Kinetics of the oxidation of diethyl sulfide in the B(OH)3-H2O2/H2O system

Abstract

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

Kinetics of the borate anion catalyzed oxidation of diethyl sulfide with hydrogen peroxide in an i-PrOH–H2O medium

“…We had found this effect of boric acid earlier for aqueous solutions (m = 0.1 M NaClO 4 ) [7]. It was shown that in water at pH 6-13 oxidation of diethyl sulfide occurs via three parallel routes: with participation of H 2 O 2 (reaction (5) For both systems the maximum catalytic effect is observed at pH » 10, however the character of the dependence of the rate constant on the acidity of the media in i-PrOH/H 2 O (Fig.…”

“…The determined values of k 5 = (7.5 ± 0.1)·10 -3 L/mol·s and k 6 = (1.5 ± 0.3)·10 -3 L/mol·s indicate the appreciable contribution of reacrion (6) to the overall rate of oxidation of Et 2 S at high pH, but the value of pK a PH = 11.8 ± 0.1 is close to that for water (11.6) [11]. According to [7] in aqueous solutions (m = 0.1 M NaClO 4 ) in the pH range 7-12.5 the oxidation of Et 2 S occurs only with the participation of H 2 O 2 (k 5 = 2.2·10 -2 L/mol·s). On the other hand in a phosphate buffer (m = 0.2) both routes are observed with k 5 = 2.7·10 -2 L/mol·s and k 6 = 0.41·10 -2 L/mol·s [2].…”

“…It was found [7] that the rate constants for the reaction of Et 2 S with MPB and DPB in water were about 2.5 and 100 times greater than for hydrogen peroxide.…”

“…However it is less effective than derivatives of hypochlorous acid in the decomposition of thioethers. One method for the activation of hydrogen peroxide is its conversion into peroxoacids with the use of such activators as bicarbonates [1, 2], silicates [3], molybdates [4,5], nitrites [6] and other compounds.It has been shown recently [7,8] that boric acid, B(OH) 3 , is an effective catalysts for the oxidation of thioethers with hydrogen peroxide in aqueous solutions. The catalytic effect of B(OH) 3 is quantitatively explained by a mechanism which includes the intermediate equilibrium formation of the anions of monoperoxoborate B(OH) 3 (OOH) -(MPB) and diperoxoborate B(OH) 2 (OOH) 2 -(DPB) according to the equations:…”

“…It has been shown recently [7,8] that boric acid, B(OH) 3 , is an effective catalysts for the oxidation of thioethers with hydrogen peroxide in aqueous solutions. The catalytic effect of B(OH) 3 is quantitatively explained by a mechanism which includes the intermediate equilibrium formation of the anions of monoperoxoborate B(OH) 3 (OOH) -(MPB) and diperoxoborate B(OH) 2 (1)…”