Data obtained for the kinetics of oxidation of diethyl sulfide (Et 2 S) by hydrogen peroxide in aqueous solution catalyzed by boric acid indicate that monoperoxoborates B(O 2 H)( ) OH 3 − and diperoxoborates B O H OH ( ) ( ) 2 2 2 − are the active species. The rates of the reactions of Et 2 S with B(O 2 H)( ) OH 3 − and B O H OH ( ) ( ) 2 2 2 − are 2.5 and 100 times greater than with H 2 O 2 .The search for new systems to effect rapid selective oxidation of sulfides and sulfoxides is important for solving the problem of the decomposition of pesticides and poisonous substances, whose active components are organic sulfides. Chloramines, commonly used for this purpose, are highly toxic and corrosive [1]. The most ecologically favorable oxidizing agent is hydrogen peroxide, which, however, has only low reactivity by itself. Bicarbonates [2, 3], molybdates [4,5], phthalates [6], and nitrites [7], which form highly reactive peroxo acids upon reaction with H 2 O 2 , are used to activate hydrogen peroxide. Boric acid, B(OH) 3 , which forms peroxoborates upon reaction with hydrogen peroxide under mild conditions, is a promising activator [8-10]. These compounds, in particular Na 2 [B 2 (O 2 ) 2 (OH) 4 ]·6H 2 O [11], are strong oxidizing agents and are used as industrial bleaches. No information is available on the kinetics and mechanisms of oxidation of thioethers by peroxyborates. The chemistry of peroxyborates is complicated. Depending on the ratio of the starting concentrations of B(OH) 3 and H 2 O 2 and the pH of the medium, either peroxoborates [B(O 2 H) n (OH) 4-n ] -(n = 1-4) or polyperoxoborates [B 2 (O 2 ) 2 (O 2 H) n (OH) 4-n ] 2-(n = 0, 2, or 4) are formed [10]. At relatively low concentrations of B(OH) 3 and H 2 O 2 (£1 mol/L) at pH 6-14, the major products are monoperoxoborate B(O H)(OH) 2 3 − (MPB) anions and diperoxoborate B(O H) (OH) 2 2 2 − (DPB)anions. Polyperoxoborates are the major products at higher reagent concentrations.In the present work, we studied the effect of boric acid on the rate of oxidation of diethyl sulfide (Et 2 S), which serves as a model for yprite, by hydrogen peroxide in aqueous solution in a broad pH range to establish the nature of the active species and find optimal reaction conditions. EXPERIMENTALDiethyl sulfide was prepared according to a standard procedure [12]. The working solutions were prepared using doubly distilled water, 35% hydrogen peroxide, and chemically-pure grade samples of boric acid, sodium perchlorate, H 3 PO 4 , and NaOH. 440040-5760/07/4301-0044

in aqueous solution and in the gas phase has opposite signs. Values for the KIE and k 6 /k 5 decrease with increasing temperature from 5 to 55°C in the gas phase, while these values increase in solution. We propose that this phenomenon is a consequence of a solvent cage effect.The reactions of hydroxyl radicals with hydrocarbons play an important role in biochemical processes and the chemistry of the atmosphere and natural water supplies [1][2][3][4]. The kinetics of the reactions of OH radicals with saturated hydrocarbons (RH)including cycloalkanes, has been studied extensively in the gas phase [1]. The absolute rate constants for reaction (1) have been measured in aqueous solution only for methane [5,6], while relative rate constants in the methane series have been measured for butane [7-10] and cyclopentane [11,12].In previous work [13,14], we carried out a systematic study of the kinetics and selectivity of the reactions of OH radicals with a wide range of normal, iso, and cycloalkanes in the oxidative system containing H 2 O 2 , Fe 2+ , Fe 3+ , and water similar to Fenton's reagent [3]. We found unusually low substrate selectivity and hydrogen kinetic isotope effect (KIE) for reaction (1) in aqueous solution in comparison with the gas-phase reactions. These findings were qualitatively explained by diffusion complications for the reactions in solution related to the cage effect [13].A new approach toward the study of the mechanisms for activation of C-H bonds of saturated hydrocarbons by oxidizing agents, metal complexes, and electrophiles developed in previous work [13,15] involves study of the temperature dependence of the KIE for cyclohexane/cyclohexane-d 6 pairs and the ratios of the reaction rates for the cyclopentane/cyclohexane pair and analysis of the compensation ratios obtained. There have been no data on the temperature dependence of the rate constants of the reactions of alkanes with OH radicals in aqueous solution.

temperature dependence of these ratios in the gas-phase reactions of cycloalkanes with peroxynitrous acid and OH radicals are identical. The same result was obtained for the reactions in aqueous solution. These data are in accord with the conclusion that OH ⋅ radicals formed in the homolysis of the HO-ONO bond are the active species in the reactions of HOONO with hydrocarbons in aqueous solution and in the gas phase.Peroxynitrous acid HOONO (PA) has been known for more than 100 years as an unstable intermediate in the oxidation of nitrites and nitrates by hydrogen peroxide in acid media [1]:(1)In vivo, the peroxynitrite anion ( -OONO) formed in the rapid recombination (k = (3.8-19)·10 9 L/mol·s [1, 2]) of nitrogen oxide (NO) and the superoxide anion (O 2 & − ) plays an important role in physiological processes, including the oxidation of alkyl and alkene C-H bonds of lipid membrane polyunsaturated fatty acids [1,3]. The anion is relatively stable in alkaline solutions, but the acid, HOONO, (pK a = 6.8) rapidly isomerizes to HNO 3 (t 1/2 » 1.2-1.3 s) [1,4]. The detailed mechanism of the isomerization is unclear [1]. Doubt has recently been cast on the previously accepted homolytic mechanism involving the intermediate formation of OH and NO 2 radicals [2]. Various workers [1,3,5,6] have explained the high activity of PA in reactions with organic compounds by 1) the formation of "OH-radical-like" species in the decomposition of HOONO, 2) by the formation of a nitronium cation NO 2 + upon catalysis by metal ions, and 3) by the direct reaction of HOONO or OONO -with the substrate.The reactions have been studied previously only with strong reducing agents. Prior to our work, only two studies have published concerning the products of the oxidation of cyclohexane, cyclohexene [7], and arenes [8] in the H 2 O 2 -HNO 2 system generating PA in situ.We were the first to undertake a systematic investigation of the kinetics of the reactions of peroxynitrite with hydrocarbons and found that alkanes, alkenes, and alkylbenzenes are oxidized by aqueous solutions of peroxynitrous acid when pH < 7.5 and by a model H 2 O 2 -HNO 2 system (pH » 4) [1,9]. These reactions in the two-phase gas/solution system 0040-5760/08/4402-0109

