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

摘要

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 (O-1(2)) 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 degrees C was found to be k (5)=(2.6 +/- 0.5)x10(-2) dm(3) 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 k(10)=(2.9 +/- 0.4)x10(-3) s(-1) at T=20 degrees C. The Arrhenius parameters determined for both reactions are consistent with the assumption that the formation of 1 occurs according to a B(AC)2-mechanism and that the acid-catalysed decomposition of 1 occurs according to an A(AC)1-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 O-1(2) is generated by the decomposition of hydrogen peroxide catalysed by chloride. The fact that in acidic solution O-1(2) 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 O-1(2). [References: 26]