Free Energy of Formation of Sulfur Trioxide in Aqueous Solution. Methods for Determining the Energy Level of an Unobservable Intermediate

Article abstract of DOI:10.1021/ja00536a008

The acid-catalyzed hydrolysis of monoaryl and monoalkyl sulfate monoanions and the uncatalyzed hydrolysis of monoaryl sulfate monoanions are generally believed to proceed via sulfur trioxide as a short-lived intermediate.SO3 is very reactive toward water, and its free energy of formation in aqueous solution has not been known.The rate of disapperance of aryl sulfate monoanions is accelerated by amines, but the rate constants for amine catalysis are known to be essentially independent of amine pK_a.This suggests a model for the reaction in which the rate-determining step for hydrolysis is trapping by water of SO3 in an "encounter complex" formed by S-O bond cleavage, and the rate-determining step for aminolysis is formation of a ternary "encounter complex" containing aryl oxide, amine, and SO3 either by diffusion of amine into contact with a binary "encounter complex" or by solvolysis of a preassociation complex.Equilibrium constants for sulfate ester formation can be estimated starting from the linear free energy relationship for transesterification of a sultone reported by William et al.An analysis based upon this model leads to a value of log K_d = -17.1 +/- 2.2 for the dehydration of undissociated sulfuric acid.For the acid-catalyzed hydrolysis of monoaryl and monoalkyl sulfate monoanions, we apply the simplest version of Marcus theory to a plot of log k vs. log K for the reaction involving neutral sulfate monoester giving neutral products, using pK_a values estimated as previously described.This treatment leads to a value of log K_d = -17.7 +/- 3.8.The two sets of data lead to a consistent value for log K_d = -17 +/- 2.