Chain mechanism, hydroperoxide

The effects of heteroatoms on autoxidation reactions are reviewed and discussed in terms of six phenomena (1) the effect on reactivity of a-hydrogens in the hydroperoxide chain mechanism in terms of electron supply and withdrawal (2) the effect on a-hydrogen acidity in base-catalyzed oxidation (3) the effect on radical ion stability in base-catalyzed redox chains (4) the possibility of heteroatom hydrogen bond attack and subsequent reactions of the resulting heteroradical (5) the possibility of radical attack on higher row elements via valence expansion (6) the possibility of radical addition to electron-deficient II and III group... 

In the familiar hydroperoxide chain mechanism for hydrocarbon autoxidation, with propagation steps,... 

The natures and precise modes of formation of the radicals X and Y have aroused considerable controversy. In the liquid phase, the noncatalytic oxidation by molecular oxygen of the majority of alkanes occurs by the hydroperoxide chain mechanism, in which the alkylperoxy... 

Inspired by Gif or GoAgg type chemistry [77], iron carboxylates were investigated for the oxidation of cyclohexane, recently. For example, Schmid and coworkers showed that a hexanuclear iron /t-nitrobenzoate [Fe603(0H) (p-N02C6H4C00)n(dmf)4] with an unprecedented [Fe6 03(p3-0)(p2-0H)] " core is the most active catalyst [86]. In the oxidation of cyclohexane with only 0.3 mol% of the hexanuclear iron complex, total yields up to 30% of the corresponding alcohol and ketone were achieved with 50% H2O2 (5.5-8 equiv.) as terminal oxidant. The ratio of the obtained products was between 1 1 and 1 1.5 and suggests a Haber-Weiss radical chain mechanism [87, 88] or a cyclohexyl hydroperoxide as primary oxidation product. 

Organic peroxides and hydroperoxides decompose in part by a self-induced radical chain mechanism whereby radicals released in spontaneous decomposition attack other molecules of the peroxide.The attacking radical combines with one part of the peroxide molecule and simultaneously releases another radical. The net result is the wastage of a molecule of peroxide since the number of primary radicals available for initiation is unchanged. The velocity constant ka we require refers to the spontaneous decomposition only and not to the total decomposition rate which includes the contribution of the chain, or induced, decomposition. Induced decomposition usually is indicated by deviation of the decomposition process from first-order kinetics and by a dependence of the rate on the solvent, especially when it consists of a polymerizable monomer. The constant kd may be separately evaluated through kinetic measurements carried out in the presence of inhibitors which destroy the radical chain carriers. The aliphatic azo-bis-nitriles offer a real advantage over benzoyl peroxide in that they are not susceptible to induced decomposition. 

The traditional chain oxidation with chain propagation via the reaction RO/ + RH occurs at a sufficiently elevated temperature when chain propagation is more rapid than chain termination (see earlier discussion). The main molecular product of this reaction is hydroperoxide. When tertiary peroxyl radicals react more rapidly in the reaction R02 + R02 with formation of alkoxyl radicals than in the reaction R02 + RH, the mechanism of oxidation changes. Alkoxyl radicals are very reactive. They react with parent hydrocarbon and alcohols formed as primary products of hydrocarbon chain oxidation. As we see, alkoxyl radicals decompose with production of carbonyl compounds. The activation energy of their decomposition is higher than the reaction with hydrocarbons (see earlier discussion). As a result, heating of the system leads to conditions when the alkoxyl radical decomposition occurs more rapidly than the abstraction of the hydrogen atom from the hydrocarbon. The new chain mechanism of the hydrocarbon oxidation occurs under such conditions, with chain... 

Hydroperoxide is the first product of hydrocarbon oxidation and plays a key role in the chain mechanism of autoxidation. Hydroperoxide possesses a weak O—O bond and decomposes... 

Different chain mechanisms of hydroperoxide decomposition are known with the participation of alkyl, alkoxyl, and peroxyl radicals [9]. 

