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The thermal decomposition of methyl disulfide Coope, John Arthur Robert


This report describes a kinetic study of the thermal decomposition of methyl disulfide. The study was made as a contribution to the chemistry of organic sulfur compounds. The homogeneous decomposition was studied by the static method in an all-glass apparatus. It was followed by observation of the accompanying pressure changes and by analysis of the reaction mixture at various stages of the reaction. Several new analytical methods were developed for this analysis. The experimental conditions were varied by varying the temperature of the decomposition, the pressure of the reactant, and the surface to volume ratio of the reaction vessel, and by adding various substances to the reacting system. The experimental results show that the reaction is composite: (1) Initially a fraction of the reactant disulfide adsorbs on the surface of the pyrex reaction vessel at a rate which agrees with the theoretical rate for a collision mechanism. (2) There follows an induction period during which the disulfide does not decompose. (3) The first decomposition is into methyl mercap-tan and a non-volatile product believed to be a thiofor-maldehyde polymer. The suggested overall reactions are: CH₃-S-S-CH₃ → CH₃SH + CH₂=S n CH₂=S → (-CH₂-S-)n (4) A later, competing, process produces hydrogen sulfide, ethylene, free sulfur and polysulfides. It is possible that the carbon disulfide formed is produced by this reaction, and it is probable that the thioaldehyde polymer is decomposed. The overall unbalanced reactions may be written: CH₃SSCH₃ + ((CH₂S)n?) → H₂S + C₂H₄ + S + (CS₂?) + (?) CH₃-S-S-CH₃ + Sₓ → CH₃-Sₓ -CH₃ (5) Secondary reactions continue long after the disulphide has been used up. The major process appears to be the degradation of the postulated polythioformaldehyde to hydrogen sulfide and possibly to carbon disulfide and ethylene. Degradation of the polysulfides appears to occur also, and there may well be further secondary reactions. The nature of the various component processes have been discussed in some detail, and lines of further research have been suggested. The structure of disulfides and polysulfides and the bonding of organic sulfur compounds have been discussed.

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