Kinetics of Pyrolysis of Alkyl Hydroperoxides and Their O–O Bond Dissociation Energies

It is shown that group additivity rules lead to very reliable estimates (±1 kcal) for the ΔH_f° of both alkyl peroxides and hydroperoxides. Previously published data for t‐BuO₂H are unique in showing a discrepancy of 6.3 kcal/mole from the estimated data. From the ΔH_f° for the ROOH and the activation energies of pyrolysis of ROOR it is shown that the bond dissociation energies D(RO–OH) are about 42 kcal/mole for alkyl R. On this basis the direct split ROOH→RO+OH is shown to contribute negligibly to the observed kinetics of decomposition of t‐BuOOH in solution. Instead the decomposition is shown to be compatible with a chain involving t‐BuO₂ and t‐BuO radicals via the fairly well established reaction: 2 t‐BuO₂→2 t‐BuO+O₂. The rate law for such a chain is shown to lead to $\frac{4}{3}$ power dependence on t‐BuO₂H in good accord with the kinetic data. Absolute values of the chain rate are also in good accord with the data. It is further shown that very reactive solvents (olefins, etc.) may convert RO₂ to RO directly, changing the rate to $\frac{3}{2}$ order with a lower activation energy (∼28 kcal). Published data from studies of tetralin‐OOH and cyclohexane‐OOH are shown to be in good accord with the proposed mechanism. Allylic hydroperoxides are discussed briefly and considered from the standpoint of good H‐atom donors. The styrene induced decomposition of cyclohexane‐OOH is considered in detail and a new, fast initiation step is discussed: $$\text{2φ}\mathrm{CH}={\mathrm{CH}}_2+\mathrm{ROOH}\mathrm{\to \phi }\mathrm{C}̇\mathrm{H}–{\mathrm{CH}}_2–{\mathrm{CH}}_2–{\mathrm{CH}}_2\mathrm{\phi +}{\mathrm{RO}}_2\mathrm{·.}$$