Hydrolysis of esters of oxy acids: pKa values for strong acids; Brønsted relationship for attack of water at methyl; free energies of hydrolysis of esters of oxy acids; and a linear relationship between free energy of hydrolysis and pKa holding over a range of 20 pK units

Abstract

By combining various kinds of evidence from the literature it is possible to derive an internally consistent set of pKa values for the strong mineral acids and the arenesulfonic acids; the values are referred to dilute aqueous solution as standard state and are expected to be correct within 0.5 log unit. Using these pKa values and literature data for hydrolysis of methyl esters of acids of the type Y—XO3Men, where Y is O, OH, OMe, alkyl, or aryl and X is Cl, S, or P, a Brønsted plot can be constructed with slope equal to 1.02 ± 0.04. From the free energies of hydrolysis for dimethyl sulfate and the methyl phosphates it is possible to calculate rate constants for the microscopic reverse reaction. These define a Brønsted line of slope 0.27 ± 0.3, from which rate constants for the formation of the esters of perchloric and various sulfonic acids may be estimated. This permits calculation of free energies of hydrolysis for these esters.Thermochemical data in the literature permit calculation of the free energies of hydrolysis of dimethyl sulfate, trimethyl arsenite, and tetraethyl orthosilicate. In the case of dimethyl sulfate the calculation (using the previously reported eq. [1]) leads to a pKa value in close agreement with theoretical expectation, confirming that eq. [1] is valid for acids of pKa ≥ −3. For tetraethyl orthosilicate the thermochemical data are less precise but are in satisfactory agreement with the predictions of eq. [1]. The free energies of hydrolysis derived from the Brønsted correlations are also in good agreement with expectation based on eq. [1].For acids where resonance phenomena are important either in the acid itself (boric acid) or in the anion (nitric acid, nitrous acid, carboxylic acids) the experimental free energies of formation fall far from the line defined by eq. [1]. It is concluded that eq. [1] is limited to species where there is no n delocalization involving the reacting oxygen, but where this condition is satisfied, the equation holds over the entire accessible range of oxy acid pKa values, i.e., from −6.4 to 16.