In the oxidized hydrocarbon, hydroperoxides break down via three routes. First, they undergo homolytic reactions with the hydrocarbon and the products of its oxidation to form free radicals. When the oxidation of RH is chain-like, these reactions do not decrease [ROOH]. Second, the hydroperoxides interact with the radicals R , RO , and R02. In this case, ROOH is consumed by a chain mechanism. Third, hydroperoxides can heterolytically react with the products of hydrocarbon oxidation. Let us consider two of the most typical kinetic schemes of the hydroperoxide behavior in the oxidized hydrocarbon. The description of 17 different schemes of chain oxidation with different mechanisms of chain termination and intermediate product decomposition can be found in a monograph by Emanuel et al. [3]. 

Along with homolytic decomposition, hydroperoxides are decomposed in an acetamide solution by the chain mechanism under the action of formed free radicals [33,35]. 

The experiments on emulsion cumene oxidation with AIBN as initiator proved that oxidation proceeds via the chain mechanism inside hydrocarbon drops [17]. The presence of an aqueous phase and surfactants compounds does not change the rate constants of chain propagation and termination the ratio (fcp(2fct)-1/2 = const in homogeneous and emulsion oxidation (see Chapter 2). Experiments on emulsion cumene oxidation with cumyl hydroperoxide as the single initiator evidenced that the main reason for acceleration of emulsion oxidation versus homogeneous oxidation is the rapid decomposition of hydroperoxide on the surface of the hydrocarbon and water drops. Therefore, the increase in the aqueous phase and introduction of surfactants accelerate cumene oxidation. 

Oxidation of organic compounds occurs by the chain mechanism via alternating reactions of alkyl and peroxyl radicals (see Chapter 2). The accumulated hydroperoxide decomposes into radicals, thereby increasing the rate of oxidation (see Chapter 4). The oxidation of an organic compound may be retarded by one of the following three ways ... 

Hydroperoxides oxidize aromatic amines more readily than analogous phenols. Thus, at 368 K cumyl hydroperoxide oxidizes a-naphthylamine and a-naphthol with ku = 1.4 x 10 4 and 1.7 x 10 5L mol-1 s 1, respectively [115,118], The oxidation of amines with hydroperoxides occurs apparently by chain mechanism, since the step of free radical generation proceeds much more slowly. This was proved in experiments on amines oxidation by cumyl hydroperoxide in the presence of /V,/V -diphcnyl-l, 4-phcnylcnediamine (QH2) as a radical acceptor [125]. The following reactions were supposed to occur in solution (80% decane and 20% chlorobenzene) ... 

The decay of amine oxidized by hydroperoxide occurs much more rapidly than free radical generation. Apparently, these reactions proceed by chain mechanism. The diatomic phenols and aryldiamines (QH2) must react with ROOH by the chain mechanism in which the semiquinone radical -QH that reduces hydroperoxide plays the key role. The following chain mechanism can be supposed [122] ... 

Phosphites can react not only with hydroperoxides but also with alkoxyl and peroxyl radicals [9,14,17,23,24], which explains their susceptibility to a chain-like autoxidation and, on the other hand, their ability to terminate chains. In neutral solvents, alkyl phosphites can be oxidized by dioxygen in the presence of an initiator (e.g., light) by the chain mechanism. Chains may reach 104 in length. The rate of oxygen consumption is proportional to v 1/2, thus indicating a bimolecular mechanism of chain termination. The scheme of the reaction... 

Oxidation to CO of biodiesel results in the formation of hydroperoxides. The formation of a hydroperoxide follows a well-known peroxidation chain mechanism. Oxidative lipid modifications occur through lipid peroxidation mechanisms in which free radicals and reactive oxygen species abstract a methylene hydrogen atom from polyunsaturated fatty acids, producing a carbon-centered lipid radical. Spontaneous rearrangement of the 1,4-pentadiene yields a conjugated diene, which reacts with molecular oxygen to form a lipid peroxyl radical. 

