Acetonyl radicals

This explanation is supported by the nature of the products formed in secondary reactions. The acetonyl radicals are considered to dimerize or to abstract hydrogen from the parent molecule forming, respectively, acetonyl acetone and acetone. 

The formation of these products is reduced or suppressed by radical scavengers such as nitric oxide or iodine and the products of the scavenging reactions with acetonyl radicals were identified as Pyruvonitrile (formed by way of nitrosoacetone) and iodoacetone. 

The chloroacetonyl radicals formed in the hydrogen abstraction reactions are also shown to combine and disproportionate, or combine and disproportionate with acetonyl radicals. 

In preliminary studies, it was considered that the 1,3-dichlorohexa-fluoropropane identified among the photolysis products was a result of the abstraction reaction. The acetonyl radical was presumed to react with CF2C1 radicals to form 1,4-dichlorohexafluorobutanone. Quantitative studies showed that the abstraction reaction was relatively unimportant at room temperature when substantial amounts of the 1,3-dichlorohexafluoropropane were formed. Further, when the temperature was raised and the abstraction reaction became important, the yield of 1,3-dichlorohexafluoropropane did not rise significantly. The yield was also independent of ketone concentration. Finally, it was shown that the intensity exponent was between 1 and 2. In high conversion photolysis, small quantities of the 1,4-dichlorohexafluorobutanone were identified but the probability that any significant amount would be photolyzed in such small concentration was very low. 

In the treatment of his data, Hoare neglected the reaction of NO with the acetonyl radical generated in reaction (15). Fortunately, however, this reaction is not important for NO consumption under the experimental conditions, so that the analysis of the data is valid. The rate constant was found to be third order for all pressures (up to 200 torr acetone or 250 torr C02). With acetone as a third body k7ak7cjk7bk15... 

In most reactions addition of acetone (when used as a photoinitiator) to the double bond took place. This addition has been shown to follow the same path as the formamide addition, leading to methyl ketones resulting from terminal addition in the case of terminal olefins, and to a mixture of two methyl ketones in the case of nonterminal olefins. The ratio of the two isomeric methyl ketones were the same as those of the appropriate amides obtained in these reactions. These ketones are assumed to be produced from addition of acetonyl radicals to the double bond ... 

The photochemical alkylation of olefins by nitriles and ketones is not straightforward, due mainly to the inefficient abstraction of hydrogen from an electron-withdrawing-substituted carbon by an electrophile such as the photocatalyst excited state. Nevertheless, various methyl ketones have been synthesized by the irradiation of a ketone/oleftn mixture dissolved in aqueous acetone. The mechanism of the reaction remains to be clarified, but a water-assisted C—C coupling between an acetonyl radical and the olefin has been postulated (Scheme 3.12). The reaction has several advantages, as it is cheap (an acetone/water mixture is used as the solvent) and occurs under mild metal-free conditions with no need for a photocatalyst [28],... 

In the chain photooxidation of acetone it seems inevitable that the acetonyl radical must be formed98 by hydrogen abstraction from acetone. The oxidation of acetonyl radicals has been postulated to occur by the following reactions ... 

Exposure of the xanthate 304 to AIBN generates an acetonyl radical, which will subsequently react with the enesulfonamide 305 to provide the pyrrole 306 via the intermediate 307 (Equation 91) <2002CC2214>. 

J. R. McNesby et al., J. Am. Chem. Soc.j 76, 823 (1954), were able to identify biacetonyl, (CH2( OCH )2, and acetyl acetone in the pyrolysis of acetone at 466 to 525°C. This indicates the int reasing importance of the acetonyl radical, as ketene, which is a major product of the pyrolysis, begins to accumulate in the system. 

It seems to be well-established that methane and ketene are formed in the main reaction, while carbon monoxide, ethylene and other products, produced in trace amounts, are probably the result of the ketene decomposition. Ethane and acetonyl acetone are chain termination products (see later), while the precursor of acetyl acetone is probably the radical formed in the reaction of the acetonyl radical and the ketene. 

On the basis of a Rice-Herzfeld mechanism, with long chains and termination by the recombination of methyl and acetonyl radicals, the overall activation energy can be calculated as... 

Though radicals react with acetone, chains are not propagated below about 450 °C. On the other hand, at higher temperatures where the thermal decomposition of acetone has been generally studied, the acetonyl radical is unstable and decomposes into ketene and methyl radical. Thus, under such conditions, the reaction is a chain process. 

