The Oxidation of Free Methyl Radicals

The gaseous oxidation of diethyl ether

A detailed study has been made of the gaseous oxidation of diethyl ether in the low-tem­perature region. The reaction has many characteristics of hydrocarbon oxidation, although there is one essential difference: excess oxygen tends to retard the rate of ether combustion. The principal products during the induction period are acetaldehyde, peracetic acid and ethanol. In the later stages of reaction, acetaldehyde and peracetic acid are consumed; ethanol and acetic acid being the principal end-products. It is assumed that an ‘ether radical’, CH 3 CHOCH 2 CH 3 , formed by the initiation process, decomposes to yield acetaldehyde and an ethyl radical. Subsequently, there are two con­current reactions. First, the ethy radicals are oxidized throughout the course of the reaction, ethanol being the principal product of this process. Secondly, acetaldehyde is oxidized to peracetic acid, the peracetic acid decomposing in the later stages of reaction to form copious amounts of acetic acid. It is thought that the peracetic acid is the compound that is primarily responsible for chain-branching.

Reaction of cytochrome P450 with cumene hydroperoxide: ESR spin-trapping evidence for the homolytic scission of the peroxide O-O bond by ferric cytochrome P450 1A2

ESR spin trapping was used to investigate the reaction of rabbit cytochrome P450 (P450) 1A2 with cumene hydroperoxide. Cumene hydroperoxide-derived peroxyl, alkoxyl, and carbon-centered radicals were formed and trapped during the reaction. The relative contributions of each radical adduct to the composite ESR spectrum were influenced by the concentration of the spin trap. Computer simulation of the experimental data obtained at various 5,5-dimethyl-1-pyrroline N-oxide (DMPO) concentrations was used to quantitate the contributions of each radical adduct to the composite ESR spectrum. The alkoxyl radical was the initial radical produced during the reaction. Experiments with 2-methyl-2-nitrosopropane identified the carbon-centered adducts as those of the methyl radical, hydroxymethyl radical, and a secondary carbon-centered radical. The reaction did not require NADPH-cytochrome P450 reductase or NADPH. It is concluded that the reaction involves the initial homolytic scission of the peroxide O-O bond to produce the cumoxyl radical. Methyl radicals were produced from the beta-scission of the cumoxyl radical. The peroxyl adduct was not observed in the absence of molecular oxygen. We conclude that the DMPO peroxyl radical adduct detected in the presence of oxygen was due to the methylperoxyl radical formed by the reaction of the methyl radical with oxygen. At a higher P450 concentration, a protein-derived radical adduct was also detected.

Photooxidation of acetone vapor

The photochemical reaction between acetone and oxygen has been studied at temperatures from 100 to 250 °C at several oxygen pressures with light of 3 130 Å. A study of the quantum yields of all the major products has shown the necessity of introducing the reaction

[Formula: see text]

[Formula: see text]to the generally accepted mechanism, with a rate constant about 80 times that of the reaction

[Formula: see text]at 100 °C. Both this reaction and the reaction of acetyl radicals with oxygen appear to have low or negative activation energies, consistent with a rate-determining process involving combination of the radical with oxygen. The reaction of acetonyl radicals with oxygen to give carbon monoxide is not important at temperatures below 150 °C, but it becomes the most important reaction at temperatures above 250 °C.

THE OXIDATION OF DI-tertiary-BUTYL PEROXIDE

The oxidation of di-t-butyl peroxide has been investigated in a static system at low conversion at 124 °C with sufficient oxygen present to suppress completely the formation of methane and ethane. The decomposition of the t-butoxy radical is unaffected by the presence of oxygen. A major product of the oxidation is formaldehyde whose yield rapidly approaches a stationary value. It is postulated that the major source of formaldehyde is the decomposition of methyl peroxy radicals, which may also abstract hydrogen from formaldehyde to form methyl hydroperoxide, and that this competition leads to the stationary concentration of formaldehyde actually observed. Methyl hydroperoxide was demonstrated to be unstable in the system and the predominant decomposition product was methanol, a compound also found in high yields in the oxidation. Experiments with added formaldehyde-C13 showed that formaldehyde can be converted to carbon monoxide in the system and indicated that formaldehyde was a likely precursor to the carbon monoxide found in the oxidation.

THE PHOTOOXIDATION OF AZOMETHANE. II

In a further investigation of the photooxidation of azomethane it was found that the yields of all products depend on the fractional conversion in a manner which suggests that some inhibitor is produced. Products from experiments at lower pressures of azomethane are obtained in yields appropriate to the higher conversions. Only very small changes in yields are observed for relatively large variations in light intensity. The implications of these facts to the mechanism of the photooxidation are discussed.