Thermolyses of 2-methyloxetan and 2,2-dimethyloxetan

Abstract

The thermolysis of 2-methyloxetan has been reinvestigated at temperatures between 429.5 and 483.2 °C and initial reactant pressures from 6.0 to 14.2 Torr. Two parallel unimolecular reactions occur producing either propene and formaldehyde or ethene and acetaldehyde. The ratio of propene to ethene is 1.31 ± 0.03 within this temperature range. The total rate constant for disappearance of reactant is given by log₁₀(k/s^–1)=(14.89 ± 0.36)–(249.8 ± 4.7 kJ mol^–1)/2.303 RT. The thermolysis of 2,2-dimethyloxetan was studied in the same apparatus at temperatures between 402.2 and 471.3 °C and at reactant pressures from 7.2 to 9.2 Torr. The reactant disappears by two parallel unimolecular paths to give either isobutene and formaldehyde or ethene and acetone. Minor products of methane, propene and isobutane were also detected. The ratio of isobutene to ethene was found to differ considerably from unity and to be strongly temperature-dependent. The following rate expressions were derived for the overall rate of disappearance of reactant (k_{2, 2}) and for the rate constants for the individual reactions producing isobutene (k₃) and ethene (k₄) log₁₀(k_2,2/s^–1)=(13.78 ± 0.24)–(226.0 ± 2.7 kJ mol^–1)/2.303 RT, log₁₀(k₃/s^–1)=(13.48 ± 0.25)–(222.1 ± 2.9 kJ mol^–1)/2.303 RT, log₁₀(k₄/s^–1)=(15.56 ± 0.34)–(270.6 ± 4.4 kJ mol^–1)/2.303 RT. The differences observed between the thermolyses of these two compounds are noted and compared with previously obtained results for other methyl-substituted oxetans. A possible explanation for the difference in Arrhenius parameters for the two paths in the case of the 2,2-disubstituted compound is suggested.