Grunwald-Winstein equation solvent parameter

Univariate LSERs may possess the conventional LEER form, as exemplified by Eq. (8-67), the Grunwald-Winstein equation, or they may simply be plots of log k against a solvent parameter such as Z, (30), or ir. Brownstein developed an LEER form for the latter type of correlation, writing... 

The extent to which a solvent stabilises ions could be measured by comparing the equilibrium constant for a standard dissociation with the value for that in a standard solvent. The Grunwald-Winstein equation (Equation 48) uses as its standard the solvolysis of /-butyl chloride (Equation 49) and assumes that the transition state has almost complete carbenium ion character and that the formation of the ion-pair is rate determining. The rate constant would therefore measure the energy of formation of the free carbenium ion. Equation (48) defines the solvent parameter, K, where is the solvolysis rate constant of /-butyl chloride in the solvent (S) and the standard solvent (ss) is 80% EtOH/H2O. Other solvolyses can be used as standards and those of 1-adamantyl species has largely supplanted the original one for reasons given later. 

The success of the single parameter Grunwald-Winstein equation is largely due to the limited range of solvent change such as variation of composition of mixtures. When different solvent types are employed extra terms are needed in the equation to fit the data. Solvolysis reactions in aliphatic nucleophilic substitution involve nucleophilic attack of the solvent and it is unlikely that solvents of different structure would have similar nucleophilicities excepting those in a series of mixed solvents such as ethanol-water (Chapter 2). The simplest treatment involves dividing the solvent action into nucleophilic and electrophilic components as shown in Equation (32). 

The extended Grunwald-Winstein equation has been applied to the 5 nI solvolyses of p- and o-bromomethylphenylacetic acids. Rates increase with solvent nucleophilicity and water content, for both substrates. Activation parameters are given. 

The 5[tertiary alkyl halides have been reviewed in the light of the three parameters, Y (the ionizing power of the solvent), N (the nucleophilicity of the solvent), and 1 (an aromatic ring or jr-bond parameter) of the Grunwald-Winstein equation. ... 

The solvolyses of p-methoxybenzyl chloride in 39 solvents are well correlated by an extended Grunwald-Winstein equation involving terms in Tci (solvent ionizing power), At (solvent nucleophilicity) and I (Kevill s aromatic ring parameter). A claim by Liu that nucleophilic participation is greater than in the solvolyses of the corresponding bromide is discussed and said to be disproved. 

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... 

A semiempirical approach, similar, for examples, to the Hammett equation, does not require full understanding of complex molecular interactions in solution. A standard reaction or phenomenon is chosen, and the parameters of this reaction or phenomenon during changes in the solvent are examined. These parameters may be represented by the rate of equilibrium constants of the reaction, but also by, for example, shifts of the maxima in various spectra. The relations of this type most frequently used are the Grunwald-Winstein (50), Swain-Scott (57), Gielen-Nasielski (52), Berson (55), and Drougard-Decroocq (54) equations. 

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. 

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