It seems to be well established that reactions (l)-(4) are the chain initiation and propagation steps of the decomposition. The question of chain termination is, however, far from being clear. In their fundamental paper. Rice and Herzfeld suggested that the recombination of the methyl and acetonyl radicals to give methyl ethyl ketone was the probable termination step. This leads to an overall order of one which is in agreement with the observations at high pressures. 

Finally, the reactions of the acetonyl radical have to be dealt with. The thermal stability of this radical is manifested by the facts that (/) the ketene formation is negligible below 200°C , and (ii) chains do not occur in the photolysis of acetone up to about 350°C . The acetonyl radicals disappear from the system by recombination reactions, viz. 

Methyl ethyl ketone was detected in experiments above lOO C. Acetonyl acetone has not been identified in the products so far. Actually, not more than a small amount of this compound is expected to be formed, since (i) the acetyl radical is unstable at higher temperatures, and (ii) the rate of formation of acetonyl radical is slow at lower temperatures. The formation of biacetonyl was observed in the investigation of the reaction between CH3 and acetone . Brin-ton ° has succeeded in detecting biacetonyl also among the products of acetone photolysis in the temperature range 200-475°C. Most of the evidence on the formation of biacetonyl in the photolysis of acetone is, however, based on material bal-ance and hence is only of secondary importance. 

Ketene, which is formed with the highest overall rate around 300 °C, comes from the decomposition of the acetonyl radical, viz. 

Reactions of vinoxy and acetonyl radicals with nitric oxide... 

The vinoxy CH2CHO and 1-methylvinoxy (acetonyl) radicals are key intermediates in the mechanisms of many reactions of importance for atmospheric and combustion chemistry. The formation of vinoxy radicals has been observed in several chemical processes. They may be formed in reactions of OH radicals with ethylene oxide (C2H4O) and with acetylene (C2H2) in the presence of 02.8 They are also produced in reactions of 0(3P) atoms with alkenes and in the reactions of reactive atoms such as F or 0(3P) with acetaldehyde.164,165 The 1-methylvinoxy (acetonyl) radical CH2C(CH3)0 is considered an important intermediate in the atmospheric oxidation of acetone initiated by the OH radical.166171 Spectroscopic studies by Washida et al.164 and Williams et al.m allow estimation of the rate constant for the reaction of acetonyl with 02. 

In the vapor phase the formation of acetonyl radicals via neutral dissociation and free radical processes (Reactions 9 and 10), is well established in both photochemistry (21) and radiation chemistry (3, 41, 42). 

In the photolysis of liquid acetone, Reaction 11, also involving an excited state of acetone, is an important source of acetonyl radicals (44). 

We have no definite evidence for the presence of (CH3)2COH radicals although a weak unidentified absorption, detected at high gain, is present in the wings of the spectrum shown in Figure 3A. Acetonyl radicals could also be produced by an ion-molecule reaction such as Reaction 12. 

The vapor-phase radiolysis of acetone is known to yield mainly CO, CH4, C2H6, C2H4, and H2 (3, 41). Methane together with acetonyl radicals results from the reaction of CH3 radicals with acetone via Reaction 15. 

Some evidence that this can also occur in our system is seen from the increased yield of acetonyl radicals in the presence of CH3I. This is consistent with the occurrence of Reaction 8 followed by Reaction 15. A similar sequence of reactions can be written for deuterated acetone. 

K. consists of a singlet superimposed on a triplet (1, 32), the triplet spectrum being assigned to the acetonyl radical. Blandamer et al. (7) and Nitta et al. (31) also observed a singlet assigned to trapped electrons in y-irradiated acetone at 77 °K. 

The solvents of choice for free-radical reactions (CH3OH, H2O, benzene) share the feature that their X—H bonds have very large BDEs, so that the solvents are unlikely to participate in the reaction. Free-radical reactions executed in ether, THF, toluene, CH2CI2, and CHCI3 are often complicated by atom abstraction from the solvent. Acetone is sometimes used as a solvent for free-radical reactions, because it can be photoexcited to act as an initiator, but it can also complicate free-radical reactions by giving up H- to another radical to afford the relatively stable 2-oxopropyl (acetonyl) radical. 

Miranda and coworkers utilized xanthate-mediated radical conditions to add a-acetyl and a-acetonyl radicals to indoles 3a and 3b, with good regioselectivity, at the C-2 position [10]. In this case, better yields were observed with indole (3a) than with the Af-benzyl-3-indolecarboxaldehyde (3b). 

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