The reaction of methyl p‐nitrophenyl sulfate with hydrogen peroxide anion determination of pK_a of hydrogen peroxide in methanol by a kinetic spectrophotometric procedure

Abstract

A kinetic spectrophotometric investigation of the reaction of the hydrogen peroxide anion with methyl p-nitrophenyl sulfate in methanol solvent resulted in the evaluation of the pKa of HOOH in methanol at 25'C as 15.8 f 0.2. Since normal kinetic procedures for the determination of the equilibrium constant K for the process CH30-+ HzOz i=t CH30H + HOT were found to be associated with high uncertainty, another procedure was devised to establish the magnitude of K. This method is based on an analysis of the changing slopes of plots of pseudo-first-order rate constants against the total base concentration as the stoichiometric amount of hydrogen peroxide is varied. The method is applicable to any system in which anionic nucleophiles generated in situ compete with solvent anions. Such B corroboration of kinetically determined equilibrium constants is believed essential. The kinetic data allow the specific rate constant kHOO-for the reaction of methyl p-nitrophenyl sulfate with hydrogen peroxide anions to be evaluated and yield the rate constant ratio kHOO-/kMeO-= 8.8 f 2.2. This confirms the existence of an (Y effect at saturated carbon in this system.

Role of Protons in Superoxide Reduction by a Superoxide Reductase Analogue

“…Rates of 2 formation are dependent on the p K a of the proton donor: for the reaction to occur at comparable rates, the concentration of EtOH has to be ∼500 times higher than that of NH 4 + . Although the p K a 's (both relative and absolute) of these proton donors are likely to differ dramatically in THF (vs H 2 O), the fact that EtOH will protonate 2 suggests that the initial protonation site is rather basic. , The proton dependence of 2 formation, along with the unusually high energy of the S → Fe(III) charge-transfer band 6 and the highly ordered distal oxygen observed by EXAFS,5a suggests that the distal peroxide oxygen is protonated and perhaps hydrogen-bonded to the thiolate. A hydrogen-bonded ring structure (Figure ) might, in fact, provide a driving force for the formation of hydroperoxide-ligated 2 .…”

The α‐Effect: A Critical Examination of the Phenomenon and Its Origin

“…Moreover, it is also not clear what type of subst rates will benefit more from such an inhomogeneous polarizability. Willit indeed be more important with carbonyl func- 40,41] have shown that in certain cases the a-effect should be at least partially attributed to stabilization of the transiton state rather than to ground state destabilization. For example, in the reaction of hydrazine and hydroperoxide anion with a series of methyl phenyl sulfates (Nu + CH 3-OS0 3Ar~N u-CH 3 + "OSO~r), the a-effect was shown by Buncel et al [23,41] to result mainly from stabilization of the transition state.…”

“…Willit indeed be more important with carbonyl func- 40,41] have shown that in certain cases the a-effect should be at least partially attributed to stabilization of the transiton state rather than to ground state destabilization. For example, in the reaction of hydrazine and hydroperoxide anion with a series of methyl phenyl sulfates (Nu + CH 3-OS0 3Ar~N u-CH 3 + "OSO~r), the a-effect was shown by Buncel et al [23,41] to result mainly from stabilization of the transition state. The argument (which will not be repeated here) considered Brensted and Hammett type correlations in terms of "anti-Hammond" motion on a More O'Ferrall-Jencks diagram and suggested a limitation on the applicability of the RSP in this case.…”