Effects of Solvent on Rate

Table 8-2 lists several physical properties pertinent to our concern with the effects of solvents on rates for 40 common solvents. The dielectric constant e is a measure of the ability of the solvent to separate charges it is defined as the ratio of the electric permittivity of the solvent to the permittivity of the vacuum. (Because physicists use the symbol e for permittivity, some authors use D for dielectric constant.) Evidently e is dimensionless. The dielectric constant is the property most often associated with the polarity of a solvent in Table 8-2 the solvents are listed in order of increasing dielectric constant, and it is evident that, with a few exceptions, this ranking accords fairly well with chemical intuition. The dielectric constant is a bulk property. 

E. Amis, Effect of Solvents on Rate and Mechanism of Chemical Reactions, Mir Publ., Moscow, 1966... 

Effect of Solvents on Rates and Routes of Oxidation Reactions... 

TABLE 1.2. Effect of Solvent on Rate Constants kg (s 20 °C) for Decay of Cations"... 

Probably the most familiar parameter used by the coordination chemist investigating the effect of solvent on rates of reaction of coordination complexes is the relative permittivity R (dielectric constant) of the medium. If the solvent can be regarded as an inert medium then the effect of the solvent can be evaluated, semiquantitatively at least, if only electrostatic forces are considered. 

Effect of solvent on rate Rate increases with increasing polarity of solvent as measured by its dielectric constant e. (Section 8.12) Polar aprotic solvents give fastest rates of substitution solvation of Nu - is minimal and nudeophilicity is greatest. (Section 8.12)... 

The effect of solvent on rates and thermodynamics of reactions can be understood to propagate through macromolecular processes to an influence of solvent on higher levels of organization. 

Two papers on the aminolysis and hydrolysis of isatin (149) have appeared. In the aminolysis study using piperidine, the reaction rate was much faster in water than in aqueous MeOH or aqueous MeCN. A multi-step mechanism was proposed. In the hydrolysis study the focus was again on the effect of solvent on rate in base hydrolysis, both ethanol and ethylene glycol being used. The reaction rate decreases with increasing ethanol content and it passes through a minimum at about 30% ethylene glycol. 

In order to explain rigorously whether the effect of solvent on rate is predominantly an effect on the transition state or the products it is necessary to determine the transfer energies of the reactants between the solvents concerned, as well as the reaction rates. This has only been done in a few cases, the most thoroughly studied example being the quaternisation of tertiary amines [12]. Some reaction rate constants for the quaternisation of triethylamine with ethyl iodide in a variety of solvents are shown in Table 12.9. 

Fig. 9. Effect of solvent on rate as measured by the response of log k versus a for a series of primary substrates in solvents of a wide range of ionizing abilities and nucleophilicities. Reproduced by permission of John Wiley and Sons.

We are now in a position to draw some general conclusions concerning the effects of solvents on rates of reaction. 

D. S. Kemp and K. G. Paul, ]. Am. Chem. Soc., 97, 7305 (1975). The Physical Organic Chemistry of Benzisoxaroles. 111. The Mechanism and the Effects of Solvents on Rates of Decarboxylation of Benzisoxazole-3-Carboxylic Acids. 

Multiparameter correlation analyses of the effect of solvent on rates and equilibria were introduced by Koppel and Palm [37], but these have been utilized extensively by Taft and his students who recognized the contribution of five factors [38]. These are (i) the abihty of the solvent to stabilize an ion or dipole by dielectric effects (n ) (ii) the polarizability of the solvent (iii) the hydrogen-bond donation (a) (iv) the acceptor ability (P) and (v) the cohesive energy density of the solvent (8 ). In the case of the solvolysis reaction of t-butyl chloride, Taft s group found that the logarithm of the rate constant in 21 solvents from water to diethyl ether could be correlated with four factors [39] ... 

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