Grunwald and Winstein

AN is known to show good correlations with the solvation energies of anions. Also, AN has good correlations with other solvent parameters defined in different reaction systems, e.g., Grunwald and Winstein s T-value [24], Kosower s Z-value [25], Dimroth and Reichardt s T Value [26,27], etc. 

There is an ongoing controversy about whether there is any stabilization of the transition state for nucleophilic substitution at tertiary aliphatic carbon from interaction with nucleophilic solvent." ° This controversy has developed with the increasing sophistication of experiments to characterize solvent effects on the rate constants for solvolysis reactions. Grunwald and Winstein determined rate constants for solvolysis of tert-butyl chloride in a wide variety of solvents and used these data to define the solvent ionizing parameter T (Eq. 3). They next found that rate constants for solvolysis of primary and secondary aliphatic carbon show a smaller sensitivity (m) to changes in Y than those for the parent solvolysis reaction of tert-butyl chloride (for which m = 1 by definition). A second term was added ( N) to account for the effect of changes in solvent nucleophilicity on obsd that result from transition state stabilization by a nucleophilic interaction between solvent and substrate. It was first assumed that there is no significant stabilization of the transition state for solvolysis of tert-butyl chloride from such a nucleophilic interaction. However, a close examination of extensive rate data revealed, in some cases, a correlation between rate constants for solvolysis of fert-butyl derivatives and solvent nucleophicity. " ... 

The quantity Y = log k - log k0 characterizes the ionizing power of the solvent (Grunwald and Winstein 1948). Here k0 is the rate constant at 25°C in the reference solvent, 80% v/v ethanol + 20% v/v water, and k is the rate constant in any other solvent studied. Representative values of Y are shown in Table 2.5 (Reichardt 1998 Grunwald and Winstein 1948 Abraham 1972, 1985 and Parker 1978). 

The empirical ionizing power parameter, Y, was introduced by Grunwald and Winstein (1948) and was defined by eqn (5) with... 

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),... 

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 ... 

Beichardt (1965). Molar trmisition energies from the visible spectrum of pyridinium N-phenol betaines in solvents. Dimroth et al. (1963). Molar transition energies from spectrum of l-ethyl-4-methoxycarbonyl pyridinium iodide (Koso rer, 1958). From rates of Sj l solvolyses of alkyl halides and tosylates (Grunwald and Winstein, 1948). From ratio of endo to exo products in the Siels-Alder addition of cyclopentadiene to... 

There is an enormous amount of information on solvolyses in a variety of solvents, and linear free energy relationships, such as those of Grunwald and Winstein (1948), successfully correlate much of it. The Grunwald-Winstein Y relationship, in terms of solvent activity coefficients, becomes... 

Solvent effects on nucleophilic substitution reactions have long been of interest (28b), and LFERs have been a major tool in studying these effects. Probably the best known such LFER is that of Grunwald and Winstein (29) ... 

Grunwald and Winstein [80] developed an index for solvents based on the model reaction for the solvolysis of -butylchloride. It is assumed that the solvolysis involves full carbenium ion development so that differences in reactivity from solvent to solvent will mirror solvating power relative to a standard solvent (80% ethanol/water). A parameter T dependent on solvent is defined by Eqn. 86 and the reaction in question will obey Eqn. 87 when carried out in a series of solvents. 

In 1948, Grunwald and Winstein attempted to define the ionizing power of a solvent by the Y parameter, based on the comparison of the rate for the solvolysis of t-butyl chloride. In 1956, Kosower made an attempt to define the polarity of a medium (solvent) by introducing the Z parameter based on the spectroscopic properties (in various solvents)... 

The Inactivity of trianisyl derivatives under solvolytic conditions has been thoroughly investigated. The bromide and chloride (38 and 39 of Table 7) were shown to react in buffered solutions at a rate which is independent of the concentration of added base or of nucleophiles, such as thiolate ions. The ratio Br/ ci = Grunwald and Winstein m... 

In a series of papers, " Grunwald and Winstein showed that the solvolysis rate constants for organic halides which exhibit values differing by more than 6 orders of magnitude in fliis parameter can generally be accurately described by the following equation ... 

In faet, the solvolysis of tert-butyl ehloride is one of the eomerstones of physieal or-ganie ehemistry. Thus, some quantitative approaehes to the kineties of spontaneous re-aetions in various solvents -and, more interesting, solvent mixtures- are based on linear free-energy relations sueh as that of Grunwald and Winstein or its extensions.These equations allow one to interpolate or extrapolate rate eonstants that eannot be readily measured, and also to derive meehanistieally signifieant information in the proeess. 

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