The Alkaline Hydrolysis of Sulfonate Esters: Challenges in Interpreting Experimental and Theoretical Data

Sulfonate ester hydrolysis has been the subject of recent debate, with experimental evidence interpreted in terms of both stepwise and concerted mechanisms. In particular, a recent study of the alkaline hydrolysis of a series of benzene arylsulfonates (Babtie et al., Org. Biomol. Chem. 10, 2012, 8095) presented a nonlinear Brønsted plot, which was explained in terms of a change from a stepwise mechanism involving a pentavalent intermediate for poorer leaving groups to a fully concerted mechanism for good leaving groups and supported by a theoretical study. In the present work, we have performed a detailed computational study of the hydrolysis of these compounds and find no computational evidence for a thermodynamically stable intermediate for any of these compounds. Additionally, we have extended the experimental data to include pyridine-3-yl benzene sulfonate and its N-oxide and N-methylpyridinium derivatives. Inclusion of these compounds converts the Brønsted plot to a moderately scattered but linear correlation and gives a very good Hammett correlation. These data suggest a concerted pathway for this reaction that proceeds via an early transition state with little bond cleavage to the leaving group, highlighting the care that needs to be taken with the interpretation of experimental and especially theoretical data

Accurate Experimental Values for the Free Energies of Hydration of H\u3csup\u3e+\u3c/sup\u3e, OH\u3csup\u3e-\u3c/sup\u3e, and H\u3csub\u3e3\u3c/sub\u3eO\u3csup\u3e+\u3c/sup\u3e

Accurate experimental values for the free energies of hydration, or the free energies of solvation, of the H+. OH-, and H3O+ ions are of fundamental importance. By use of the most accurate value for the free energy of solvation of H+, the known value for the free energy of solvation of water, and the known values for the gas phase and aqueous phase deprotonation of water, the corresponding experimental free energy of solvation for OH- is -106.4 +/- 0.5 kcal/mol. Similarly, by use of the known values for DeltaG(f)(0) for H3O, H2O+, and OH-, the known values for DeltaG(s) for H+ and OH-, and the known value for the aqueous phase autoionization of water, we obtain an experimental free energy of solvation value for H3O+ of -103.4 +/- 0.5 kcal/mol. These values are in excellent agreement with the commonly accepted values and with the value for DeltaG(s),(OH-) obtained from embedding, clusters of OH-(H2O)(n) in a dielectric continuum