Grunwald-Winstein parameters

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... 

Aqueous ethanolyses of adamantylideneadamantyl halides show Grunwald-Winstein sensitivity parameters (m) of 0.74 ( 0.06), 0.90 ( 0.01), and 0.88 ( 0.03) for the chloride, bromide, and iodide compounds, respectively. All reaction products are formed with retention of both the ring structure and the stereochemistry of the reaction centre. Observed common-ion rate depressions are consistent with a reaction pathway via a free solvated homoallylic carbenium ion. 

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... 

See also Dimroth-Reichardt et parameter Grunwald-Winstein equation. 

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 values of Z and 7 values belong to a class of solvent parameters based on various standard physical processes (electromagnetic transition, dielectric constant, NMR chemical shift, etc.. In the context of similarity these parameters are not directly useful for mechanistic studies because the reference process is a physical property rather than a chemical reaction. The parameters Z and are often linearly related to the Grunwald-Winstein Y value (Figure 14) and provide a secondary definition of Y values which are inaccessible via the chemical definition of those parameters. 

Construct a Grunwald-Winstein plot and comment briefly on the magnitude of the slope. The y, Ad parameter is determined from the solvolysis of 1 -adamantyl chloride. 

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). 

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 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. 

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