Solvent independent transition-state structures. Part III. Sulfonyl transfer reactions

A kinetic spectrophotometric study has been performed on the reactions of p-nitrophenyl benzene-sulfonate (PNPBS) with m-chlorophenoxide, p-chlorophenoxide, phenoxide, p-methylphenoxide and p-methoxyphenoxide ions in DMSO-H₂O mixtures at 25.0°C. Systematic variation of the DMSO content showed that the second-order rate constants (k₂) changed only moderately upon addition of DMSO up to 20 mol% for reaction of p-chlorophenoxide, phenoxide and p-methyl-phenoxide. On the other hand, the rates of reaction for all of the phenoxide ions studied increased markedly in solvent compositions greater than 40 mol% DMSO. The nature of the transition state (TS) for these reactions was probed via Brønsted-type linear free energy relationships (LFERs). Traditional Brønsted-type plots (method 'A'), in which the remote substituent on the aryloxide 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 mole. Novel Brønsted-type plots (method 'B'), in which variation in phenoxide pKa was imparted through solvent changes while the remote substituent was held constant, were also constructed for the reactions of each of the five phenoxide nucleophiles. Method A yielded a set of lines whose slopes (β_nuc values) decreased steadily from 0.75 in 40 mol% DMSO to 0.60 in 90 mol% DMSO. Method B yielded a single straight line, β_nuc = 0.60, with satisfactory linearity for all data points (r² = 0.995, 95% Cl = ±0.021), although closer examination suggested slight curvature with β_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 Brønsted-type LFERs can be directly translated into a TS structure. Considerations based on the Marcus equation suggest that linearity in a Brønsted-type plot may result for a set of reaction series with differing intrinsic barriers provided that ΔG^# ₀ varies linearly with ΔG₀.