Chain propagation aldehydes

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). 

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... 

The polar carbonyl group interacts with the polar transition state of the reaction between the peroxyl radical and the C—H bond of the aldehyde. This interaction lowers the activation energy of this reaction (see Section 8.1.4). As a result, all the three factors, viz., the strong RC(0)00—H bond formed, the weak C—H bond of the oxidized aldehyde, and the polar interaction in the transition state, contribute to lowering the activation energy of the reaction RC(0)00 + RCH(O) and increasing the rate constant of the chain propagation reaction (see Section 8.1.4). 

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). 

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

The important role of reaction enthalpy in the free radical abstraction reactions is well known and was discussed in Chapters 6 and 7. The BDE of the O—H bonds of alkyl hydroperoxides depends slightly on the structure of the alkyl radical D0 H = 365.5 kJ mol 1 for all primary and secondary hydroperoxides and P0—h = 358.6 kJ mol 1 for tertiary hydroperoxides (see Chapter 2). Therefore, the enthalpy of the reaction RjOO + RjH depends on the BDE of the attacked C—H bond of the hydrocarbon. But a different situation is encountered during oxidation and co-oxidation of aldehydes. As proved earlier, the BDE of peracids formed from acylperoxyl radicals is much higher than the BDE of the O—H bond of alkyl hydroperoxides and depends on the structure of the acyl substituent. Therefore, the BDEs of both the attacked C—H and O—H of the formed peracid are important factors that influence the chain propagation reaction. This is demonstrated in Table 8.9 where the calculated values of the enthalpy of the reaction RjCV + RjH for different RjHs including aldehydes and different peroxyl radicals are presented. One can see that the value A//( R02 + RH) is much lower in the reactions of the same compound with acylperoxyl radicals. 

They proposed a polymerization scheme closely related to other well-known chemical reactions of metal alkoxide with carbonyl compounds (20). In Scheme 2, complex [A] is converted to [B] by hydride ion transfer from the alkoxyl group to the carbon atom of aldehyde (Meerwein-Ponndorf reduction). Addition of one molecule of monomer to the growing chain requires transfer of the alkoxide anion to the carbonyl group to form a new alkoxide [C]. Repetition of these two consecutive processes, i.e., coordination of aldehyde and transfer of the alkoxide anion, constitutes the chain propagation step. 

Radical X , which initiates the reaction, is regenerated in a chain propagation sequence that, at the same time, produces an organic peroxide. The latter can be cleaved to form two additional radicals, which can also react with the unsaturated fatty acids to set up the autocatalytic process. Isomerization, chain cleavages, and radical coupling reactions also occur, especially with polyunsaturated fatty acids. For example, reactive unsaturated aldehydes can be formed (Eq. 21-14). 

The ultimate products of the oxidation of any hydrocarbon are carbon dioxide and water vapour, but there are many relatively stable partially oxidised organic species such as aldehydes, ketones and carbon monoxide that are produced as intermediate products during this process, with ozone produced as a by-product of the oxidation process. Figure 10 shows a schematie representation of the free radical catalysed oxidation of methane, whieh is analogous to that of a hydrocarbon. As previously discussed, the oxidation is initiated by reaction of the hydrocarbon with OH and follows a mechanism in with the alkoxy and peroxy radicals are chain propagators and OH is effectively catalytic, viz... 

The nature of the cool flames which propagate in the Lj lobe are similar to those of the Lq lobe and this mechanism is also thought to be caused by primary peroxidation, but in this case it is catalysed by hydrogen donors. It was seen in Sect. 3.2.2(a)(iv) that aldehydes are formed in the primary chain-propagation cycle and hence alkylhydro-peroxides may be formed by the reaction sequence (14) + (21) + (32), (where AH is an aldehyde)... 

The Fischer Tropsch kinetics of product formations are best understood as a non trivial surface polymerization. The basic kinetic interrelations are well described by an ideal model with carbon number independent probability of chain propagation. The pecularities of real Fischer Tropsch systems are then described as deviations from the ideal model. In this paper (because of space limitations) only the Anderson two slope distributions are discussed and explained by a readsorption extension of the ideal model. The full model including chain branching and formation of olefins, alcohols and aldehydes is being published shortly (ref. 28). 

