Empirical Parameters of Solvent Polarity from Kinetic Measurements

3 Empirical Parameters of Solvent Polarity from Kinetic Measurements 

Since reaction rates can be strongly affected by solvent polarity cf. Chapter 5), the introduction of solvent scales using suitable solvent-sensitive chemical reactions was obvious [33, 34]. One of the most ambitious attempts to correlate reaction rates with empirical parameters of solvent polarity has been that of Winstein and his co-workers [35-37]. They found that the SnI solvolysis of 2-chloro-2-methylpropane (t-butyl chloride, t-BuCl) is strongly accelerated by polar, especially protic solvents cf. Eq. (5-13) in Section 5.3.1. Grunwald and Winstein [35] defined a solvent ionizing power parameter Y using Eq. (7-13), 

(7-14), k and ko are the specific rate constants for the SnI solvolysis of RX (in this case t-BuCl) in a given solvent and in the standard solvent, respectively, m is the sensitivity of the specific rate of solvolysis of RX to changes in the solvent ionizing power (T), T is a parameter characteristic of the given solvent, and c is the intercept (zero for an ideally behaved solvolysis). Eq. (7-14) is expected to be applicable to reactions very similar to the standard reaction, that is, SnI substitutions. The similarity between Y and m of Eq. (7-14), and a and g of the Hammett equation (7-6) is obvious. Y values are known for some pure, mainly protic solvents and for various binary mixtures of organic solvents with water or a second organic solvent [35, 36]. Typical Y values are 

Sn2 reaction of tris(n-propyl)amine with iodomethane at 20 °C [49, 50]. 

To account for nucleophilically solvent-assisted processes, Grunwald, Winstein et al. [42] later provided a four-parameter equation of the type shown in Eq. (7-15) b 

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The elucidation of a substitution reaction mechanism depends on reliable kinetic and thermodynamic data obtained by measuring changes in the reaction rate as a function of a chemical property (e.g., concentration, pH, ionic strength, solvent polarity) or physical quantity (e.g., temperature). The determination of an empirical rate law, the observation of steric or electronic effects induced by the entering, spectator, or leaving groups, and the estimation of activation parameters from variable-temperature experiments (i.e.. A// and Aj ) contribute to the adjudication of a plausible mechanism for a given reaction. 

A number of empirical parameters have been recommended for expressing the polarity of organic solvents (K 33). Most of them originate from spectroscopic determination or thermodynamic and kinetic measurements. Among these parameters only those which have been discussed in relation with NMR spectroscopy will be dealt with here. First af all we shall consider solvent parameters which have been derived from direct NMR measurements... 

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