Dimethyl sulphoxide, acidity

Aravamudan and Venkappayya75 oxidized dimethyl sulphoxide in acetate buffer of pH 4 to 4.5 and with a reaction time of only 1 min. They then added potassium iodide and acid and titrated with thiosulphate the iodine liberated by unused reagent. They reported that cerium(IV) and Cr(VI) were much less effective oxidizing reagents for the sulphoxide. A very similar procedure was used by Rangaswama and Mahadevappa76 to determine dimethyl sulphoxide and numerous other compounds with chloramine B. 

Bohme77 employed excess monoperphthalic acid in diethyl ether to oxidize dibenzyl and benzyl ethyl sulphoxides. Reaction time was 24 h at - 15 to + 10 °C, after which he added potassium iodide and water and titrated the iodine set free with thiosulphate. Dickenson78 oxidized dimethyl sulphoxide in malt, wort or beer with Na2S2Os. In... 

Goheen and Bennett9 showed that regular nitric acid could be used, in about two molar excess, for the oxidation of dimethyl sulphoxide to dimethyl sulphone in 86% yield. The reaction temperature was 120-150°C with a reaction time of about 4 hours. The mechanism for this reaction was postulated to involve initially a protonated sulphoxide species (which has been shown to be present in other strongly acidic systems101 ) followed by nucleophilic attack by nitrate, and the loss of nitrogen dioxide as shown in equations (4) and (5). 

If acetic anhydride is employed in place of the sulphuric acid, only the sulphone is formed12,13 whilst if nitroethane or acetic acid are employed, no oxidation at sulphur occurs. A patent has been secured for the industrial oxidation of dimethyl sulphoxide to the sulphone with nitric acid14. This procedure yielded 84% of the sulphone in a continuous process which was prone to detonation at water concentrations below 14%. 

Sodium tungstate has also been used as a catalyst in the oxidation of dimethyl sulphoxide to the sulphone36. The kinetics of this reaction have been studied in great detail and it has been shown that oxygen transfer to the sulphoxide takes place via two peroxytungstic acid species (HW05 and HWOg ). 

The first report of the use of bromine for the oxidation of sulphoxides appeared in 1966116. Diphenyl sulphone was isolated in 0.5-1% yield when the sulphoxide was treated with bromine in aqueous acetic acid for several hours. The yield was increased to about 5% by quenching the reaction with sodium carbonate. A kinetic study117 of a similar reaction involving dimethyl sulphoxide showed no significant yield improvement but postulated that the mechanism proceeds via an equilibrium step forming a bromosulph-onium type intermediate which reacted slowly with water giving dimethyl sulphone as indicated in equation (35). 

Nickel(IV) complexes react with dimethyl sulphoxide in acidic solution to give the sulphone and nickel(II) ions. The kinetics of this reaction have been studied and found to be very complex in nature. The reaction probably proceeds by initial complexation of the dimethyl sulphoxide to the nickel(IV) species followed by electron transfer and oxygen atom transfer producing the observed products149. 

Dimethyl sulphoxide has also been oxidized electrochemically, using either a platinum anode or a dimensionally stable anode containing iridium and selenium in 1 M sulphuric acid solution158. The former electrode requires a potential close to that required for oxygen evolution whilst the latter needed a potential 0.5 volts lower. Thus the dimension-... 

Gollnick and Stracke176 investigated the very complex mechanism involved in the photolysis of dimethyl sulphoxide and concluded that disproportionation is probably the route for the major sulphone-producing reaction. Other oxidized species such as methanesulphonic acid are also produced and are also probably formed by a series of disproportionation reactions, for example equation (62). Thus photolysis of dimethyl sulphoxide is not a synthetically useful reaction due to the large number of compounds produced. 

The base catalysed autoxidation of dimethyl sulphoxide and methyl phenyl sulphoxide at 80 °C produces low quantities of methanesulphonic acid in both cases and benzenesul-phonic acid in the latter case189 (equation 71 and 72). There is no evidence of sulphone formation in either reaction. Dimethyl sulphoxide oxidation to methanesulphonic acid also occurs in the presence of trace quantities of acid and oxygen. Again the reaction would not be synthetically useful190. 

Inhibition of Hyaluronic Acid Degradation by Dimethyl Sulphoxide, S. A. Barker, S. J. Crews, J. B. Marsters, and M. Stacey, Nature, 207 (1965) 1388-1389. 

Douglas investigated heats of formation of dimethyl sulphoxide (and also of the sulphone) and proposed in a footnote that it could be determined by 5-min reaction with potassium permanganate/sulphuric acid, then adding excess iron(II) sulphate and finally titrating with permanganate. The same principle was used by Krishnan and Patel to determine dimethyl sulphoxide in various complexes (with perchlorates of titanyl, zirconyl and thorium), and by Krull and Friedmann to determine the same compound but using only dilute sulphuric acid and 5-min reaction. 

A determination of dimethyl sulphoxide by Dizdar and Idjakovic" is based on the fact that it can cause changes in the visible absorption spectra of some metal compounds, especially transition metals, in aqueous solution. In these solutions water and sulphoxide evidently compete for places in the coordination sphere of the metal ions. The authors found the effect to be largest with ammonium ferric sulphate, (NH4)2S04 Fe2(S04)3T2H20, in dilute acid and related the observed increase in absorption at 410 nm with the concentration of dimethyl sulphoxide. Neither sulphide nor sulphone interfered. Toma and coworkers described a method, which may bear a relation to this group displacement in a sphere of coordination. They reacted sulphoxides (also cyanides and carbon monoxide) with excess sodium aquapentacyanoferrate" (the corresponding amminopentacyanoferrate complex was used) with which a 1 1 complex is formed. In the sulphoxide determination they then titrated spectrophotometrically with methylpyrazinium iodide, the cation of which reacts with the unused ferrate" complex to give a deep blue ion combination product (absorption maximum at 658 nm). 

Peroxomonophosphoric acid (PMPA) oxidizes dimethyl sulphoxide in high yield in water and aqueous ethanol . In neutral solution the reaction mechanism was thought to be very complex but actually occurs by two different mechanisms that are very similar to those for sulphoxide oxidation by peracids in acidic and basic media. In an alkaline medium the mechanism involves nucleophilic attack by a phosphorus-containing species (probably POs ) on the sulphur atom of the sulphoxide, followed by O—O bond scission yielding the sulphone (equation 24). In acidic solution, on the other hand, the sulphoxide is the nucleophilic species as detailed in equation (25). It should be noted however that there is some evidence that these mechanisms are oversimplified since there are other nucleophilic species (such as H2P05 and HPO ") present in aqueous solutions of PMPA over a wide pH range . 

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