Kinetics and Mechanism of Peroxymonocarbonate Formation

The kinetics and mechanism of peroxymonocarbonate (HCO4−) formation in the reaction of hydrogen peroxide with bicarbonate have been investigated for the pH 6−9 range. A double pH jump method was used in which 13C-labeled bicarbonate solutions are first acidified to produce 13CO2 and then brought to higher pH values by addition of base in the presence of hydrogen peroxide. The time evolution of the 13C NMR spectrum was used to establish the competitive formation and subsequent equilibration of bicarbonate and peroxymonocarbonate following the second pH jump. Kinetic simulations are consistent with a mechanism for the bicarbonate reaction with peroxide in which the initial formation of CO2 via dehydration of bicarbonate is followed by reaction of CO2 with H2O2 (perhydration) and its conjugate base HOO− (base-catalyzed perhydration). The rate of peroxymonocarbonate formation from bicarbonate increases with decreasing pH because of the increased availability of CO2 as an intermediate. The selectivity for formation of HCO4− relative to the hydration product HCO3− increases with increasing pH as a consequence of the HOO− pathway and the slower overall equilibration rate, and this pH dependence allows estimation of rate constants for the reaction of CO2 with H2O2 and HOO− at 25 °C (2 × 10−2 M−1 s−1 and 280 M−1 s−1, respectively). The contributions of the HOO− and H2O2 pathways are comparable at pH 8. In contrast to the perhydration of many other common inorganic and organic acids, the facile nature of the CO2/HCO3− equilibrium and relatively high equilibrium availability of the acid anhydride (CO2) at neutral pH allows for rapid formation of the peroxymonocarbonate ion without strong acid catalysis. Formation of peroxymonocarbonate by the reaction of HCO3− with H2O2 is significantly accelerated by carbonic anhydrase and the model complex [Zn(II)L(H2O)]2+ (L = 1,4,7,10-tetraazacyclododecane).