OXIDATION OF 4A,4B-DIHYDROPHENANTHRENES .I. KINETICS OF THERMAL REACTION OF 9,10-CYCLOPENTANO-4A,4B-DIHYDROPHENANTHRENE WITH OXYGEN

The thermal reaction of 9,10-cyclopentano-4a,4b-dihydrophenanthrene (PH2) with oxygen, both in the presence and in the absence of 2,6-di-t-butyl-4-methylphenol (SH), acting as inhibitor, takes place through the self-initiated HO2- chain. The products are hydrogen peroxide and phenanthrene (P). The following distinct steps were recognized: initiation, PH2 + O2 → PH + HO2 (Ka); propagation, PH· + O2 → P + HO2 (kb); PH2 + HO2 →PH- + H2O2 (kd); termination, SH + HO2 →inactive products (kc); 2HO2 → H2O2 + O2(Ae). Three types of kinetics were found, according to the concentration of the inhibitor. (I) At high concentrations of the inhibitor, [SH] > 0.04 M, the reaction chain is composed of steps a, b, and c. The rate of the reaction of PH2 is then controlled by step a and is of the first order with respect to both [O2] and [PH2]. At -31°, ka is 0.65 1. mole-1 min-1. The activation energy for step a is 6.2 kcal/mole. The preexponential factor is 103,7l. mole-1 sec-1. Step a is accelerated in polar solvents, indicating a polar transition state. (II) In the absence of inhibitor the reaction is represented by steps a, b, d, and e. The rate expression for the disappearance of PH2 is then to a good approximation of order 1/2 with respect to [O2], and of order 3/2 with respect to [PH2], (III) At intermediate concentrations of the inhibitor, [SH] < 0.04 M, the rate of disappearance of PH2 is to a good approximation of the second order with respect to [PH2], of the first order with respect to [O2], and inversely proportional to [SH], The exact kinetic expressions (differential and integrated) in cases II and III were confirmed experimentally. The ratio kdke is 3 × 102 and does not vary with the temperature. The ratio kc/kd is 1.08 × 10-2at -31°and 1.57 × 10-2at -52°. Steps b-e and probably step a are exothermic. The dissociation energy of the PH-H bond is estimated as equal to or lower than 47 kcal/mole. © 1969, American Chemical Society. All rights reserved.