Electrocyclic versus homolytic decompositions of peroxides: Solvent and phase effects

The thermal decomposition of isopropyl peroxide in solvents is shownto be a competition between unimolecular homolysis (kr) and an electrocyclic reaction yielding H2 plus acetone (kH). The value of kH increases markedly with increasing solvent polarity: PhCH3 < i‐PrOH < MeOH < H2O. Log kH correlates linearly with Et (or Z). The effect on kr is similar but less pronounced. Log kr's for both i‐Pr2O2 and t‐Bu2O2 are shown to correlate with the molar solubility parameter δ. The negligible contribution of the electrocyclic reaction to thermal decompositions of peroxides in the vapor phase is thus not due to peculiar behavior of that reaction, which clearly has a polartransition state. It is argued that the transition state for unimolecular homolysis has no polar character despite the apparent sensitivity of homolysis rates to solvent polarity.