Abstract
The kinetics of the thermal decomposition reaction of cis-6-phenyl-5,6-(2-phenylpropyliden)-3,3-pentamethylene-1,2,4-trioxacyclohexane (Ia) were investigated in benzene and methanol solutions in the temperature and concentration ranges of 353.3 – 413.2 K and (1.1 – 13.1)x10-3 M, respectively. First-order rate constant values were obtained for up to at least ca. 20% conversions of that cyclic peroxide. The activation parameter values for the initial unimolecular homolysis of that molecule, results supported by the effect of the addition of di-tert-butyl-p-cresol as a free radical scavenger, indicate a stepwise reaction mechanism which is in keeping with the reaction products analysis. The corresponding activation parameters for the reaction of Ia in methanol (ΔH# = 20.2 ± 0.6 kcal mol-1; ΔS# = 0.1 ± 1.6 cal mol-1 K-1; ΔG# = 20.2 ± 0.6 kcal mol-1) and in benzene (ΔH# = 15.4 ± 0.2 kcal mol-1; ΔS# = -13.2 ± 0.5 cal mol-1 K-1; ΔG# = 20.5 ± 0.2 kcal mol-1) solutions are compared with values obtained for cis-6-phenyl-5,6-(2-phenylpropyliden)-3,3-tetramethylene-1,2,4-trioxacyclohexane (Ib) thermolysis in the same solvents. The thermolysis kinetics of Ia are less sensitive to solvent changes compared to the behaviour already reported for the analogous reactions of Ib. Because both molecules in solution are flexible structures due to their configurations, the relatively small solvent effect found on the former trioxane reaction is attributed to the extent of the chain of methylene groups attached on C-3 of the corresponding molecular rings. Furthermore, the pertinent substituent effect on the peroxidic bond strength of those molecules in solution was evaluated.
Highlights
The activation parameter values for the initial unimolecular homolysis of that molecule, results supported by the effect of the addition of di-tert-butyl-p-cresol as a free radical scavenger, indicate a stepwise reaction mechanism which is in keeping with the reaction products analysis
Cyclic peroxides such as the substituted 1,2,4,5-tetroxanes undergo thermolysis, both in the gas [1] and solution phase [2] by a stepwise mechanism, where the activation parameters values of the initial unimolecular homolysis fall within a limited range, significant steric [3] and solvent effects [4] have been observed
The low values of the observed rate constant values for Ia thermolysis confirm the stability of the cis-fused 1,2,4-trioxanes
Summary
Cyclic peroxides such as the substituted 1,2,4,5-tetroxanes undergo thermolysis, both in the gas [1] and solution phase [2] by a stepwise mechanism, where the activation parameters values of the initial unimolecular homolysis fall within a limited range, significant steric [3] and solvent effects [4] have been observed. The experimental activation energies for their thermolyses are usually in good agreement with calculated values based on a peroxidic bond homolysis of the corresponding molecular rings [5]. Because the cis-6-phenyl-5,6-(2-phenylpropyliden)-3,3-pentamethylene-1,2,4-trioxacyclohexane (Ia) and the cis-6-phenyl-5,6-(2-phenylpropyliden)-3,3-tetramethylene-1,2,4-trioxacyclohexane (Ib) molecules (Scheme 1) have different cyclic atomic groups on the corresponding C-3 atoms it is relevant to report the activation parameters for both thermolyses and to compare the effects of the reaction solvents. The decomposition reactions of Ia in methanol and benzene solutions have been investigated to learn about on the thermal stability in solution of this cyclic peroxide, which is related to the chemotherapy with Qinghaosu and other synthetic drugs of the same type
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