The Grunwald-Winstein Equation Class II

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 value of m is a measure of the carbenium ion character of the transition state relative to that of the standard (1-adamantyl) reaction where m is unity (Equation 50). 

Unfortunately no carbenium ion-forming equilibrium is known where it is sure that the positive charge is localised solely on the central carbon and which would be convenient to use as a reference reaction for SnI-type processes. Further difficulties in the interpretation of the Grunwald-Winstein m values arise because the solvent itself could almost certainly be involved in the mechanism of the standard reaction. Internal return could also interfere with the value of the rate constant if decomposition of the carbenium ion became rate limiting as a result 

The solvent structure can be altered by solutes and, for example, heats of solution of Z-butyl chloride in ethanol-water mixtures are markedly non-linear in solvent composition. Enthalpies and entropies of 

The value of m (0.34) for the solvo lysis of ethyl bromide in 80% ethanol-water compared with the value of m = 0.94 for the solvolysis of Z-butyl bromide is consistent with a smaller charge separation in the transition state for the former reaction. 

---