Common intermediates in the OH-radical-induced oxidation of cyanide and formamide

Abstract

OH radicals generated in the pulse radiolysis of N₂O-saturated water react with formamide (by H-abstraction) and with cyanide (by addition and rearrangement) to give the same radicals, the main radical being the ^·CONH₂ radical 1 (85–95%), the other most likely being the HC(O)NH^· radical 2 (5–15%). Quantum-chemical calculations support the preferred formation of 1 from formamide as well as from cyanide. Radical 1 reduces tetranitromethane (TNM) to the nitroform anion (2.8 × 10⁸ dm³ mol⁻¹ s⁻¹). In the presence of oxygen, it rapidly adds oxygen (2.7 × 10⁹ dm³ mol⁻¹ s⁻¹) to give the corresponding peroxyl radical ^·OOCONH₂ (3) which absorbs more strongly in the wavelength region 250–400 nm than radical 1 [ε(320 nm) ≈ 180 dm³ mol⁻¹ cm⁻¹]. Peroxyl radical 3 deprotonates in basic solution (pK_a ≈ 9.6), and its anion rapidly eliminates O₂^·− (10⁶ s⁻¹) to give cyanic acid. Product studies under γ-radiolysis conditions show that in the absence of O₂ less than half of radical 1 decays by disproportionation to produce cyanate and formamide. In basic solution and in the presence of O₂, the G values of cyanate and H₂O₂ confirm that all of the peroxyl radical 3 decays by O₂^·−-elimination.