Acetyl radical oxidation

Because of the mixture of VOCs in the atmosphere, the composition of smog reaction products and intermediates is extremely complex. formed via reaction 16, is important because when dissolved in cloud droplets it is an important oxidant, responsible for oxidising SO2 to sulfuric acid [7664-93-9] H2SO4, the primary cause of acid precipitation. The oxidation of many VOCs produces acetyl radicals, CH CO, which can react with O2 to produce peroxyacetyl radicals, CH2(C0)02, which react with NO2... 

When n = 0 or 1, the system appeared to be too rigid to allow the radical pair created upon hydrogen abstraction to form a carbon-carbon bond. Hence a considerable amount of chlorine appears in the product from radical abstraction from the solvent, carbon tetrachloride. When n = 2 the radicals are able to form a carbon-carbon bond. After a five-step workup of the crude irradiation product including reduction with LiAlH4, acetylation, dehydration, oxidation with ruthenium tetroxide, and hydrolysis a 16% yield of previously unreported 12-keto-3a-chlorestanol was obtained. However,... 

Anodic substitution reactions of aromatic hydrocarbons have been known since around 1900 [29, 30]. The course of these processes was established primarily by a study of the reaction between naphthalene and acetate ions. Oxidation of naphthalene in the presence of acetate gives 1-acetoxynaphthalene and this was at first taken to indicate trapping of the acetyl radical formed during Kolbe electrolysis of... 

The acetyl radicals produced in this reaction must be oxidized by cobalt (III) to acetic acid and/or anhydride in a fast step since only traces of other possible by-products (biacetyl and acetone) were found. 

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. 

For the vanadium-catalyzed reaction, the involvement of radical species has been suggested [4], as shown in Scheme 4. The oxovanadium(V) species could abstract an H atom from CH4 to form the methyl radical CH3, which could react with CO to give the acetyl radical CH3CO. Oxidation of CH3CO to CH3CO+ by V(V)=0 would give acetic acid. 

Depending on the conditions, metal-catalyzed autoxidation of acetaldehyde can be utilized for the manufacture of either acetic acid or peracetic acid.321 In addition, autoxidation of acetaldehyde in the presence of both copper and cobalt acetates as catalysts produpes acetic anhydride in high yield.322 b The key step in anhydride formation is the electron transfer oxidation of acetyl radicals by Cu(II), which competes with reaction of these radicals with oxygen ... 

In a study of a nonchain photooxidation of acetone one is concerned with the oxidation of methyl radicals and acetyl radicals. Figure 4 shows that under simple conditions the yields of formaldehyde and methanol are equal and this is a feature of other systems (to be described later) that produce methyl radicals. 

In addition, considerable quantities of carbon dioxide are found and, since these are not normally found in large quantities in the oxidation of methyl radicals, one can assume that they originate from the oxidation of acetyl radicals. The oxidation of acetyl radicals is a very surface-dependent reaction (McDowell and Sharpies82) and in the presence of a readily abstractable hydrogen atom at room temperature (e.g., in acetaldehyde) the main product appears to be peracetic acid. 

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... 

The work on acetone photooxidation shows that acetyl radicals may yield acetic acid or carbon dioxide, carbon monoxide, and products of methyl radical oxidation. The carbon dioxide came mostly from the carbonyl group but the carbon monoxide was not so specific in origin. 

Three experiments performed using oxygen containing about 12% Qi6Qig indicated that the carbon dioxide was formed predominantly from carbonyl group oxidation, while the formaldehyde and methanol were formed by methyl radical oxidation and the acetic acid was formed by oxidation of an acetyl radical. The results for carbon monoxide were unreliable. 

In view of the difficulty observed by all workers on the photooxidation of biacetyl in obtaining reproducible results, it is difficult to write a satisfactory mechanism. The products are much the same whether 3130 or 4358 A. radiation is used and can be summarized as oxidation products of acetyl radicals or carbon monoxide plus the oxidation products of methyl radicals. 

