Activation of hydrogen peroxide by p-nitrophenyl chloroformate. Indication for in situ formation of carbon trioxide and evidence for singlet oxygen generation by the chloride-catalysed decomposition of hydrogen peroxide in acidic solution

Abstract

The kinetics of the activation of hydrogen peroxide by p-nitrophenyl chloroformate (NPCF) in tetrahydrofuran (THF) have been investigated by measuring the phosphorescence of the released singlet molecular oxygen (¹O₂) at 1270 nm. The results are consistent with the assumption that the reaction between hydrogen peroxide and NPCF under pseudo first-order conditions with respect to hydrogen peroxide proceeds via a consecutive, pseudo first-order reaction. In the first pseudo first-order reaction O-p-nitrophenyl monoperoxycarbonic acid 1 and hydrochloric acid are formed. The corresponding second-order rate constant at T = 20 °C was found to be k₅ = (2.6 ± 0.5) × 10⁻² dm³ mol⁻¹ s⁻¹. In the second pseudo first-order reaction 1 undergoes an acid-catalysed decomposition to give, in the end, besides p-nitrophenol, carbon trioxide (CO₃), the anhydride of the unstable monoperoxycarbonic acid. The pseudo first-order rate constant of this catalysis was determined to be k₁₀ = (2.9 ± 0.4) × 10⁻³ s⁻¹ at T = 20 °C. The Arrhenius parameters determined for both reactions are consistent with the assumption that the formation of 1 occurs according to a B_AC2-mechanism and that the acid-catalysed decomposition of 1 occurs according to an A_AC1-mechanism. The assumption that 1 in the NPCF–H₂O₂ system undergoes an acid-catalysed decomposition to give CO₃ is confirmed by the results of the measurements carried out with the 1H-1,2,4-triazolecarboxylic acid p-nitrophenyl ester (TCNP)–H₂O₂ system in THF in the presence of sulfuric acid. Moreover, in the NPCF–H₂O₂–THF system ¹O₂ is generated by the decomposition of hydrogen peroxide catalysed by chloride. The fact that in acidic solution ¹O₂ is formed by a chloride-catalysed decomposition of hydrogen peroxide is confirmed by measurements carried out with hydrochloric acid–H₂O₂ systems in THF and in water, respectively. For the latter system it is found that the oxygen liberated is exclusively formed as ¹O₂.