The initiated oxidation of 2,4dimethylpentane in the neat liquid phase at 100°C with 760 torr 0 gives more than 90% of a mixture of 2,4-dihydroperoxy-2,4-dimethylpentane and 2-hydroperoxy-2,4-dimethyIpentane in a ratio of 7 : 1. The rate of oxidation depends closely on the [initiator]"*, consistent with a mechanism in which chain termination occurs mostly by interactions of two 2-hydroperoxy-2,4-dimethyI-4-pentylperoxy radicals. 2,4-Dimethylpentane oxidizes only one sixth as fast as isobutane at the same rate of initiation at 100°C. In cooxidations of the same hydrocarbons? it is 0.71 as reactive as isobutane toward any of the peroxy radicals involved. 2,4-Dimethylpentane oxidizes 7.5 times a s fast at 125°C as at 50°C for the same rate of initiation, but the ratio of dihydroperoxide to monohydroperoxide increases only from 5 to 7, corresponding to a difference in activation energy between intramolecular and intermolecular abstraction of 1 kcal/mole. The overall activation energy (E, -E1,.J is 10.7 kcal/mole, close to the value of 12 kcal/mole found for isobutane.
Ring closure of 2-hydroperoxy-2,4-methyl-4-pentyl radicals to oxetane, not detected during oxidation, was observed when this radical was generated at 100°C in the near-absence of oxygen. The rario of rate constants for oxetane formation and addition of oxygen to the 2,4-dimethv1-2-hydroperoxy-4-pentyl radical is about 5.4X 10-5 M at 100°C. Thus, ring closure to oxetane is too slow to compete with addition of oxygen above -200 torr. At 1 OO"C> '2,3-dimethylbutane gave no evidence of any intramolecular abstraction. However, 2.3-dimethylpentane did give at least 1 2yo 2,4-glycol or hydroxvketone.
Absolute values for Ep, E,, k,, k,, and k c were derived. * T h e 0-0 bond strength in R 0 2 H is estimated to be -43 kcaI/moie, compared to 37 kcal/mole in R 0 2 R . , see ref. [ 161.