Stoichiometric solvation effects. Part 2. A new product–rate correlation for solvolyses of p-nitrobenzenesulfonyl chloride in alcohol–water mixtures

Abstract

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 of p-nitrobenzenesulfonyl chloride in aqueous ethanol and methanol at 25 °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: kww, where water acts as both nucleophile and general base; kwa, water acts as a nucleophile and alcohol acts as a general base; kaw, alcohol acts as a nucleophile and water acts as a general base. Inclusion of a fourth rate constant, kaa, 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; kaa can be calculated from the observed first-order rate constants in pure alcohols. Independent values of kaw and kwa can be obtained from kaa 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.