrode reaction may be expressed by the Bronsted-type relation... 

The influence of solvent on both selectivity and reaction rate is detectable. Phenolic solvents were found to have a promoting effect on the reaction rate and to increase the rate of straight/branched nitriles. 

The influence of solvent on the rate at which a chemical reaction takes place was already made clear, in the final stages of the XIX centuiy, with the Menschutkin reaction... 

The interest of chemists in this topic originated in two findings reported more than a century ago that exposed the influence of solvents on the rate of esterification of acetic acid by ethanol, estabhshed in 1862 by Berthelot and Saint-Gilles, and on the rate of qua-temization of tertiary amines by alkyl halides, discovered in 1890 by Menschutkin. In his study, Menschutkin found that even so-called inert solvents had strong effects on the reaction rate and that the rate increased by a factor about 700 from hexane to acetophenone. Subsequent kinetic studies have revealed even higher sensitivity of the reaction rate to the solvent. Thus, the solvolysis rate of tert-butyl chloride increases 340,000 times from pure ethanol to a 50 50 v/v mixture of this alcohol and water," and by a factor of 2.88x10 " from pentane to water. Also, the decaiboxylation rate of 6-nitrobenzisoxazol 3-carboxylate increases by a factor of 9.5x10 from water to HMPT. ... 

As already pointed out, the influence of solvents on ionic equilibria and on the rates of chemical reactions cannot be exactly determined. Thus, we must be satisfied with some generalization, based on the simple model of separation of two opposite electric charges in different media. Let us assume a charge separation from an equilibrium distance (r ) of 1 A (10 m) to infinity in a medium of known relative permittivity e, = s/eq, where Cq is the permittivity in vacuum (Eq = 8.8542xlO C J m ), and e is the permittivity of the dielectric under study. Relative permittivity is broadly known as the dielectric constant (D) it is 1.0 for vacuum. The distance of 1 A is the distance encountered in reality. A model for the charge separation often quoted is the ionization of HCl in the gas phased ... 

In the sections below a brief overview of static solvent influences is given in A3.6.2, while in A3.6.3 the focus is on the effect of transport phenomena on reaction rates, i.e. diflfiision control and the influence of friction on intramolecular motion. In A3.6.4 some special topics are addressed that involve the superposition of static and transport contributions as well as some aspects of dynamic solvent effects that seem relevant to understanding the solvent influence on reaction rate coefficients observed in homologous solvent series and compressed solution. More comprehensive accounts of dynamics of condensed-phase reactions can be found in chapter A3.8. chapter A3.13. chapter B3.3. chapter C3.1. chapter C3.2 and chapter C3.5. 

What is the influence of ligands on the Lewis acid on the rate and selectivity of the Diels-Alder reaction If enantioselectivity can be induced in water, how does it compare to other solvents Chapter 3 deals with these topics. 

In order to obtain more insight into the local environment for the catalysed reaction, we investigated the influence of substituents on the rate of this process in micellar solution and compared this influence to the correspondirg effect in different aqueous and organic solvents. Plots of the logarithms of the rate constants versus the Hammett -value show good linear dependences for all... 

The influence of solvent polarity on the rate of quatemization is well known and recent measurements have supported the general view that the more polar solvents produce a faster reaction. Fuoss and his colleagues determined the rate of reaction in a number of solvents and discovered that the process was twice as fast in... 

Many high-pressure reactions are done neat, but if a solvent is used, the influence of pressure on that solvent is important. The melting point generally increases at elevated pressures, which influences the viscosity of the medium (viscosity of liquids increases approximately two times per kilobar increase in pressure). Controlling the rate of diffusion of reactants in the medium is also important. In most reactions, pressure is applied (5-20kbar) at room temperature, and then the temperature is increased until reaction takes place. 

House, J. E. (2007). Principles of Chemical Kinetics, 2nd ed. Elsevier/Academic Press, San Diego, CA. Chapters 5 and 9 contain discussions of factors affecting reactions in solution and the influence of solubility parameter of the solvent on reaction rates. 

However, before the kinetics we need to discuss one more electrochemical matter, namely how the dielectric constant D of the whole reaction mixture affects the rate-constants kpl+, kp and. Because of the irregular shape of the cations, it is unlikely that this influence will obey the appropriate Laidler equations (k varies as 1/D). For the the appropriate equation would be that for a dipole-dipole reaction, which predicts an increase of the rate-constant with increasing D. The effect of solvents on the kpl+ is discussed by Plesch (1993). 

However, it is known, that in homolytical processes certaine influence on reaction rate has also so-called "cage effect", which is described by density of medium cohesion energy. That was confirmed by generalization of data concerning to influence of solvents upon decomposition rate of benzoyl peroxide [2] or oxidizing processes [3, 4], That is why the data analysis from work [1] is seemed as expedient by means of five parameter equation ... 

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