Acetyl radicals, decomposition

Herr and Noyes, they assumed that the effect was due to the hindrance of diffusion of acetyl radicals to the wall. However, at these comparatively high pressures, an explanation based on the dependence of acetyl radical decomposition seems to be preferable. 

Because the activation energy for acetyl radical decomposition is greater than that for its oxidation there is a shift in the balance of the ratio of reaction rates (vtt/vs) towards decomposition as the temperature is raised. 

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. 

In words, we describe the process as initiated by the decomposition of acetaldehyde to form the methyl radical CH3 and the formyl radical CHO. Then methyl attacks the parent molecule acetaldehyde and abstracts an H atom to form methane and leave the acetyl radical CH3CO, which dissociates to form another methyl radical and CO. Finally, two methyl radicals combine to form the stable molecule ethane. 

The existence of methane and ethane in the reaction products, however, suggests that some decomposition of type B is also taking place or that the acetyl radical is decomposing at room temperature. 

Oxidation of Acetaldehyde. When using cobalt or manganese acetate the main role of the metal ion (beside the initiation) is to catalyze the reaction of peracetic acid with acetaldehyde so effectively that it becomes the main route to acetic acid and can also account for the majority of by-products. Small discrepancies between acetic acid efficiencies in this reaction and those obtained in acetaldehyde oxidation can be attributed to the degradation of peracetoxy radicals—a peracetic acid precursor— by Reactions 14 and 16. The catalytic decomposition of peracetic acid is too slow (relative to the reaction of acetaldehyde with peracetic acid) to be significant. The oxidation of acetyl radical by the metal ion in the 3+ oxidation state as in Reaction 24 is a possible side reaction. Its importance will depend on the competition between the metal ion and oxygen for the acetyl radical. 

When acetaldehyde is oxidized in the presence of copper (II), the noncatalytic reaction between acetaldehyde and peracetic acid may be the main route to acetic acid. Since this reaction is slow, one would expect the presence of a significant concentration of peroxide in the reactor product, and we have confirmed this experimentally. Acetic acid can also be produced by oxidizing acetyl radicals by copper (II) the copper(I) formed could be easily reoxidized by oxygen. The by-products when using copper (II) acetates must be produced mainly by degradation of peracetoxy radicals by Reaction 14 and 16 since peracetic acid decomposition is negligible and the reaction of acetaldehyde with peracetic acid produces essentially only acetic acid. 

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-... 

It is generally considered that the rate of decomposition of this radical is faster than that for the acetyl radical (Steacie117). This, as explained earlier, is confirmed by the experimental evidence of Dunn and Kutschke.48 In the presence of oxygen there is competition between reaction (45) and (53)... 

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 increased efficiency of the decomposition (reaction 30), as compared with that observed in other methods of preparation of the acetyl radical, is probably dne both to a higher exothermicity of the initial reaction and to the fact that all the energy released is contained in a single prodnct molecnle. 

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. 

The participation of free radicals in the thermal decomposition of acetone has been proved by Patat and Sachsse by the para-ortho technique. Talrose et al identified methyl and acetyl radicals by mass spectrometry. 

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. 

Below about 200 °C, the photo-decomposition of acetaldehyde becomes involved as a result of the increased stability of the formyl and acetyl radicals. The occurrence of new elementary steps, in addition to those already mentioned, renders the kinetics of the reaction rather complex. 

The kinetics of the photolysis is much more complex at lower temperatures than at around 300 °C. The role of rate-determining step, i.e. the hydrogen atom transfer reaction (20) at high temperatures, is taken over by the decomposition of the acetyl radical as the temperature decreases. At the highest temperatures, the chains are terminated almost exclusively by the recombination of the methyl radicals, while at medium and low temperatures the disproportionation step (26) as well as self combination of the formyl and acetyl radicals are dominant. The first-order wall reaction of the radicals, such as reactions (22) and (31), may also play an important role, especially at low light intensities and pressures. On account of the aforesaid, it seems almost impossible to attempt a general discussion of the kinetics of the reaction. Instead, only selected questions will be dealt with in detail. 

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. 

If the hypothetical life-time of the excited acetyl radical is characterized by the average period of one vibration, then the acetone molecule directly decomposes into two methyl radicals and a CO molecule in this case there is no point in speaking about the acetyl radical at all. If, however, decomposition into CH3 and... 

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). 

Considerable amounts of hydrogen and ketene were observed by Lossing among the products of the mercury-sensitized photolysis at 55 °C. He concluded that these were formed as the result of the mercury-sensitized decomposition of the acetyl radicals rather than in primary processes analogous to IV and V. 

Let us discuss first the questions related to the decomposition of the acetyl radical as well as to the self- and cross-combination reactions of the CH3 and CH3CO radicals. Most of the experimental results can be interpreted by taking into accoimt the following steps... 

We prefer to explain the results of Herr and Noyes as well as those of Anderson and Rollefson on the pressure effect by the pressure dependence of the decomposition of the CH3CO radical. According to the Benson equation , based on the detailed theory of unimolecular reactions, the acetyl radical is in the fall-off region below a few atmospheres. O Neal and Benson demonstrated that the results, obtained in the absence of added foreign gases at low light intensities - , are in accordance with the assumption of a pressure-dependent decomposition and a homogeneous recombination of the acetyl radicals. 

The effect of added CO2 on the formation of CO was investigated by Calwell and Hoare - with about 10 quanta.l . sec absorbed intensities, and at temperatures of 50-200 °C. The value of co increased with increasing presstire at all temperatures investigated however, above 120 °C the pressure effect was less pronounced and was more complex in character. It has been suggested that, in the temperature range 120-200 °C, the inert gas effect is due to the acceleration of the decomposition of electronically excited acetone molecules. However, below 100 °C, the increase of co with increasing inert gas pressure is mainly related to the pressure dependence of the homogeneous decomposition of the acetyl radicals. 

There is another competition between the homogeneous decomposition (22) and the homogeneous recombination reactions (21) and (24) of the acetyl radical. Taking into consideration the difference in the reaction orders, with respect to radical concentration, the value of (chjCO)2 expected to increase and that of co to decrease with increasing intensity. The C2Hg/CO ratio should also increase with intensity. All these expectations are supported by the experimental results. 

A considerable fraction of the methyl radicals, recombining to ethane, is formed by the decomposition of acetyl radicals. Thus, the increase in importance of the... 

Martin et that attempts to trap the acetoxy radical with iodine, galvinoxyl or diphenylhydrazyl were either unsuccessful or ambiguous. However, isotopic labeling experiments show that cage recombination does occur and the mechanism of acetyl peroxide decomposition may be formulated as... 

Secondary isotope studies are consistent with the suggested mechanism for acetyl peroxide decomposition. It was concluded from the data given in Table 90 that little or no methyl radical character was developed in the rate-determining step . This view has been contested with data from a study of and 0-iso-tope effects in the decomposition of acetyl peroxide. The carbon isotope effect in... 

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