Mo containing Y zeolites were also tested for cyclohexene oxidation with oxygen as oxidant and t-butyl hydroperoxide as initiator [86]. In this case the selectivity for cyclohexene oxide was maximum 50%, 2-cyclohexene-l-ol and 2-cyclohexene-l-one being the main side products. The proposed reaction scheme involves a free radical chain mechanism with intermediate formation of cyclohexenyl hydroperoxide. Coordination of the hydroperoxide to Mo + in the zeolite and oxygen transfer from the resulting complex to cyclohexene is believed to be the major step for formation of cyclohexene oxide under these conditions. 

The known reaction product of the oxidation of cyclohexene was assigned as the hydroperoxide 27 by Criegee in 1936. The oxidation of cumene to the hydroperoxide 28 proceeds by a chain mechanism (equations 35, 36), and the conversion of the hydroperoxide by acid to phenol and acetone, in what has become a commercially important process, was reported by Hock and Lang in 1944. 2s... 

The catalysis of the selective oxidation of alkanes is a commercially important process that utilizes cobalt carboxylate catalysts at elevated (165°C, 10 atm air) temperatures and pressures (98). Recently, it has been demonstrated that [Co(NCCH3)4][(PF6)2], prepared in situ from CoCl2 and AgPF6 in acetonitrile, was active in the selective oxidation of alkanes (adamantane and cyclohexane) under somewhat milder conditions (75°C, 3 atm air) (99). Further, under these milder conditions, the commercial catalyst system exhibited no measurable activity. Experiments were reported that indicated that the mechanism of the reaction involves a free radical chain mechanism in which the cobalt complex acts both as a chain initiator and as a hydroperoxide decomposition catalyst. 

The dissociation process is described by a free radical chain mechanism. The thermo-oxidative dissociation is initiated by the oxidation of the aliphatic moieties by a subsequent cleavage of the hydroperoxides formed. With increasing time of oxidation the temperature of the onset of degradation is lower as compared with that for a purely thermal degradation. 

There is further evidence for the role played by methyl hydroperoxide in the low temperature combustion of acetone. Knox (23) showed that if one assumes a simple basic chain mechanism for oxidation, then the acceleration constant, < , which characterizes the exponential acceleration to maximum rate, is given by... 

The major product of this chain is the alkyl hydroperoxide. The secondary products which are observed are the results of the reactions of alkyl radicals in the system with the hydroperoxide or of the secondary spontaneous breakdown of the hydroperoxide if the temperature is sufficiently high. This chain mechanism predominates in the temperature regime from about 30° to about 250°C., for the gas phase or in relatively inert solvents. 

On the one hand, this complicates the problem of understanding such fast oxidations, but on the other, it points up the possibility of a much wider range of control and selectivity than would be possible if only a single chain-carrying species were involved. It may well be that the autoxidations of cumene and acetaldehyde are the only technical processes in operation to which the classical hydroperoxide (or peracid) long-chain mechanisms truly apply. 

On the other hand, the Gif-tert-butyl hydroperoxide (TBHP) systems seem to be much less complicated. These have been extensively studied by many groups and all workers agree that the reaction proceeds via radical pathways based on the reactivity of tert-butylperoxy and tert-butyloxy radicals [22-24]. Minisci et al. [22] suggested a Haber-Weiss radical chain mechanism [25] that accounts for the observed selectivities. 

Unsaturated fatty acids in foods are very susceptible to oxidation by oxygen in the air during processing and storage. The oxidation results initially in the formation of fatty acid hydroperoxides by a free radical chain mechanism. The hydroperoxides are subject to several further reactions forming secondary products such as aldehydes, ketones, and other volatile compounds, many of which are odorous and cause rancid flavor in the food. This development of rancid flavor limits the storage stability of a large number of food products. 

Gray61 and Nalbandyan94 studied the mercury photosensitized oxidation of ethane in a flow system and showed that ethyl hydroperoxide was the chief initial product. Gray61 similarly obtained considerable amounts of methyl hydroperoxide in the photosensitized oxidation of methane. However, Watson and Darwent127 pointed out that the chain mechanism they proposed ... 

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