The liquid‐phase oxidation of 2,4‐dimethylpentane

The initiated oxidation of 2, 4‐dimethylpentane in the neat liquid phase at 100°C with 760 torr O2 gives more than 90% of a mixture of 2,4‐dihydroperoxy‐2,4‐dimethylpentane and 2‐hydroperoxy‐2, 4‐dimethylpentane in a ratio of 7:1. The rate of oxidation depends closely on the [initiator]1/2, consistent with a mechanism in which chain termination occurs mostly by interactions of two 2‐hydroperoxy‐2, 4‐dimethyl‐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 as fast at 1.25°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 (Ep – Et/2) is 10.7 kcal/mole, close to the value of 12 kcal/mole found for isobutane. Absolute values for Ep, Et, kp, kr, and kt were derived. 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 ratio of rate constants for oxetane formation and addition of oxygen to the 2, 4dimethyl‐2‐hydroperoxy‐4‐pentyl radical is about 5.4 × 10−5 M at 100°C. Thus, ring closure to oxetane is too slow to compete with addition of oxygen above ˜200 torr. At 100°C, 2, 3‐dimethylbutane gave no evidence of any intramolecular abstraction. However, 2, 3‐dimethylpentane did give at least 12% 2, 4‐glycol or hydroxyketone.