STOICHIOMETRIC SOLVATION EFFECTS .2. A NEW PRODUCT-RATE CORRELATION FOR SOLVOLYSES OF P-NITROBENZENESULFONYL CHLORIDE IN ALCOHOL-WATER MIXTURES

For reactions involving nucleophilic attack in alcohol-water mixtures, a linear relationship between the reciprocal of product selectivities (S) and the molar ratios of alcohol and water solvents can be derived, if it is assumed that the reactions are second-order in protic solvent (e.g., with one molecule of solvent acting as a nucleophile and the other as a general base). The relationship {1/S = (slope) ([alcohol]/[water]) + intercept} fits the products of solvolyses or p-nitrobenzenesulfonyl chloride in aqueous ethanol and methanol at 25 degrees C (determined by refrigerated RP-HPLC) within the range from water to 80% v/v alcohol-water. From the slopes and intercepts of these product plots and the one observed rate constant for hydrolysis in pure water, the observed first-order rate constants in alcohol-water mixtures up to 90% (v/v) can be calculated satisfactorily, further supporting the validity of the derived linear relationship; the kinetic model includes three third-order rate constants: k(ww), where water acts as both nucleophile and general base; k(wa), water acts as a nucleophile and alcohol acts as a general base; k(aw), alcohol acts as a nucleophile and water acts as a general base. Inclusion of a fourth rate constant, k(aa), where the alcohol acts as a nucleophile and a second molecule of alcohol acts as a general base, is necessary to account for solvolyses in 90-99% alcohol-water; k(aa) can be calculated from the observed first-order rate constants in pure alcohols. Independent values of k(aw) and k(wa) can be obtained from k(aa) and the slopes and intercepts of linear relationships between S and the molar solvent ratio [water]/[alcohol] within the range 90-99% alcohol-water. The dominant effect of solvent stoichiometry and the absence of other substantial medium effects is confirmed by the approximately constant third-order rate constants, calculated from the observed first-order rate constants in acetonitrile-, acetone- and dioxane-water mixtures.