Abstract
Using a strictly qualitative approach, the present work suggests a possible explanation for the observed antioxidant action of dithiocarbamates. It has been shown that dithiocarbamates react with hydroperoxides to form a new oxidized intermediate which in most cases is stable. These have been identified as semisulfinates: (see PDF for diagram) With continued oxidation, a normal sulfinate is formed, the structure of which is probably best represented as follows where M is a divalent metal and R is, e.g., a simple alkyl group: (see PDF for diagram) The stability of the above compound will depend upon the metal M and the nature or size of the alkyl group. These compounds further decompose into a thiuram monosulfide and the corresponding metal sulfate. Particularly active or sensitive dithiocarbamates are oxidized directly to the metal sulfate and thiuram monosulfide. In these cases, the ephemeral formation of the semi-sulfinate and normal sulfinate as intermediates is probable. In the presence of excess hydroperoxide, the thiuram monosulfide undergoes further oxidation to form a carbamyl thiocarbamyl disulfide: (see PDF for diagram) Compounds of this structure are moisture sensitive and readily undergo hydrolysis. In the presence of zinc oxide and water such as would be found on the surface of a conventional rubber stock, it is suggested that a further reaction occurs to form a zinc dithiocarbamate which will then undergo the sequence of reactions described above until the dithiocarbamate is completely consumed. It is suggested that these reactions account for the antioxidant behavior of dithiocarbamates in a hydrocarbon system such as rubbers and petroleum products where the oxidizing agent is a hydroperoxide or some peroxygen compound which, if not destroyed, would in turn degrade the product to one of no utility.