Envlron. Sci. Technol. 1988, 20, 1017-1022
Kinetics of Oxidation of Dimethyl Sulfide by Hydrogen Peroxide in Acidic and Alkaline Medium Yusuf Gbadebo Adewuyl and Gregory R. Carmlchael* Chemical and Materials Engineering Department, University of Iowa, Iowa City, Iowa 52242
The kinetics of oxidation of dimethyl sulfide (Me$) by hydrogen peroxide has been investigated spectrophotometrically in acidic and neutral medium. The rate law and other data indicate the reaction is first order with respect to both H202and MezSand is subject to catalysis by strong acids. The catalytic effect was found to be pronounced at a pH value of 1or less. Differences in activation energies for acidic solutions (pH Hz02> HO,. The production of HOC (H20z OH- s HO, + HzO) is pronounced only in strong alkaline medium, pH >11 (28). Equation 19, the rate-controlling step for the uncatalyzed reaction, represents a termolecular displacement with water, the electrophilic agent, and sulfide, the nucleophilic agent. The hydrogen peroxide could be presumed to be largely solvated prior to oxidation. Overberger and Cummins (22) observed that the uncatalyzed oxidation of sulfides by H20z was promoted in solvents capable of hydrogen bonding with H202,and they also found alcohols and water to be equally effective in solvating hydrogen peroxide. It is generally true that reactions producing products more polar than the reactants are favored in polar solvents (29). However, solvation of sulfoxide by solvent results in a negative charge displacement toward the solvation site with reduction in the nucleophilicity of the sulfur atom and decreased oxidation of sulfoxide to sulfone. The overall rate expressions can be obtained by applying steady-state approximations to the intermediate I and I‘ (i.e., dI/dt = dI’/dt = 0): -d [Me,S] = kH[HOOH2+] [MezS] dt
+
and
for the cases of acid-catalyzed and uncatalyzed reactions, respectively. The overall rate law over a range of pH values can thus be described by the relation
where k = kfH + k H K ~ [ H +and ] hi, is the observed rate constant in uncatalyzed neutral solution and the product kHKH is the catalytic constant for the acid-catalyzed reaction. The existence of two independent rate laws is a characteristic of H202reactions with electron pair donors including the thiosulfate, chloride, and hypochloride and Environ. Sci. Technol., Vol. 20, No. 10, 1986
1021
sulfides (RSR), and dimethyl disulfides (RSSR). Hydrogen peroxide will oxidize SO2to sulfate (30), HzS to sulfate and sulfur (311,RSH and RSSR to sulfonic acid (RS03H)and sulfate (32,331,and RSR to sulfoxides and sulfones. The products of oxidation are all odorless, and hence, HzOzmay provide economic means for odor and wastewater quality control. Registry No. Me#, 75-18-3; HzO,, 7722-84-1;HC1,7647-01-0; HZSOI, 7664-93-9.
Literature Cited
1 0
I
I
I
I
I
I
I
1
2
3
4
5
6
7
PH
Figure 10. Plot of rate constant vs. pH at 20
OC.
the bromide and hypobromide (20).
S u m m a r y and Conclusions Oxidation of dimethyl sulfide by hydrogen peroxide is bimolecular (unit order with respect to H20zand Me$) and is subject to acid catalysis. The observed rates are found to be pH dependent. As shown in Figure 10, the rate coefficient is fairly constant between pH 2 and pH 6, increases drastically at pH 1and below, and decreases substantially at pH 7. The presence of 0.1 N HC1 solution increased the observed rate constant by a factor of 3-5 times the value in neutral solution depending on the temperature. The observed catalysis is not simply due to an increase in ionic strength (21). For solutions of the same normality, the catalytic power of HCl was found to be double that of H2S04at 20 "C. The low catalytic ability observed with H2S04solutions compared to HCl was partly attributed to the depletion of H202in solution due to a secondary reaction with H2S04to form peroxornonosulfuric acid (H,SO,) which is fairly stable in solution. Two different reaction mechanisms were found to be operative as indicated by the differences in activation energies. For acidic solutions (pH
