SOLVENT INDEPENDENT TRANSITION-STATE STRUCTURES .3. SULFONYL TRANSFER-REACTIONS

A-kinetic spectrophotometric study has been performed on the reactions of p-nitrophenyl benzenesulfonate (PNPBS) with m-chlorophenoxide, p-c hlorophenoxide, phenoxide, p-methylphenoxide and p-methoxyphenoxide io ns in DMSO-H2O mixtures at 25.0 degrees C. Systematic variation of the DMSO content showed that the second-order rate constants (k(2)) chang ed only moderately upon addition of DMSO up to 20 mol% for reaction of p-chlorophenoxide, phenoxide and p-methylphenoxide. On the other hand , the rates of reaction for all of the phenoxide ions studied increase d markedly in solvent compositions greater than 40 mol% DMSO. The natu re of the transition state (TS) for these reactions was probed via Bro nsted-type linear free energy relationships (LFERs). Traditional Brons ted-type plots (method 'A'), in which the remote substituent on the ar yloxide was varied and the solvent composition was held constant, were constructed for the reactions of the phenoxides with PNPBS in each of the following solvent compositions: 40%. 50%, 70% and 90% DMSO by mol e. Novel Bronsted-type plots (method 'B'), in which variation in pheno xide pK(a) was imparted through solvent changes while the remote subst ituent was held constant, were also constructed for the reactions of e ach of the five phenoxide nucleophiles. Method A yielded a set of line s whose slopes (beta(nuc) values) decreased steadily from 0.75 in 40 m ol% DMSO to 0.60 in 90 mol% DMSO. Method B yielded a single straight l ine, beta(nuc) = 0.60, with satisfactory linearity for all data points (r(2) = 0.995, 95% Cl = +/- 0.021), although closer examination sugge sted slight curvature with beta(nuc) varying from 0.54 for p-CH3O to 0 .66 for m-Cl. These results point to a discrepancy in the traditional view that Bronsted-type LFERs can be directly translated into a TS str ucture. Considerations based on the Marcus equation suggest that linea rity in a Bronsted-type plot may result for a set of reaction series w ith differing intrinsic barriers provided that Delta G(0)(#), varies l inearly with Delta G(0).