The main inhibiting action of dialkyl sulfoxides in the autoxidation of hydrocarbons derives from their ability to form sulfenic acids on thermal decomposition. The sulfenic acids are extremely active radical scavengers, the rate constant of the reaction of sulfenic acid and peroxy radicals being at least 107 l.mole−1 · S−1 at 60°C. In the AIBN‐initiated oxidation of tetralin in the presence of sulfoxides one peroxy radical is scavenged per molecule of sulfenic acid. This inhibition can be described kinetically as a reduction of the rate of initiation by an amount equal to the rate of decomposition of the sulfoxide.
KONINKLIJKE/SHELL-LABORATORIUM, AMSTERDAMThe autoxidation of n-octyl mercaptan and thiophenol in tert-butanol containing varying amounts of potassium tert. butoxide, yields as the only products disuffide and/or acids (sulfinic and sulfonic). It appears that disulfide is formed from unionized mercaptan, whereas the acids originate in the mercaptide ion. A mechanism for the formation of the acids is presented, in which the sulfenate anion (RSO-) is an important intermediate. The primary species formed in the reaction of rnercaptide ion with oxygen is thought to react preferentially with unionized mercaptan to give the disulfide.
I . IntroductionAutoxidation (oxidation with molecular oxygen) of mercaptans in alkaline solution and oxidation by other mild oxidants are generally known to yield disulfides. Oxidation to oxygenated products -sulfinic and sulfonic acids -requires the action of concentrated nitric acid or permanganate.Recent investigations in this laboratory have shown that sulfinic and sulfonic acids can also be formed by the action of oxygen in alkaline systems 1 : 2. However, attempts to obtain reproducible data for this reaction have so far not been successful owing to simultaneous formation of disulfide which is strongly catalysed by unavoidable traces of metal ions.Since the effect of metals appeared to be very much reduced in tertbutanol * we have studied the autoxidation in this solvent with potassium tert-butoxide as the base. Another important advantage of this system is the low acidity of the solvent. Hence mercaptan can be converted almost completely into its anion by the base. As a result, nearly quantitative formation of acids can be attained. * A short survey of the effect of some metal ions in this solvent is given in the Appendix.
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