Hydrolysis and first ionization constant of monoperoxyphthalic acid

Abstract

The ionization constant for the carboxy proton of monoperoxyphthalic acid (MPPA) has been determined as 1.1 × 10^–3, based on PK_a1, 2.96 ± 0.05 at 21 °C and ionic strength 0.024 mol dm^–3 obtained by a distribution method. In the pH range 1–4 the observed rate constant for hydrolysis of MPPA increases by a factor of > 2 presumably as a result of intramolecular catalysis by the carboxylate function in the peroxy acid monoanion. At pH > 4, trace-impurity-induced decompositions of both MPPA and hydrogen peroxide prevented satisfactory studies of the hydrolysis reaction. At pH < 1 the hydrolysis rate constant at first decreases, as a result of an activity coefficient effect, to reach a minimum value at log₁₀[H⁺]ca. 0.7, and then increases rapidly with the onset of an acid-catalysed reaction at higher acidity.