Thermochemistry of the R−O₂ Bond in Alkyl and Chloroalkyl Peroxy Radicals

Abstract

Earlier experimental results on the kinetics of relaxation to equilibrium in R + O2 ⇄ RO2 reactions (R = CH3, C2H5, i-C3H7, t-C4H9, CH2Cl, CHCl2, and CCl3) are reanalyzed using an improved kinetic mechanism which accounts for further reactions of the RO2 adduct. Reaction enthalpy ΔH°298(R + O2 ⇄ RO2) values are obtained from the third-law treatment of the temperature dependencies of the equilibrium constants KP(T) resulting from the reinterpretation of the original kinetic data. The R−O2 bond strengths for alkyl and chloroalkyl radicals (R = ·C(CH3)iCljHk, i + j + k = 3) can be represented by a linear function of the numbers of C−C (NC-C) and C−Cl (NC-Cl) bonds at the C atom forming the C−O bond: −ΔH°298(R + O2 ⇄ RO2) = HCH3 + hC-CNC-C + hC-ClNC-Cl (HCH3 = 139.9, hC-C = 5.7, hC-Cl = −16.1 kJ mol-1). The values of ΔH°298(R + O2 ⇄ RO2) are used to calculate the enthalpies of formation for the RO2 peroxy radicals and ROOH hydroperoxides.