The photolysis of methyl disulfide vapor in the pressure range 2-25 Torr a t wavelengths between 2 300 and 2 800 and a t 2 288 A has been examined and the effect of temperature, pressure, added inert gases, ethyl disulfide, and nitric oxide determined.The primary process is a direct production of two CH3S radicals which have excess energy and which can be observed as methyl thionitrite when NO is present during the decomposition. When pure disulfide is photolyzed the inajor observable product is methanethiol, although this material accounts for only a small fraction of the primarily produced thiyl radicals whose principal fate is recombination in a substrate-reforming reaction producing excited disulfide molecules. The latter species are deactivated by added gases, or by the substrate itself. The mode of mercaptan formation is by abstraction of H atoms from the substrate by excited CH3S radicals with a n apparent activation energy of 1.5 kcal.
The photolysis of methyl sulfide vapor has been investigated as a function of substrate pressure, exposure time, and temperature in the wavelength range 2000 to 2300 Å. The effects of added propane, sulfur hexafluoride, and 2-methylpentane have been studied. The principal products of the decomposition are CH4, C2H6, CH3SSCH3, and CH3SH. The data indicate direct C—S bond scission in the primary process giving rise to "hot" CH3 and CH3S radicals. A mechanism in which disproportionation of methylthiyl radicals is a very minor process is proposed.The reactions of CH3 and CH3S radicals with methyl sulfide have been examined also by photolyzing CH3SSCH3 and CH3COCH3 in the presence of CH3SCH3. Quantum yields at 2288 Å have been determined as a function of several variables. A number of rate parameters for the reactions of methyl radicals and methylthiyl radicals have been calculated.
The gas phase photolysis of methyl disulfide and ethyl disulfide and their mixtures, in the presence and absence of ethylene, has been studied at 25 "C and 4 =.2300-2800 A. The,pure substrates give predominantly the corresponding thiol, whereas co-photolys~s y~elds methyl ethyl d~sulfide in appreciably larger yields.When ethylene is added, the substrates individually show a reduction in RSH rate and the formation of relatively small amounts of sulfides. I n their co-photolysis, added CzH4 does not appreciably alter the rate of methyl ethyl disulfide formation, indicating a low efficiency of RS scavenging by the olefin in this system.Isopropanol and 2,3-dimethyl butane do not increase the thiol yield from the pure substrates.Canadian Journal of Chemistry, 46, 2462Chemistry, 46, (1968 It has been established previously that in both the vapor (1) and liquid (2) phase, thiyl radicals, whose principal fate is recombination, are formed in the photolysis of methyl disulfide. Co-photolysis of liquid methyl disulfide and ethyl disulfide yields CH3SSC2H5 predominantly via the chain stepwhich leads to a photochemical equilibrium. Reactions yielding products other than disulfides are negligible. The attack of methylthiyl radicals in the gas (3, 4) and liquid (5, 6) phase on olefink double bonds involves the reversible formation of a thiaalkyl radical. With disulfides as substrates the absence of a labile hydrogen prevents chain propagation, and product yields should indicate the effectiveness of attack on the olefin as an alternative to RS recombination.We report here the results of a further investigation of disulfide photolyses to determine if a photochemical equilibrium characterizes the system in the gas phase, and to examine the effects of added olefin and two addends potentially providing a site for H-atom abstraction.In the first series of experiment^,^ 3 Torr each of CH3SSCH3 and C,H5SSC2H5 were photolyzed. Product yields are given in Fig. 1. As observed in the liquid phase (2) the yield of methyl ethyl disulfide is appreciable even after relatively short exposure times. In the gas phase, however, CH3SH and C,H5SH are also formed, and this decomposition mode becomes increasingly important.When methyl disulfide is photolyzed with added ethylene, the yield of methanethiol, the major product of pure substrate decomposition, decreases with increasing P(C2H,). A new product, methyl ethyl sulfide, in yields directly proportional to olefin pressure, is also formed.
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