Study of Thermal Decomposition Mechanism of Thiirane and Thiolane by Low-pressure Pyrolysis

Low-pressure pyrolysis technique was used to study the thermal decomposition of thiirane and thiolane at pressure 10^-2 Pa and the temperature up to HOOK (Table 1), Thiirane decomposed above 900 K (Fig. 1) to yield ethylene stoichiometrically (Table 2). RRK unimolecular reaction theory was applied to interprete the results. Arrhenius A factor was estimated as shown in Table 3. The best fit to the experimental rate constants k_uni was obtained when the following high pressure Arrhenius parameters were assumed (Fig. 2). k=10^13.2exp(-171(kJ·mol^-1)/RT) s^-1 The estimation of thermodynamic properties of 2-thioethyl biradical (Table 4) indicated that the obtained activation energy is too low to form the biradical as an intermediate. To verify whether this low activation energy is characteristic for the decomposition of thiirane, we studied thiolane under similar conditions. Thiolane decomposed above 900 K (Fig. 3) to yield propylene and ethylene as main products (Table 5). By using the procedure as employed for thiirane, the A factor was estimated as shown in Table 6, and the high pressure Arrhenius parameters were obtained (Fig. 4). k=10^15.6exp(-301(kJ·mol^-1)/RT) s^-1 The estimation of thermodynamic properties of the 4-thiobutyl biradical (Table 6) indicated that the obtained activation energy is in excellent agreement with the enthalpy change for the formation of 4-thiobutyl biradical. Accordingly the following biradical mechanism may well explain the decomposition of thiolane. Thus we conclude that the unusually low activation energy is characteristic to thiirane. We proposed an alternative mechanism in which thiirane decomposes through triplet state rather than through biradical formation as reported previously.