Study on the Pyrolytic Mechanism of Thietane Using Low Pressure

The kinetics and mechanism of the pyrolysis of thietane have been studied below 1100 K at 10^-2 Pa by using the very low pressure pyrolysis technique (Table 1). Thietane decomposed above 900 K (Fig. 1) to yield ethylene and thioformaldehyde. The amount of ethylene produced was nearly equal to that of thietane reacted (Table 2), meaning the stoichiometry of this reaction as follows. Assuming that biradical mechanism is valid in this system, unimolecular reaction rate constant K_uni (Table 3, Fig. 2) obtained is considered to be separated to the following three cases. Combining the estimated entropy change of each case (Table 4), high pressure Arrhenius parameters were given as follows (Table 5), by means of RRKM unimolecular theory (Table 6). These activation energy values are closed to those, which are estimated on the assumption that thietane decomposes via biradical. From these values, the activation energy for decomposition and the ring closure of biradical C-C-C-S are estimated to be the same, about 10~20 kj.mol^-1. It is concluded that thietane decomposes via biradical mechanism in both thermolysis and photolysis, as is in contrast to the case of cyclobutane.