Grunwald-Winstein scale

In Chapters 1 and 2 we covered molecular polarizabilities, dipoles, and conformations. We are now ready to explore how these properties dictate the properties of solvents, the interactions of solutes with the solvent, and the interactions between solutes. Since the vast majority of reactions performed by organic chemists occurs in solution, the choice of solvent can play an extremely important role in controlling the reactions. We need to choose solvents that not only solubilize the reactants, but also accelerate the desired reaction and /or impede undesirable reactions. Moreover, we can change the solvent to probe reaction mechanisms and look for the existence of various intermediates (see Grunwald-Winstein scales in Chapter 8). Finally, the interactions between the molecules of a solvent, and the interactions between solvent and solute, are some of the same interactions that occur between enzyme and substrate, antibody and antigen, and synthetic receptors and various target molecules— all topics of the next chapter. 

The specific rates of solvolysis of benzyl p-toluenesulfonate and nine benzylic-ring-substituted derivatives (324) have been satisfactorily correlated using Aij and Tots scales within the extended Grunwald-Winstein equation. The reactions of Z-phenylethyl X-benzenesulfonates (325) with Y-pyridines (326) in acetonitrile at 60 °C have been studied at high pressures. The results indicated that the mechanism of the reaction moves from a dissociative 5)vr2 to an early-type concerted 5)vr2 with increasing pressure. 

For situations where solvent nucleophilicity may be a factor, Kevill (8) favors the use of the extended Grunwald-Winstein equation (equation 1). Scales of NOTs and OTs values based upon the use of methyl tosylate and 2-adamantyl tosylate as model SN2- and SNl-reacting substrates have been developed (15, 16). Also Y scales have been developed for other anionic leaving groups using 1-adamantyl or 2-adamantyl derivatives (17-19), where Sn2 reaction is impossible or severely hindered. 

When using the Grunwald-Winstein or Schleyer scales to compare solvent ionizing power, we must remember that the reference reactions are heterolysis reactions. The polarity and the protic/aprotic nature of the solvent are mixed in these. scales as appropriate for the respective reference reactions. However, some substitution reactions may be influenced differently than the reference reactions by the protic/aprotic nature of the solvent, or its polarity. For example, Sn2 reactions are very sensitive to the protic/aprotic nature of the solvent, due to differential solvation of the nucleophiles. If the solvent is protic, it can hydrogen bond to the nucleophile and thereby substantially diminish its reactivity. The reference reactions for the Grunwald-Winstein or Schleyer scales reflect no effects on the nucleophiles because they are SnI reactions. 

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),... 

The general SPP scale of solvent dipolarity/polarizability and the specific SB and SA scales of solvent HBA basicity and HBD acidity, respectively, are orthogonal to one another and they can be used in the correlation analysis of solvent effects in single- or, in combination with the others, in two- or three-parameter correlation equations, depending on the solvent-influenced process under consideration see also Section 7.7. Examples of the correlation analysis of a variety of other solvent-dependent processes by means of SPP, SB, and SA values, including those used for the introduction of other solvent polarity parameters, can be found in references [335-337, 340-342]. In particular, comparisons with Kamlet and Taft s n scale [340] and Winstein and Grunwald s Y scale [341] have been made. 

Y polarity scale. A solvent polarity scale proposed by Grunwald and Winstein [Grunwald and Winstein, 1948] based on solvolytic rate ko of r-butyl chloride in 80 % aqueous ethanol at 25 °C. The Y polarity value for a given solvent is calculated by ... 

A brief review has been presented of the correlation analysis of solvolysis rates 50 years later, i.e. since Grunwald and Winstein proposed their eponymous equation in 1948.111 -pije authors then propose a method of correlation analysis involving multiple regression on solvent scales SPP (polarity-polarizability), SA (acidity) and SB (basicity). These scales are based on the solvatochromism of suitable probes and were initially for pure (i.e. one-component) solvents, but have now been extended to binary solvent mixtures. This enabled the authors to present a correlation for the solvolysis rate constants of r-butyl chloride in 27 pure solvents and 147 binary solvent mixtures, having a correlation coefficient r = 0.990 and a standard error of the estimate s = 0.40. The most important term in the equation is that involving SPP next comes... 

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