Chain propagation in the oxidation

In addition to two peroxyl radicals, H02 and R1R2C(0H)00 , participating in chain propagation in the oxidized alcohols, there are three reactions that are guilty of chain termination in the oxidized alcohols. The most probable reaction between them is disproportionation. 

Rate Constants for Chain Propagation in the Oxidation of Aldehydes... 

Of these reactions, the reaction of the peroxyl radical with phosphite is the slowest. The rate constant of this reaction ranges from 102 to 103 L mol 1 s 1 which is two to three orders of magnitude lower than the rate constant of similar reactions with phenols and aromatic amines. Namely, this reaction limits chain propagation in the oxidation of phosphites. Therefore, the chain oxidation of trialkyl phosphites involves chain propagation reactions with the participation of both peroxyl and phosphoranylperoxyl radicals ... 

The alkylperoxy radical isomerization and alkene-hydroperoxy radical addition theories of chain propagation in the oxidation of alkyl radicals at ca. 300°C. have been combined into a single comprehensive reaction scheme. 

Chain propagation in an oxidized aldehyde is limited by the reaction of the acylperoxyl radical with the aldehyde. The dissociation energy of the O—H bond of the formed peracid is sufficiently higher than that of the alkyl hydroperoxide. For example, in hydroperoxide PhMeCHOOH, Z)0 H = 365.5 kJ mol-1 and in benzoic peracid... 

Zolotova NV, Denisov ET. Mechanism of propagation and degenerate chain branching in the oxidation of polypropylene and polyethylene. J Polym Sci Polym Chem Ed 1971 9 3311-20. 

Secondary cage recombination of peroxy radicals [698]. In a solid polymer, a pair of polymer peroxy radicals (POO 2) is trapped in the polymer matrix. When a radical pair, produced by photoinitiation, escapes the initial cage, the probability of its recombination remains high even after several propagation steps. This phenomenon, known as secondary cage recombination, has a pronounced effect on the kinetics of oxidation and on the distribution of kinetic chain lengths in the oxidation process. 

It can be seen that the decomposing peroxide gives rise to a number of radicals, viz, (CH3)3C0 , C H3, and R, but the chain reaction fails to be initiated in this system. At the same time, the addition of oxygen to the peroxide-RH system will initiate chain oxidation with chain propagation through the cycle of reactions ... 

Peroxyl radicals can undergo various reactions, e.g., hydrogen abstraction, isomerization, decay, and addition to a double bond. Chain propagation in oxidized aliphatic, alkyl-aromatic, alicyclic hydrocarbons, and olefins with weak C—H bonds near the double bond proceeds according to the following reaction as a limiting step of the chain process [2 15] ... 

Chain propagation in oxidized 1,2-substituted ethylenes proceeds via addition of dioxygen followed by the elimination of the hydroperoxyl radical [156] ... 

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... 

The kinetic analysis proves that formation of very active radical from intermediate product can increase the reaction rate not more than twice. However, the formation of inactive radical can principally stop the chain reaction [77], Besides the rate, the change of composition of chain propagating radicals can influence the rate of formation and decay of intermediates in the oxidized hydrocarbon. In its turn, the concentrations of intermediates (alcohols, ketones, aldehydes, etc.) influence autoinitiation and the rate of autoxidation of the hydrocarbon (see Chapter 4). 

The reaction of hydrogen atom abstraction by the alkylhydroxyperoxyl radical from alcohol limits chain propagation in oxidized alcohol [8,9]. 

The analysis of alcohol reactivities in reactions with the peroxyl radicals will be discussed later. The values of the rate constants of chain propagation in oxidized alcohols are collected in Table 7.4. 

A different situation in the oxidation of these two alcohols is seen. The hydroperoxyl radical is the main chain propagating species in oxidized 2-propanol the portion of alkylhydroxy-peroxyl radicals in this reaction is less than 2.5%. In oxidized cyclohexanol, on the contrary, the stationary state concentrations of both radicals are close and both of them take important part in chain propagation. 

Chain propagation in oxidized alcohols proceeds with the participation of a mixture of H02 and R(0H)02 radicals. Using the data of Table 7.4 and Table 7.7 we can compare the activity of the H027H0R02 mixture and alkylperoxyl radicals toward the same alcohol (R OO is the peroxyl radical of cyclohexene, T = 333 K). 

The chain unit in the thermal and photochemical oxidation of aldehydes by molecular dioxygen consists of two consecutive reactions addition of dioxygen to the acyl radical and abstraction reaction of the acylperoxyl radical with aldehyde. Experiments confirmed that the primary product of the oxidation of aldehyde is the corresponding peroxyacid. Thus, in the oxidation of n-heptaldehyde [10,16,17], acetaldehyde [4,18], benzaldehyde [13,14,18], p-tolualdehyde [19], and other aldehydes, up to 90-95% of the corresponding peroxyacid were detected in the initial stages. In the oxidation of acetaldehyde in acetic acid [20], chain propagation includes not only the reactions of RC (0) with 02 and RC(0)00 with RC(0)H, but also the exchange of radicals with solvent molecules (R = CH3). 

As mentioned above, the parameters of co-oxidation can be used for the estimation of the BDE of the O—H bond formed in the chain propagation reaction. The ratio... 

The nonsaturated esters with tt-C=C bonds and without activated a-C—H bonds (esters of acrylic acid (CH2=CHCOOR) and esters of vinyl alcohols (RC(0)0CH=CH2)) are oxidized by the chain mechanism with chain propagation via the addition of peroxyl radicals to the double bond. Oligomeric peroxides are formed as primary products of this chain reaction. The kinetic scheme includes the following steps in the presence of initiator I and at p02 sufficient to support [02] > 10 4 mol L-1 in the liquid phase [49]. 

This catalyst makes the increase in the oxidation rate of alkylaromatic hydrocarbons possible due to the intense participation of the catalyst itself (Co2+, Co3+, Br, and Br ) in chain propagation. 

Catalysis by strong bases in the oxidation of organic compounds in aprotonic solvents, principally, is similar. The slow reaction of chain propagation of the type R02 + RH... 

It was in 1924 when Christiansen [3] put forward the conception of the chain mechanism of oxidation and explained the action of antioxidants via chain termination by the antioxidant [3]. Three years later, Backstrom and coworkers [4—6] experimentally proved the chain mechanism of benzaldehyde oxidation (see Chapter 1) and the mechanism of antioxidant (hydroquinone) action via chain termination. The systematic study of the oxidation kinetics of esters of nonsaturated acids was performed by Bolland and ten Have [7,8], They observed in the kinetic experiments that substrates are oxidized by the chain mechanism with chain propagation via the cycle of reactions (see Chapter 2). 

We see that the participation of the inhibitor radical in chain propagation decreases the reaction order of the initiation rate, i.e., initiator diminishes the dependence of the reaction rate on the inhibitor concentration. The retardation of oxidation can be performed with the antioxidant concentration [InH] > 2k(,k 10[R H]/fk7kH. The kinetics of chain oxidation obeys the equation (for t[Pg.494]

The question why the aminyl radicals ensure cyclic chain termination in those systems in which the hydroperoxyl and hydroxyalkylperoxyl radicals are formed, but not in the oxidation of hydrocarbons where alkylperoxyl radicals are the chain-propagating species deserves special attention [22 24]. Indeed, the disproportionation of the aminyl and peroxyl radicals... 

The sequence [Eqs. (17)—(20)] is of great importance in the oxidation reaction mechanisms of any hydrocarbon in that it provides the essential chain branching and propagating steps as well as the radical pool for fast reaction. 

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