Acetyl radicals, from decomposition

Three reports of free radical decomposition kinetics have appeared. Rate coefficients for the unimolecular dissociation of the acetyl radical have been obtained as a function of temperature from 420 to 500 K, and pressure from 1 to 6 torr by monitoring the rate of decay of CH3CO produced by photolysis of 2-butanone or 2,4-pentanedione [125] ... 

The photooxidation of diethyl ketone at 120 and 175°C. using oxygen labeled with 2.8 at.-% of oxygen-18 has been reported more recently.48 The two experiments performed at room temperature yielded negligible amounts of carbon monoxide all the carbon dioxide came from the carbonyl group. The experiments performed at higher temperatures were in substantial agreement with those of Noyes and Finkelstein.80 90% of the carbon monoxide came from the decomposition of the propionyl radical [reaction (45)], in contrast to the acetyl radical which yields only 10-20% of the carbon monoxide under conditions that are apparently comparable. 75% of the carbon dioxide came from the car-... 

Reactions involving abstraction from acetaldehyde are just as likely in diethyl ketone oxidation as reactions involving abstraction from formaldehyde in acetone oxidation. The acetyl radical so produced will oxidize as in the oxidation of acetone to give mostly carbon dioxide (from the -carbon atom of diethyl ketone71), but a little decomposition seems to occur since some carbon monoxide does not come from the original carbonyl group of the ketone.71... 

In marked contrast with all the other experiments, hydrogen atoms abstract the aldehydic hydrogen from acetaldehyde to form the acetyl radical, CHg. CO. The occurrence of this reaction is presumably due to the relatively weak CH bond (82 kcal mole ) (Benson, 1965) and the absence of an efficient addition reaction, in contrast to the olefins. A small amount of methyl radical is also observed and must arise from decomposition of the acetyl radical (reaction 29). 

The decompositions of the formyl and acetyl radicals are certain to be in the fall-off region at the pressures used in the investigations of the acetaldehyde pyrolysis. However, the complexity of the mechanism impedes any conclusion to be drawn from this system. 

Below, some of the results concerning the pressure dependence of the radical decomposition and recombination processes will be discussed. No deviation was observed from the second-order kinetics of the termination step in the experiments of Grahame and Rollefson , while Dodd S found it necessary to consider the pressure dependence of the methyl recombination at around 10 torr. Dorman and Buchanan came to the conclusion that the decomposition of the formyl radical is in its pressure-dependent region below a few atm, whilst that of the acetyl radical seems to be pressure-dependent below about 50 or 150 torr. The results of Style and Summers also indicate the formyl radical decomposition to be pressure-dependent under the conditions where the photolysis of acetaldehyde was usually studied. 

It was first suggested by Spence and Wild that the decomposition originating from various electronic states leads to different products. Heicklen and Noyes ° observed that, at 3130 A, the ratio C2Hg/CO is smaller in the presence of biacetyl than in its absence. This led them to conclude that the excited singlet molecules, which are not quenched by biacetyl, dissociate preferably into two methyl radicals and CO, while thermalized triplet molecules decompose rather into methyl and acetyl radicals. 

From the value of found in the presence of iodine as well as from that of 4>co obtained in the presence of HI , it follows that a < 0.01 at 3130 A. On the basis of the assumption that all the CO formed in the presence of iodine originates from the decomposition of the excited acetyl radicals, the following values were obtained at 100 °C a 0.02 (3130 A) and a 0.11 (2654 A). These values are to be considered as upper limits. The considerable amount of CO found at 2537 A in the presence of iodine obviously supports the high values given for a, at 2537 A, by Herr and Noyes as well as by Rubin and Leach (see above). 

The competitive method employed for determining relative rates of substitution in homolytic phenylation cannot be applied for methylation because of the high reactivity of the primary reaction products toward free methyl radicals. Szwarc and his co-workers, however, developed a technique for measuring the relative rates of addition of methyl radicals to aromatic and heteroaromatic systems. - In the decomposition of acetyl peroxide in isooctane the most important reaction is the formation of methane by the abstraction of hydrogen atoms from the solvent by methyl radicals. When an aromatic compound is added to this system it competes with the solvent for methyl radicals, Eqs, (28) and (29). Reaction (28) results in a decrease in the amount... 

The polarization of biphenyl, deserves special comment. If, as indicated in Scheme 2, its immediate precursor is a radical pair consisting of two phenyl radicals, then it should be formed without detectable net polarization since if Ag = 0. Analogous results have been reported in the decomposition of other peroxides for example, ethane formed from acetyl peroxide shows net emission. To account for this, it has been suggested (Kaptein, 1971b, 1972b Kaptein et al., 1972) that nuclear spm selection which occurs in the primary radical pair—in... 

Acetylation occurs at the 2-position of allene systems (Scheme 8.14). The intermediate 7t-allyl complex breaks down via the nucleophilic displacement of the cobalt carbonyl group by the hydroxide ion to produce the hydroxyketone (7) [ 11 ]. An alternative oxygen-initiated radical decomposition of the complex cannot, however, be totally precluded. The formation of a second major product, the divinyl ketone (8), probably arises from direct interaction of the dicobalt octacarbonyl with the allene and does not require the basic conditions. 

With ketones which can eject radicals more stable than methyl, fragmentation competes more successfully with all physical processes than in acetone, and unsymmetrical ketones preferentially eject the more stable alkyl radical.309 Thus both methyl ethyl ketone310 and methyl isopropyl ketone311 yield chiefly acetyl and ethyl or isopropyl radicals. Half of the diethyl ketone molecules excited by 3130-A irradiation at 25° decompose from the excited singlet state before they can undergo intersystem crossing, and another 40% fragment from the triplet state.312 Both fluorescence and phosphorescence are extremely weak. The more rapid decomposition in both excited states relative to that observed in acetone almost eliminates competition from physical-decay processes. 

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