Kinetic and mechanistic analysis of oxidation of 2-furoic hydrazide by hexachloroirradate(IV) in a wide pH range

Abstract

Oxidation of 2-furoic hydrazide (FH) by hexachloroiridate(IV) ([IrCl6]2−) was studied kinetically in a wide pH range in aqueous solution of 1.0 M ionic strength. The oxidation reaction followed well-defined second-order kinetics: − d[IrCl62−]/dt = k′[FH]tot[IrCl62−], where [FH]tot denotes the total concentration of FH and k′ stands for the observed second-order rate constants. The established k′–pH profile displays that k′ increases drastically with pH and a plateau region exists between pH 4 and 6. A stoichiometric ratio of Δ[FH]tot/Δ[IrCl62−] = 1/4 was revealed by spectrophotometric titrations. 1H NMR spectroscopic studies indicated that FH was cleanly oxidized to 2-furoic acid. The kinetic data suggest a reaction mechanism in which all the three protolysis species of FH react with [IrCl6]2− in parallel, forming the rate-determining steps. Two stabilized hydrazyl radicals are generated in the rate-determining steps, in which a single electron is transferred to [IrCl6]2−. The two hydrazyl radicals react rapidly in consecutive steps requiring 3 mol of Ir(IV) to form 2-furoic acid as the final product. Rate constants of the rate-determining steps were deduced through a simulation of the rate expression to the k′–pH dependency data. Values of these rate constants demonstrate that the three protolysis species of FH have a huge reactivity span, changing by about 109 times toward reduction in [IrCl6]2− and that FH can be readily oxidized in neutral and basic media. Rapid scan spectra and the measured activation parameters suggest that an outer-sphere electron transfer is probably taking place in each of the rate-determining steps. This is the first kinetic study on the oxidation reactions of FH and provides concurrently the protolysis constants of FH (pKa1 = 3.04 ± 0.08 and pKa2 = 11.6 ± 0.1) at 25.0 °C and 1.0 M ionic strength.