Kinetics and mechanism of the reaction between dimethyl trisulfide and cyanide

Abstract: Organically bound sulfur in the form of mono-, di- and polysulfide bridges constitutes a significant fraction of this element in recent and ancient sediments. In water columns of lakes, the concentrations of organo-sulfur compounds are much lower, and they are present in the form of malodorous dimethyl polysulfides. Currently, information regarding reactivity of organic polysulfides towards nucleophiles that are stronger than hydroxyl anions is lacking. In this work, the reaction kinetics of dimethyl trisulfide (DMTS) with the strong nucleophile and important environmental pollutant cyanide as a function of concentrations of reactants, pH and temperature were studied. It was found that the reaction rate constant as well as the activation energy of the reaction strongly depend on pH. The experimental data agree well with the existence of two distinct pathways: slow reaction between protonated cyanide and DMTS under acidic and neutral conditions and fast reaction between cyanide anion and DMTS under highly alkaline conditions. However, reactions of DMTS with the iron cyanide complexes hexacyanoferrate(ii) and hexacyanoferrate(iii) have no environmental significance since they are slower than the rate of DMTS decomposition. Under environmentally relevant conditions, rates of reactions of free cyanide with DMTS will be lower than the reaction with inorganic polysulfides or tetrathionate, but faster than the reaction with thiosulfate. Examples of application of kinetic parameters for calculation of rates of cyanide consumption in industrial and non-polluted natural aquatic systems as well as a protocol for quantification of organic polysulfide sulfur based on reaction with cyanide are provided.