The reactions of chain propagation are presented through steps (2), (3), (7) for the oxidation of the aldehyde group and steps (4), (5) and (6) for the reactions with participation of the double bond. 

It can be seen from Figure 6.5 that at 40% conversion of the aldehyde the contributions of steps (12)-(16) become negative, and the opposite is true for steps (1) and (8) on radical generation. The most probable explanation of this fact is in the significant increase in the contribution of the chain propagation step (3). In this case increase in the concentration of... 

The effectiveness of a modifier depends on its chemical structure, concentration, temperature, and pressure. A concentration-independent measure for its effectiveness is the chain transfer constant, defined as the ratio of kinetic coefficients for the transfer reaction to this substance and radical chain propagation reaction. Usually the effectiveness of chain transfer agents is increased with rising temperature and reduced pressure. The chain transfer constant of modifiers falls from aldehydes, which are more effective than ketones or esters, to hydrocarbons. Unsaturated hydrocarbons typically have higher transfer constants than saturated hydrocarbons and a strong effect on polymer density must be considered because of the ability to copolymerize that give a higher frequency of short-chain branches in the polymer. 

Oxidation begins with the breakdown of hydroperoxides and the formation of free radicals. These reactive peroxy radicals initiate a chain reaction that propagates the breakdown of hydroperoxides into aldehydes (qv), ketones (qv), alcohols, and hydrocarbons (qv). These breakdown products make an oxidized product organoleptically unacceptable. Antioxidants work by donating a hydrogen atom to the reactive peroxide radical, ending the chain reaction (17). 

Autooxidation. Liquid-phase oxidation of hydrocarbons, alcohols, and aldehydes by oxygen produces chemiluminescence in quantum yields of 10 to 10 ° ein/mol (128—130). Although the efficiency is low, the chemiluminescent reaction is important because it provides an easy tool for study of the kinetics and properties of autooxidation reactions including industrially important processes (128,131). The light is derived from combination of peroxyl radicals (132), which are primarily responsible for the propagation and termination of the autooxidation chain reaction. The chemiluminescent termination step for secondary peroxy radicals is as follows ... 

The mechanism proposed by Emmons thus corresponds in part to the decomposition of the trialkyl-oxaziranes by ferrous salts. By radical attack on the 7V-alkyl group of the oxazirane, the radical 32 is formed which rearranges with ring opening to 33. Radical 33 propagates the chain by attack on a further molecule of oxazirane. It takes up an H-atom and is decomposed to ketone and ammonia. The aldehyde produced from the M-alkyl group is converted to tar. 

Packer and Richardson (1975) and Packer et al. (1980) made use of the fact that electrons can be generated in water by y-radiation from a 60Co source (Scheme 8-29) to induce a free radical chain reaction between diazonium ions and alcohols, aldehydes, or formate ion. It has to be emphasized that the radiolytically formed solvated electron in Scheme 8-29 is only a part of the initiation steps (Scheme 8-30) by which an aryl radical is formed. The aryl radical initiates the propagation steps shown in Scheme 8-31. Here the alcohol, aldehyde, or formate ion (RH2) is the reducing agent (i.e., the electron donor) for the main reaction. The process is a hydro-de-diazoniation. 

Anionic copolymerization involving similar monomer pairs which propagate through a similar chain end occurs rather easily. Examples are the copolymerization involving aldehydes, isocyanate and ketenes, although these are not always random... 

Another factor complicating the situation in composition of peroxyl radicals propagating chain oxidation of alcohol is the production of carbonyl compounds due to alcohol oxidation. As a result of alcohol oxidation, ketones are formed from the secondary alcohol oxidation and aldehydes from the primary alcohols [8,9], Hydroperoxide radicals are added to carbonyl compounds with the formation of alkylhydroxyperoxyl radical. This addition is reversible. 

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