The relative rate constants (k RH /k EtH ), the temperature dependence of these constants at from 5 to 55°C, and the activation parameters were found for reactions of propane, butane, pentane, hexane, isobutane, cyclopentane, and cyclohexane with peroxynitrous acid (HOONO) in water. The similarity of these results to the data for the reaction of alkanes with OH radicals confirms that the active species in the reactions of HOONO with hydrocarbons in water are OH radicals formed in the homolysis of the HO-ONO bond.Peroxynitrous acid (HOONO) is an unstable intermediate in the oxidation of nitrites to give nitrates by hydrogen peroxide in acid media [1]. The history of peroxynitrite goes back more than a century but extensive systematic studies of the kinetics and mechanisms of the reaction of this species have been undertaken only in recent decades (see our review of recent investigations [2]). HOONO and its anion -OONO play an important role in biochemical processes, including the oxidation of alkyl and olefinic C-H bonds of polyunsaturated fatty acids in lipid membranes [2,3]. The relatively stable anion at physiologic pH values is protonated to give HOONO (pK a = 6.8), which rapidly isomerizes to HNO 3 (t 1/2 » 1 s) [2,4]. In vivo, -OONO is formed upon rapid recombination of nitric oxide (NO) and superoxide (O 2 & -) [5].The high and polyfunctional activity of HOONO in reactions with organic compounds is attributed to 1) the formation of "OH-radical-like" species upon the decomposition of HOONO [2, 6], 2) the formation of the nitronium cation NO 2 + upon catalysis by metal ions [2,7], and 3) the direct reaction of HOONO or -OONO with substrates [2,8]. Possessing a high oxidation potential, strong electrophilicity, and strong nucleophilicity, peroxynitrite reacts with many compounds [2]. The variety of pathways and mechanisms for the reactions of peroxynitrite and the nature of its active sites are a function not only of the reaction conditions (pH, presence of traces of transition metals, CO 2 , hydrogen peroxide, and nitrite), but also the properties of the substrates. In the case of a high rate of reaction of compounds with peroxynitrite, exceeding the rate of its decomposition, peroxynitrous acid or its anion may act as reagents. Radical mechanisms involving × OH-like active species (hydroxyl radical, radical pair or excited form of peroxynitrous acid) may obtain in the case of substrates with low reactivity [2]. We were the first to undertake a systematic study of the kinetics of the reactions of peroxynitrite with hydrocarbons and found that alkanes, alkenes, and alkylbenzenes are oxidized by aqueous solutions of peroxynitrous acid at pH < 7.5 and the 112 0040-5760/10/4602-0112

The relative rate constants (k rel ) were measured for the oxidation of alkanes (RH) by peroxynitrous acid (HOONO) in aqueous solutions at 25°C. The k rel values for the reactions of RH with HOONO and with OH radicals in the series of C 2 -C 7 alkanes and the isotopic effects (c-C 6 H 12 /c-C 6 D 12 ) agree. However, the k rel value for methane was lower than for its reaction with ⋅ OH. Possible reaction mechanisms are discussed.Peroxynitrous acid HOONO (PA) has been known for more than 100 years as an intermediate product in the oxidation of nitrites to nitrates by hydrogen peroxide in acidic media [1].(1)Interest in the chemistry of PA has increased in the last 15 years in connection with the discovery of the in vivo formation of peroxynitrite anion ( -OONO) during the rapid recombination of NO ⋅ and O 2 & − particles (k = 6.7·10 9 L/mol·s) [2] and the establishment of its important role in physiological processes [1,3]. The high reactivity of PA in reactions with organic substrates and biomolecules is usually attributed to the formation of "hydroxyl-like" radicals during homolysis of the ONO-OH bond [1,3,4]. Having high one-and two-electron oxidation potentials (E 0 = 1.70 and 1.37 V respectively [5] at pH 7), peroxynitrite can also react with strong reducing agents by direct attack on the substrate molecule [6]. In spite of numerous investigations in this region the nature of the active particles in reactions with the participation of PA remains the subject of pointed discussions [1,7] (in particular, the title of one of the papers is "Peroxynitrous acid -where is the hydroxyl radical?" [7]). There are two opposing views about the nature of the active particle in these reactions, i.e., either the OH radical or the PA in molecular form. Experimental and calculated evidence has been presented both for and against the homolytic mechanism in the reactions of PA with various substrates [7,8]. Difficulties in the study of these reactions are due to the instability of PA in neutral and acidic media and the large number of transformation paths [1]. Earlier we found that alkanes, alkenes, and arenes are oxidized by aqueous solutions of PA [9] and by solutions of H 2 O 2 -HNO 2 in an acetate buffer [10,11] and in the phosphate system (H 3 PO 4 -KH 2 PO 4 ) [12,13], in which HOONO is 0040-5760/06/4205-0303