In oxidation systems where formaldehyde, a primary product, is attacked by radicals, hydrogen atoms will only be produced at very low oxygen concentrations, i.e., where reaction (79) can compete with reaction (80). Even at temperatures as high as 500°C., formaldehyde is known to cause chain ending by the production of the formyl radical.8 It can therefore be safely assumed that formyl radicals normally react according to reaction (80). As with the oxidation of the acetyl radical, the acid (in this case formic acid) can be found in large quantities in the products or it can be entirely absent. By comparison, it is likely that this depends on the condition of the surface and the availability of hydrogen atoms for abstraction. 

Reaction (87) would be followed by oxidation of the radicals produced [e.g., reactions (80) and (18) or (80) and (13) ], so that at the end of the sequence acetyl radicals would be produced by abstraction from acetaldehyde [e.g., reaction (100) or (98)]. The propagation reactions which follow are said to be ... 

The mechanism proposed by Calvert and Hanst88 originally used ozone as a chain carrier in the methyl oxidation. This has now been shown to be unlikely. They agree with McDowell et al.81 that reaction of a radical with acetaldehyde to give acetyl radicals ends the initiation sequence. The propagation steps are also identical with those proposed by McDowell et al.81 Chain ending steps include... 

Although the formyl radical, HCO, has been trapped and identified by e.s.r. (Adrian et al., 1962 Cochran et al., 1966 Brivati et al., 1962) the corresponding acetyl radical, CH3CO, which is an important intermediate in hydrocarbon oxidation had not been identified conclusively. In fact several different e.s.r. spectra have been attributed to this radical. 

Acetylation is a very common metabolic reaction which occurs with amino, hydroxyl or sulfhydryl groups. The acetyl group is transferred from acetyl-coenzyme A 2ind the reaction is catalysed by acetyltransferases. An important aspect of this kind of substitution is the genetic polymorphism of one acetyltransferase in humans, who are divided into fast and slow acetylators. In a few cases, the conjugates are further metabolized to toxic compounds, as is seen with isoniazid. Some evidence exists that acetylation of the antitubercular isoniazid leads to enhanced hepatotoxicity of the drug. ° Acetylation followed by hydrolysis and cytochrome P-450-dependent oxidation yields free acetyl radicals or acylium cations which may acetylate the nucleophilic macromolecule functions (Fig. 32.16). 

Turro, George, and co-workers [70] have also observed and studied an acyl radical in solution by TRIR methods in their investigation of the photochemistry of (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (Scheme 2.4). More recently, Vasenkov and Frei [71, 72] reported the generation of the acetyl radical by photolysis of 1-naphthyl acetate (Scheme 2.5) or pinacolone (Scheme 2.2 R = Me, R = f-Bu) in zeolite NaY. Through the use of step-scan FTIR spectroscopy, they observed a carbonyl stretching mode at 2125 cm , interestingly... 

Initially, acetaldehyde is oxidized to peracetic acid via hard acetyl radical intermediates (Eq. 2-85). The peracetic acid then oxidizes acetaldehyde to acetic acid. 

There are other products which are also typical of photochemical air pollution. One which is highly characteristic of photochemical air pollution processes is perojgracetyl nitrate (PAN). The formation route is via acetyl radicals (CH3CO) formed from a mrmber of routes, most notably oxidation of ethanal ... 

The direct a-oxidation of propionate to pyruvate has been questioned for some time. The degree of randomization found in glucose upon administration of propionate-2-C or -3-C was found to be more complete than for the correspondingly labeled lactate. Shreeve, in studying the formation of acetyl groups from propionate, observed that the acetyl radicals in N-acetyl phenylaminobutyric acid derived from propionate-2-C was 100% randomized while that derived from the correspondingly labeled lactate or pyruvate was only about 20% randomized. 

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. 

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