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

Earlier experimental results on the kinetics of relaxation to equilibrium in R + O 2 ⇄ RO 2 reactions (R = CH 3, C 2 H 5, i -C 3 H 7, t -C 4 H 9, CH 2 Cl, CHCl 2, and CCl 3 ) are reanalyzed using an improved kinetic mechanism which accounts for further reactions of the RO 2 adduct. Reaction enthalpy Δ H ° 298 (R + O 2 ⇄ RO 2 ) values are obtained from the third-law treatment of the temperature dependencies of the equilibrium constants K P ( T ) resulting from the reinterpretation of the original kinetic data. The R−O 2 bond strengths for alkyl and chloroalkyl radicals (R = ·C(CH 3 ) i Cl j H k, i + j + k = 3) can be represented by a linear function of the numbers of C−C ( N C - C ) and C−Cl ( N C - Cl ) bonds at the C atom forming the C−O bond: −Δ H ° 298 (R + O 2 ⇄ RO 2 ) = H CH 3 + h C - C N C - C + h C - Cl N C - Cl ( H CH 3 = 139.9, h C - C = 5.7, h C - Cl = −16.1 kJ mol -1 ). The values of Δ H ° 298 (R + O 2 ⇄ RO 2 ) are used to calculate the enthalpies of formation for the RO 2 peroxy radicals and ROOH hydroperoxides.