Kinetics and mechanism of oxidation of 2,4-dinitrophenylhydrazine, p-nitrophenylhydrazine, and p-tolylhydrazine with potassium hexacyanoferrate(III) in acidic perchlorate media

Abstract

The kinetics of oxidation of 2,4-dinitrophenylhydrazine (dnph), p-nitrophenylhydrazine (pnph), and p-tolylhydrazine (pth) with hexacyanoferrate(III) have been studied in acidic perchlorate solutions. The oxidation of pnph has no dependence on [H⁺] in the [HClO₄] range 0.15–2.0 mol dm^–3. In the oxidation of dnph complex formation with [Fe(CN)₆]^3– occurs, and the reaction is independent of [H⁺] in the [HClO₄] range 0.025–2.5 mol dm^–3. The oxidation of pth in the [HClO₄] range 0.01–1.6 mol dm^–3 has been found to obey the rate law –d[Fe(CN)₆^3–]/dt= 3[Fe(CN)₆^3–][pth]{(k₁″/K₁″[H⁺])+k₂″+k₃″K₂″[H⁺]}, where K₁″ and K₂″ are the first and second protonation constants of pth, and k₁″, k₂″, and k₃″ are the second-order rate constants for the unprotonated, monoprotonated, and diprotonated species respectively. The oxidations occur via aryldiazene and diazonium ion intermediates, to produce substituted azobenzenes and anilines as final products. In the oxidations 2–3 mol of [Fe(CN)₆]^3– are consumed by each mol of arylhydrazine, depending on the conditions; under specific conditions, the stoicheiometry is exact.