Superoxo radical scavenging action by common analgesic drug paracetamol: A model kinetic study

In acid perchlorate media ([H+] = 1.0–3.0 M), each mole of paracetamol, HOC6H4NHCOCH3 (APAP), at ambient temperature quantitatively reduces two mole of the metallo-superoxo complex, μ-superoxo-bis[pentaamminecobalt(III)]5+, [(NH3)5Co(III)(μ-O2)Co(III)(NH3)5]5+ (1). Here, complex 1 is reduced to [(NH3)5Co(OH2)]3+, Co2+, O2 and NH4+ and APAP itself is oxidised to quinone oxime and acetic acid. With a large excess of APAP over 1, the reduction follows first-order kinetics. The observed first-order rate constant (ko) increases linearly with increasing [H+] as well as with TAPAP (TAPAP being the analytical concentration of APAP). The protonated form of APAP, viz. APAPH+ seems to be the kinetically reactive reductant species. The enrichment of aqueous reaction media with D2O retards the reaction and thus it appears that the reaction proceeds through an electroprotic mechanism. A relatively small ΔH≠ (57 ± 2 kJ M−1) and moderately negative ΔS≠ (−68 ± 8 JK−1 M−1) supports a compact transition state.