Borate‐Catalyzed Reactions of Hydrogen Peroxide: Kinetics and Mechanism of the Oxidation of Organic Sulfides by Peroxoborates

Abstract

The kinetics of the oxidation of substituted phenyl methyl sulfides by hydrogen peroxide in borate/boric acid buffers were investigated as a function of pH, total peroxide concentration, and total boron concentration. Second-order rate constants at 25 degrees C for the reaction of methyl 4-nitrophenyl sulfide and H(2)O(2), monoperoxoborate, HOOB(OH)(3) (-), or diperoxoborate, (HOO)(2)B(OH)(2) (-), are 8.29 x 10(-5), 1.51 x 10(-2) and 1.06 x 10(-2) M(-1) s(-1), respectively. Peroxoboric acid, HOOB(OH)(2), is unreactive. The Hammett rho values for the reactions of a range of substituted phenyl methyl sulfides and hydrogen peroxide, monoperoxoborate or diperoxoborate are -1.50 +/- 0.1, -0.65 +/- 0.07 and -0.48 (two points only), respectively. The rho values for the peroxoborates are of significantly lower magnitude than expected from their reactivity compared to other peroxides. Nevertheless the negative rho values indicate positive charge development on the sulfur atom in the transition state consistent with nucleophilic attack by the organic sulfides on the peroxoborates as with the other peroxides. The kinetic parameters, including the lack of reactivity of peroxoboric acid, are discussed in terms of the differences in the transition state of reactions involving peroxoboron species with respect to those of other peroxides.

Borax‐Catalyzed and pH‐Controlled Selective Oxidation of Organic Sulfides by H2O2: An Environmentally Clean Protocol

“…Insofar as the oxidation of organic sulfides by peroxoborate is concerned, the reactions generally proceed in a manner similar to other peroxobased oxidants, for instance, by development of a positive charge on the sulfur atom in the transition state with the sulfide behaving as nucleophile, yet there is a succinct difference. The difference lies in mono and diperoxborates having the transition state earlier along the reaction coordinate [2] with relatively less positive charge being developed on the sulfur atom. This in turn emphasizes that proton transfer would play a significant role in the peroxoborate oxidation of organic sulfides.…”

“…The discovery of hydrogen carbonate catalyzed oxidation of organic sulfides [1] through peroxocarbonate (HCO 4 -) and the subsequent seminal contributions of Davies and coworkers [2] showing that borate is a better catalyst than HCO 4 -with rapidly formed peroxoborates, HOOB(OH) 3 -and (HOO) 2 B(OH) 2 -, being more reactive than HCO 4 -, and extending the pH functional range to the 8-12 region are very important developments in the realm of oxidation chemistry of organic sulfur. The examples to bear with this contention include oxidants like H 2 O 2 , [3] periodate, [4] peracids, [5] permonosulfate, [6] and peroxymonocarbonate.…”

Photochemistry of Peroxoborates: Borate Inhibition of the Photodecomposition of Hydrogen Peroxide

“…In this respect, we have recently shown that the electrophilic oxidation of organic sulfides by hydrogen peroxide is catalyzed in borate/boric acid buffers by the formation of peroxoborates. [1] Relevant equilibria are given in Equations (1) and (2), where K BOOH is the formation constant of monoperoxoborate, [2] and K BOH is the formation constant of borate (i.e. the acid dissociation constant of boric acid).…”

“…For runs above pH 9.5, small aliquots were removed from the reaction vessel at measured time intervals and analysed for total peroxide using the titanium IV method. [1] Values of k obs were determined using Equation (6) with the value of l for the reaction vessel that was determined as described in the next paragraph. For runs below pH 9.5, where the peroxide absorbs weakly at 254 nm, the absorbance of the reaction solution at 220 nm was measured directly, and k obs values determined using Equation (9).…”

“…The total peroxide concentration, [P], about 2.2 10 À3 m, unless stated otherwise, was determined as the titanium IV complex. [1] Absorbance at 254 nm, A 254 , was measured by using a Nicolet Evolution 300 spectrophotometer with a silica cell of optical path length, l, 1 cm. Equation (3) was used to calculate the apparent molar absorptivity, e 254 , due to the various peroxide species present.…”