Oxidation of S IV by Hydrogen Peroxide

FIGURE 7.17 Second-order rate constant for oxidation of S(IV) by hydrogen peroxide defined according to d[S(VI)]/ = fc[H202(aq)] [S(IV] as a function of solution pH at 298 K. The solid curve corresponds to the rate expression (7.84). Dashed lines are arbitrarily placed to encompass most of the experimental data shown. 

The reaction proceeds via a nucleophilic displacement by hydrogen peroxide on bisulfite as the principal reactive S(TV) species (McArdle and Hoffmann 1983) 

The latter reaction becomes faster as the medium becomes more acidic. 

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In addition to the S(IV) —H202 reaction, the reactions of other peroxides such as peroxymonosulfate, peroxyacetic acid, and methyl hydroperoxide with S(IV) are also sensitive to specific-acid catalysis (Hoffmann and Calvert, 1985). The rate of oxidation of S(IV) by HSO / is comparable to the rate of oxidation of S( IV) by hydrogen peroxide (Betterton and Hoffmann, 1988). We have proposed a general mechanism for the ROOH-S(IV) reaction in which the rate-determining step involves the acid-catalyzed decomposition of a peroxide-bisulfite intermediate. The rate expression applicable for this mechanism is... 

Continuous-flow e.s.r. studies have been made on the oxidation of vanadium(iv) by hydrogen peroxide.Both the VO + ion and the intermediate complex formed, [OVOO] +, have been monitored over a wide range of initial V, H2O2, and H+ concentrations. In the presence of excess H2O2, a mechanism accounting for the deviation from pseudo-first-order kinetics may be represented as... 

The OH radical participates in a number of aqueous-phase reactions at least 16 reactions producing OH and 19 consuming it have been proposed by various investigators. The principal sink for dissolved OH under conditions typical of remote continental clouds is reaction (142). Other sinks are the reactions with hydrogen peroxide, formic acid, and S(IV). The main aqueous-phase sources of OH are reaction (146) and the photolysis of dissolved hydrogen peroxide. Secondary sources are the photolysis of NO and the oxidation of S(IV) by HOz. 

FIGURE 7.16 Rate of aqueous-phase oxidation of S(IV) by ozone (30 ppb) and hydrogen peroxide (I ppb), as a function of solution pH at 298 K. Gas-aqueous equilibria aie assumed for all reagents. R/ so. represents the aqueous phase reaction rate per ppb of gas-phase SOj. R/L represents rate of reaction referred to gas-phase SO2 pressure per (gm 3) of cloud liquid water content. 

Differences in electrophysiological and contractile properties of mammalian cardiac tissues bathed in bicarbonate- and HEPES-buffered solutions, Acto Physiol. Scand.YT, 11-18, 2003 Mash, H.E., Chin, Y.P., Sigg, L. et al., Complexation of copper by zwitterionic aminosulfonic (good) buffers. Anal. Chem. 75, 671-677, 2003 Sokolowska, M. and Bal, W., Cu(ll) complexation by non-coordinating M2-hydroxyethylpiperazine-iV -ethanesulfonic acid (HEPES buffer), J. Inorg. Biochem. 99, 1653-1660, 2005 Zhao, G. and Chasteen, N.D., Oxidation of Good s buffers by hydrogen peroxide, Anal. Biochem. 349, 262-261, 2006 Hartman, R.F. and Rose, S.D., Kinetics and mechanism of the addition of nucleophiles to alpha,beta-unsaturated thiol esters, J. Org. Chem. 71, 6342-6350, 2006. 

More decisive evidence is provided by the interconvertibility of N-aryl-JV-arylamidinothioureas (28) and Hector s bases by oxidation-reduction.63, B4,67b The former compounds are accessible (as salts) (i) by the condensation of arylthioureas (24) with arylcyanamides (23),63 (ii) by the extrusion of sulfur from the recently described40,41 s-diaryldithioformamidine hydrobromidesB4a (22), (tit) by the oxidation of arylthioureas (24) with 0.5 moles of hydrogen peroxide in the presence of mineral acids,B4a and (iv) by the mild reduction of Hector s bases by hydrogen sulfide in acid media.B4a The first of these four reactions limitB the structure of the products to the three alternatives 25, 28, and 30. Of these, 25 is excluded by the non-identity of the product with authentic67 N-phenyl-i -phenylamidinothiourea (25 R = Ph). The monosulfide structure (30) is not reconciled as readily with the observedB4a hydrolytic fission of the products into diaryl-guanidines (29) and thiocyanic acid as is structure 28. Indeed, as in the case of thioamides and nitriles (see Section II, C, 1), the present condensation may involve the primary formation of an intermediate diimido-monosulfide (30) and its isomerization to 28. 

A third example of a polymeric ligand with pH-sensitive solubility is 97. This ligand was prepared by ring-opening metathesis polymerization of the 1,4,7-triazacyclononane-containing monomer 96 by the chemistry shown in Eq. 40 [132]. This polymer was capable of forming Mn(IV) complexes that oxidize alkenes and cycloalkanes with hydrogen peroxide. This basic polymer s solubility is affected by pH, as is the case with the other polymers 93 and 95 described above. 

This polymer was capable of forming Mn(IV) complexes that oxidize alkenes and cycloalkanes with hydrogen peroxide. This basic polymer s solubility is affected by pH, as is the case with the other polymers 93 and 95 described above. 

Oxidation of thioureas. The well-established synthesis of 1,2,4-thiadiazole derivatives by the oxidation of arylthioureas has been further studied by an examination of the oxidation products of mixtures of equimolar quantities of two thioureas. Oxidation of a mixture of syw-diarylthiourea and thiourea by hydrogen peroxide in acidified ethanol yields 3-amino-4-aryl-5-arylimino-A -1,2,4-thiadiazolines (42 R = H). These compounds are considered to be formed by the cyclization of amidino-thioureas (43), which can be isolated from the oxidation of mixtures of l-alkyl-3-arylthioureas and thiourea. The overall mechanism resembles that postulated for the formation of Hector s Bases. The oxidation of binary mixtures of sym-diaryl- and iV-alkyl-thioureas similarly furnishes the trisubstituted thiadiazolines (42 R = alkyl). ... 

Returning to the explanation of induced reactions, we can say the following. Friend s proposal , according to which the error in the H2O2 determination is caused by reaction (83) catalyzed by manganese(II) or cerium(III) formed in the primary reaction between hydrogen peroxide and permanganate or cerium(IV) cannot be accepted. The reaction between the ions mentioned and peroxydisulphate at room temperature is very slow, and, furthermore, the increase in acidity —in contrast to its effect on the induced reaction —promotes the oxidation. There is... 

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