Pyrolysis of Azetidinones. Part 2. Kinetics and Mechanism of Thermolysis of β-Lactams and β-Thiolactams

The kinetics of the gas-phase thermolysis reaction of seven β-lactams and their thione analogues was investigated over the temperature range 533-603 K for the β-lactams, and 463-542 K for the β-thiolactams. The average values of the energy of activation (Ea/kJ mol-1) and Arrhenius log A/s-1 were respectively, 170.8 ± 18.6 and 12.4 ± 1.6 for the lactams, and 131.7 ± 18.2 and 11.0 ± 2.0 for the thione analogues. The entropy of activation (ΔS#), was negative for eleven of the substrates, and small positive for three.The rate constants (k/s-1) were calculated at 510 K, and compared for the two series of azetidinones. The effects of substituents on rates, and the novel role played by the (C=O) and (C=S) moieties on the relative reactivities of the cyclic amides are rationalized on the basis of a formal retro[2+2]cycloaddition mechanism used earlier to explain the products of the gas-phase thermolysis reaction of the present azetidinones.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author