Solvent and substituent effects on the kinetics of thermolysis of cis-fused 1,2,4-trioxanes

The kinetics of the thermal decomposition reaction of cis-6-phenyl-5,6-(2-p henylpropyliden)-3,3-pentamethylene-1,2,4-trioxacyclohexane (Ia) were inves tigated in benzene and methanol solutions in the temperature and concentrat ion ranges of 353.3 - 413.2 K and (1.1 - 13.1) x 10(-3) M, respectively. Fi rst-order rate constant values were obtained for up to at least ca. 20% con versions of that cyclic peroxide. The activation parameter values for the i nitial unimolecular homolysis of that molecule, results supported by the ef fect 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 reacti on products analysis. The corresponding activation parameters for the react ion of la in methanol (DeltaH(#) = 20.2 +/- 0.6 kcal mol(-1); DeltaS(#) = 0 .1 +/- 1.6 cal mot(-1) K-1; DeltaG(#) = 20.2 +/- 0.6 kcal mol(-1)) and in b enzene (AW = 15.4 +/- 0.2 kcal mol(-1); DeltaS(#) = -13.2 +/- 0.5 cal mol(- 1) K-1; DeltaG(#) = 20.5 +/- 0.2 kcal mol(-1)) solutions are compared with values obtained for cis-6-phenyl-5,6-(2-phenylpropyliden)-3,3-tetramethylen e-1,2,4-trioxacyclohexane (Ib) thermolysis in the same solvents. The thermo lysis kinetics of la are less sensitive to solvent changes compared to the behaviour already reported for the analogous reactions of Ib. Because both molecules in solution arc flexible structures due to their configurations, the relatively small solvent effect found on the former trioxane reaction i s attributed to the extent of the chain of methylene groups attached on C-3 of the corresponding molecular rings. Furthermore, the pertinent substitue nt effect on the peroxidic bond strength of those molecules in solution was evaluated.