Reactions of thiyl radicals. VI. Photolysis of ethanethiol

Abstract

The photolysis of ethanethiol vapor in the pressure range 10 to 350 Torr and the effect of % decomposition, wavelength, and of added inert gas and ethylene on the rate of formation of the products has been determined. In addition, the effect of temperature on the photolysis of methanethiol–ethylene–inert gas and ethanethiol–ethylene–inert gas systems has been studied.For the pure substrate, the primary process is S—H bond cleavage and the hydrogen atom produced abstracts the sulfyhydryl hydrogen of the substrate. The quantum yield of molecular hydrogen formation is unity, independent of pressure. C2H5S radicals, formed either in the primary process or as a result of abstractive attack by H atoms or radicals on the substrate, combine to form an excited disulfide molecule. The major condensable product, C2H5SSC2H5, arises via collisional deactivation of this excited species which can also sensitize the decomposition of the substrate to give C2H4, C2H6, and H2S, the other products observed. With added inert gas the rates of formation of the minor products decrease whereas hydrogen and disulfide remain unchanged.With added ethylene, the major products are H2, C2H5SSC2H5, and C2H5SC2H5, the addition product. Diethyl sulfide is formed in a chain reaction by the attack of C2H5S on the olefinic double bond to form the composite radical, followed by H abstraction from the substrate. The hydrogen rate decreases due to the addition of H atoms to ethylene. The rate constant ratio, k22/k4, for the reactions [22] and [4], respectively,[Formula: see text]is pressure dependent, indicative of the formation of hot hydrogen atoms in the primary process of the photolysis of methanethiol and ethanethiol. The Arrhenius parameters A22/A4 and E22−E4 for the methanethiol–ethylene and ethanethiol–ethylene systems have been determined. Kinetic treatments of the proposed mechanisms have been undertaken.