Solvent nucleophilicity. A scale based on triethyloxonium ion solvolysis

First-order rate coefficients have been obtained for the solvolysis of triethyloxonium hexafluorophosphate in a variety of organic and aqueous-organic solvents. These have been used to set up a scale of solvent nucleophilicities based upon the four-parameter (two-term) Grunwald-Winstein equation. The required corrections for variation in solvent influence upon the leaving group were estimated from data available for tert-butyldimethylsulfonium ion solvolysis. A previously established scale, based upon methyl p-toluenesulfonate solvolysis, can be brought into good agreement with this scale if a revised value is used for methyl p-toluenesulfonate sensitivity to the electrophilic influence of the solvent. The scale is applied to previously studied solvolyses of alkyl p-toluenesulfonates and chlorides. Determination of solvent nucleophilicities is complicated by concurrent solvent influence upon the leaving group. Previous attempts to establish a nucleophilicity scale for solvolytic reactions have used data from initially neutral substrate^;^,^ to a first approximation, the overall influence of the solvent could be considered as a nucleophilic push at the cy carbon and an electrophilic pull a t the leaving group. Two-term (fourparameter) equations have been proposed2t4 for the combination of these two effects within a linear free energy relationship. Subsequent ~ o r k ~ , ~ * has favored the earlier form ~ l a t i o n , ~ expressed in the equation log ( k / k o ) = IN + mY (1) I n eq 1, k represents the specific rate in a given solvent, ko the specific rate in 80% ethanol, and, for a given substrate, I and m represent the sensitivities of the solvolysis to N and Y , the 0002-?863/?9/1501-3916$01.00/0 solvent nucleophilicity and the solvent ionizing power. The equation was developed as an extension of the more familiar one-term (two-parameter) GrunwaldWinstein equation9 (eq 2) from S N 1 to S N ~ solvolyses.