Kinetics and mechanism of the thermolysis of a five-membered-ring peroxide, 3,3,5,5-tetramethyl-1,2-dioxolane

Article abstract of DOI:10.1021/jo00322a028

The kinetics of the thermolysis of 3,3,5,5-tetramethyl-1,2-dioxolane (1) was studied in benzene solution with a free-radical chain inhibitor (2,6-di-tert-butyl-p-cresol) in the gas phase. The peroxide 1 was susceptible to induced decomposition both in solution without the inhibitor and in the gas phase without conditioned reactor walls. However, under optimum conditions, first-order kinetics were observed in both the gas phase and in solution. Activation parameters for the thermolysis of 1 in benzene solution with the inhibitor at 500 K are E_a = 44.6 ± 0.9 kcal/mol, log A = 15.85 ± 0.42, ΔH^≠ = 43.6 ± 0.9 kcal/mol, and ΔS^≠ = 11.0 ± 1.9 eu. In the gas phase, the parameters at 500 K are E_a = 45.5 ± 0.3 kcal/mol, log A = 15.72 ± 0.13, ΔH^≠ = 44.5 ± 0.3 kcal/mol, and ΔS^≠ = 10.4 ± 0.6 eu. These parameters closely approach calculated activation parameters for 1 which are based on a stepwise biradical decomposition mechanism: E_a = 48.6 kcal/mol, log A = 16.55, ΔH^≠ = 47.6 kcal/mol, and ΔS^≠ = 14.2 eu at 500 K. Considering the susceptibility of 1 to induced decomposition, which will lower the activation parameters, the close approach of the experimental to the calculated parameters indicates that 1 undergoes decomposition by a stepwise biradical route. Thus, there is no mechanistic discontinuity between the stepwise biradical mechanism observed with simply substituted 1,2-dioxetanes (four-membered-ring peroxides) and the five-membered-ring peroxide 1.