Studies on Sulfate Esters. III. A Comparison of the Solvolyses of Salicyl Sulfate and Sulfur Trioxide

The solvolyses of o- and p-carboxyphenyl sulfates in aqueous methanol have been investigated. The hydronium ion catalyzed solvolysis of the para substrate and the carboxyl-catalyzed solvolysis of the ortho isomer yield a similar methyl sulfate/sulfate ratio; the hydronium ion catalyzed solvolysis of the o-sulfate gives a significantly lower fraction of methyl sulfate. The threefold selectivities of the former for methanol are comparable to the observed partitioning of monomeric sulfur trioxide in aqueous alcohols. Additional mechanistic evidence is obtained from the effect of various aqueous organic solvents in acclerating the carboxyl group catalyzed solvolysis of the ortho isomer. Collectively the data support transition states of considerable sulfur trioxide character for catalysis by hydronium ions and the o-carboxyl moiety. The possibility of Lewis acid-base interactions influencing the product composition as in the case of the ortho isomer is discussed. A rationale, derived in part from the results on sulfur trioxide partitioning, is proposed for the alcohol selectivity of certain phosphorylating agents and extended hypothetically to sulfating reagents.