The liquid‐phase oxidation of 2,4‐dimethylpentane

Abstract

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.

Oxidative Degradation and Stabilisation of Mineral Oil-Based Lubricants

“…Their reactivity is also influenced by steric effects: primary and secondary peroxy radicals show a three to five times higher reactivity than tertiary peroxy radicals [7]. A more favourable route of hydrogen abstraction by a peroxy radical [8][9][10] occurs via an intramolecular propagation outlined in Reaction sequence (4.4), where x is equal to 1 or 2, R 1 is a terminal alkyl group and R 2 hydrogen or an alkyl group.…”

Review on the thermo-oxidative degradation of polymers during processing and in service

“…The propagation reaction is a repeating reaction; the average number of propagation cycles (kinetic chain length) has been studied by a number of researchers [25][26][27][28][29][30][31][32][33][34]. These studies were performed in model hydrocarbon systems [33,34], in polymer solutions [31,32] and in solid polymers [25][26][27][28][29][30]. In these studies, external radical sources, such as irradiation or peroxides, were used to initiate the oxidation.…”

Free-Radical Rearrangements

“…Unless the C-H bond is "activated," this isomerization will be endothermic (D[ROO-H] ~ 88 kcal/mol) and hence will proceed more rapidly in the reverse direction. In the autoxidation of alkanes yielding tertiary peroxy radicals which can isomerize by migration of tertiary hydrogen, e.g., 2,4-dimethylpentane (640,641) The reasonable suggestion has been made (646) that difunctional prod ucts are disfavored for the straight-chain alkanes because the isomeriza tion of their peroxys will be more readily reversible than will be the case with the branched alkanes. The critical oxygen pressure required to trap all 365 that are formed will, presumably, increase as the peroxy radical isomerization becomes more endothermic.…”

“…However, even en dothermic isomerizations can be readily observed during the autoxidation of suitable substrates because the carbon-centered radical 365 will be rapidly trapped by the molecular oxygen present in the system. Moreover, others (641) have calculated that all 365 should be trapped by oxygen under the experimental conditions normally used for alkane autoxidations. The overall process can be represented as follows: The ratios of the rate constants for peroxy isomerization and intermolecular hydrogen abstraction are highly dependent on the structure of the substrate being oxidized.…”