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Radical cations in the OH-radical-induced oxidation of thiourea and tetramethylthiourea in aqueous solution

Hydroxyl radicals were generated radiolytically in N2O-saturated aqueous solutions of thiourea and tetramethylthiourea. The rate constant of the reaction of OH radicals with thiourea (tetramethylthiourea) has been determined using 2-propanol as well as NaN3 as competitors to be 1.2 x 1010 dm3 mol-1 s-1 (8.0 x 109 dm3 mol-1 s-1). A transient appears after a short induction period and shows a well-defined absorption spectrum with λ(max) = 400 nm (ε = 7400 dm3 mol-1 cm-1); that of tetramethylthiourea has λ(max) = 450 nm (ε = 6560 dm3 mol-1 cm-1). Using conductometric detection, it has been shown that, in both cases, OH- and a positively charged species are produced. These results indicate that a radical cation is formed. These intermediates with λ(max) = 400 nm (450 nm) are not the primary radical cations, since the intensity of the absorbance depends on the substrate concentration. The absorbance build-up follows a complex kinetics best described by the reversible formation of a dimeric radical cation by addition of a primary radical cation to a molecule of thiourea. The equilibrium constant for this addition has been determined by competition kinetics to be 5.5 x 105 dm3 mol-1 for thiourea (7.6 x 104 dm3 mol-1 for tetramethylthiourea). In the bimolecular decay of the dimeric radical cation (thiourea, 2k = 9.0 x 108 dm3 mol-1 s-1; tetramethylthiourea, 1.3 x 109 dm3 mol-1 s-1), formamidine (tetramethylformamidine) disulfide is formed. In basic solutions of thiourea, the absorbance at 400 nm of the dimeric radical cation decays rapidly, giving rise (5.9 x 107 dm3 mol-1 s-1) to a new intermediate with a broad maximum at 510 nm (ε = 750 dm3 mol-1 cm-1). This reaction is not observed in tetramethylthiourea. The absorption at 510 nm is attributed to the formation of a dimeric radical anion, via neutralization of the dimeric radical cation and subsequent deprotonation of the neutral dimeric radical. The primary radical cation of thiourea is deprotonated by OH- (2.8 x 109 dm3 mol-1 s-1) to give a neutral thiyl radical. The latter reacts rapidly with thiourea, yielding a dimeric radical, which is identical to the species from the reaction of OH- with the dimeric radical cation. The dimeric radical cations of thiourea and tetramethylthiourea are strong oxidants and readily oxidize the superoxide radical (4.5 x 109 dm3 mol-1 s-1 for thiourea and 3.8 x 109 dm3 mol-1 s-1 for tetramethylthiourea), phenolate ion (3 x 108 dm3 mol-1 s-1 for tetramethylthiourea), and even azide ion (4 x 106 dm3 mol-1 s-1 for thiourea and ~106 dm3 mol-1 s-1 for tetramethylthiourea). With O2, the dimeric radical cation of thiourea reacts relatively slowly (1.2 x 107 dm3 mol-1 s-1) and reversibly (2 x 103 s-1).