Radical Cations in the OH-Radical-Induced Oxidation of Thiourea and Tetramethylthiourea in Aqueous Solution
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- 作者：Wen-feng Wang ; Man Nien Schuchmann ; Heinz-Peter Schuchmann ; Wolfgang Knolle ; Justus von Sonntag ; and Clemens von Sonntag
- 刊名：Journal of the American Chemical Society
- 出版年：1999
- 出版时间：January 13, 1999
- 年：1999
- 卷：121
- 期：1
- 页码：238 - 245
- 全文大小：239K
- 年卷期：v.121,no.1(January 13, 1999)
- ISSN：1520-5126
文摘
Hydroxyl radicals were generated radiolytically in N2O-saturated aqueous solutions of thiourea andtetramethylthiourea. The rate constant of the reaction of OH radicals with thiourea (tetramethylthiourea) hasbeen determined using 2-propanol as well as NaN3 as competitors to be 1.2 × 1010 dm3 mol-1 s-1 (8.0 × 109dm3 mol-1 s-1). A transient appears after a short induction period and shows a well-defined absorption spectrumwith 
max = 400 nm (
= 7400 dm3 mol-1 cm-1); that of tetramethylthiourea has max = 450 nm ( = 6560dm3 mol-1 cm-1). Using conductometric detection, it has been shown that, in both cases, OH- and a positivelycharged species are produced. These results indicate that a radical cation is formed. These intermediates withmax = 400 nm (450 nm) are not the primary radical cations, since the intensity of the absorbance depends onthe substrate concentration. The absorbance build-up follows a complex kinetics best described by the reversibleformation of a dimeric radical cation by addition of a primary radical cation to a molecule of thiourea. Theequilibrium constant for this addition has been determined by competition kinetics to be 5.5 × 105 dm3 mol-1for thiourea (7.6 × 104 dm3 mol-1 for tetramethylthiourea). In the bimolecular decay of the dimeric radicalcation (thiourea, 2k = 9.0 × 108 dm3 mol-1 s-1; tetramethylthiourea, 1.3 × 109 dm3 mol-1 s-1), formamidine(tetramethylformamidine) disulfide is formed. In basic solutions of thiourea, the absorbance at 400 nm of thedimeric radical cation decays rapidly, giving rise (5.9 × 107 dm3 mol-1 s-1) to a new intermediate with abroad maximum at 510 nm ( = 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 cationof thiourea is deprotonated by OH- (2.8 × 109 dm3 mol-1 s-1) to give a neutral thiyl radical. The latter reactsrapidly 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 strongoxidants and readily oxidize the superoxide radical (4.5 × 109 dm3 mol-1 s-1 for thiourea and 3.8 × 109dm3 mol-1 s-1 for tetramethylthiourea), phenolate ion (3 × 108 dm3 mol-1 s-1 for tetramethylthiourea), andeven azide ion (4 × 106 dm3 mol-1 s-1 for thiourea and ~106 dm3 mol-1 s-1 for tetramethylthiourea). WithO2, the dimeric radical cation of thiourea reacts relatively slowly (1.2 × 107 dm3 mol-1 s-1) and reversibly(2 × 103 s-1).
max = 400 nm ( = 7400 dm3 mol-1 cm-1); that of tetramethylthiourea has max = 450 nm ( = 6560dm3 mol-1 cm-1). Using conductometric detection, it has been shown that, in both cases, OH- and a positivelycharged species are produced. These results indicate that a radical cation is formed. These intermediates withmax = 400 nm (450 nm) are not the primary radical cations, since the intensity of the absorbance depends onthe substrate concentration. The absorbance build-up follows a complex kinetics best described by the reversibleformation of a dimeric radical cation by addition of a primary radical cation to a molecule of thiourea. Theequilibrium constant for this addition has been determined by competition kinetics to be 5.5 × 105 dm3 mol-1for thiourea (7.6 × 104 dm3 mol-1 for tetramethylthiourea). In the bimolecular decay of the dimeric radicalcation (thiourea, 2k = 9.0 × 108 dm3 mol-1 s-1; tetramethylthiourea, 1.3 × 109 dm3 mol-1 s-1), formamidine(tetramethylformamidine) disulfide is formed. In basic solutions of thiourea, the absorbance at 400 nm of thedimeric radical cation decays rapidly, giving rise (5.9 × 107 dm3 mol-1 s-1) to a new intermediate with abroad maximum at 510 nm ( = 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 cationof thiourea is deprotonated by OH- (2.8 × 109 dm3 mol-1 s-1) to give a neutral thiyl radical. The latter reactsrapidly 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 strongoxidants and readily oxidize the superoxide radical (4.5 × 109 dm3 mol-1 s-1 for thiourea and 3.8 × 109dm3 mol-1 s-1 for tetramethylthiourea), phenolate ion (3 × 108 dm3 mol-1 s-1 for tetramethylthiourea), andeven azide ion (4 × 106 dm3 mol-1 s-1 for thiourea and ~106 dm3 mol-1 s-1 for tetramethylthiourea). WithO2, the dimeric radical cation of thiourea reacts relatively slowly (1.2 × 107 dm3 mol-1 s-1) and reversibly(2 × 103 s-1).