1. J. Wallace, W. Bartok, and A. Schrieshiem Esso Research and Engineering Company Linden. New Jersey
A Convenient Synthetic Technique to Oxidize Mercaptans to Disulfides
Recently, we have studied the base catalyzed oxidation of mercaptans to disulfides in the presence of molecular oxygen.' In the initial stages of this work, it was necessary to have some type of accurate and simple technique that would enable oxygen consumption to be measured as a function of time at atmospheric pressure. The purpose of the present note is t o report on this technique which appears to be useful as a synthetic tool in the oxidation of alkyl, aralkyl, and aromatic mercaptans to disulfides. The atmospheric oxidation apparatus (see figure) consisted of a glass reaction vessel (D) and an oxygen source. The reaction vessel was a specially adapted heavy-walled 500-ml Erlenmeyer flask equipped with a side-arm 0.5 mm in diameter and 2 mm in length.
Oxidation apparatus: A, 0 2 gar bag; B, wet test meter; C, drying tower; D, reodor; E, magnetic stirrer; F, water ~ondens~r;G, conrtont temperolure bath.
Oxygen gas was supplied to the reaction vessel from a partially filled collapsible polyethylene gas balloon ( A ) of ca. 14-liter capacity through a wet-test gas meter (B), connected to a drying tower (C) packed with indicating Drierite, and finally through an overhead Friedrich's condenser ( P ) and into the reaction flask. The amount of oxygen initially admitted into the gas balloon had t o he minimized so that the fluctuation of barometric pressure would not affect the readings on the gas meter. The reaction flask was thermostated within + 0 . 5 T and stirred magnetically throughout. In the actual utilization of this equipment 0.1 mole of the mercaptan was added to the reaction flask which contained the desired solvent-base system and a Teflon-covered stirring bar-in a nitrogen dry box. The reaction flask was sealed with a suitable ground glass cap, removed from the dry-box and attached t,o the oxidation apparatus which had been pnrged previously with nitrogen. The entire system was then purged with oxygen to displace the nitrogen through the reaction flask side-arm, the side-arm was sealed, and the wet-test meter was set to read zero when equilibrium preRsure was estabilished. The reaction was then started by setting the Teflon-coveredmagnetic stirring bar in motion a t 1100 rpm. The reaction was Presented in part at the 140th meeting of the Am~ricanChemical Society, Chicago, Illinois, September, 1961. WALLACE, T. J. AND SCHRIESHEIM, A,, J . 079. Chem., 27,1514 (1962).
allowed to proceed until no appreciable oxygen consumption could be detected on the wet-test meter. The disulfides are obtained most conveniently by extraction with ether and then evaporating the ether extract. Further purification can be accomplished by distillation or recrystallization from alcohol depending on the physical state of the disulfide. Various types of mercaptans have been oxidized according to the above procedure in 100 ml solutions of 2 M. sodium methoxide in methanol or sodium hydroxide in water. Pertinent data are summarized in the table. Generally, good to excellent yields of each disulfide are obtained in both solvent systems. The methanol system is preferred however, since the reactions are usually complete in 2.5 to 11 hrs, depending on the mercaptan used. Under ordinary conditions, the reaction time in aqueous NaOH varies from 11.5 to 23 hrs t o obtain comparable yields. If one uses one-quarter aliquots of the original reaction mixtures, this gives a useful, small scale preparative technique requiring about 1 t o 3 hrs in methanol solution. Another alternative is to use trace amounts of an oxidation catalyst such as cobalt phthalocyanine (see table). In aqueous sodium hydroxide, e.g., a comparable yield of n-hutyl disulfide is obtained in about one-eighth of the time interval (1.5 hrs) required for the non-catalyzed reaction. The above technique has found wide use in a large number of atmospheric oxidation reactions studied in these laboratories. An additional advantage of this system is the fact that it can even be used a t elevated t,emperatures. If volatile gases are evolved during the course of the reaction (e.g., Cot) then the appropriate correction should be applied t o the measured uptake. The apparatus could be apparent rate of 0% also used at subatmospheric or superatmospheric pressures by placing the gas bag in a rigid enclosure equipped with a pressure gauge. In this modification, of course, the remainder of the equipment must be supplied with vacuum or pressure-proof parts. Thus, it should be of use to others who are engaged in similar work. Oxidation of Various Mercaptans in Basic Media Mercaptm
~.. .
Solvent" Water Water Water Water Methanol Methand Methanol
% Yield( of disulfide
Reaction time (hr.)
79 61 83 67 85 77 84
NaOH was em~lovedas the base in water while NaOMe was employed in th&ðanol solvent. Wohalt phthalocyanine (100 m g ) added as a catalyst. Based on the weight of disulfide after purilicstion. a
Volume
40, Number 1 , Jonuory 1963
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39
