Free radicals and reaction chains

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. 

In contradiction to Bairstow and Hinshelwood , Gantz and Walters found that iodine catalyzes the thermal decomposition of acetone between 470 and 517 °C. The rate is roughly proportional to the square root of the acetone pressure. Inhibitors, such as NO, C2H4 or C3H6, retard the reaction. 

It is likely that the iodine-catalyzed decomposition is a chain reaction initiated by iodine atoms. The iodine-catalyzed decomposition has also been studied with mixtures of acetone and acetone-c/g. The isotope distribution of the acetone in the pyrolyzed sample has been found to approach the composition expected on statistical grounds. The rapid isotope mixing indicates the presence of chains. 

Relatively more attention had been paid to the study of the sensitized thermal decomposition of acetone. In the temperature range 350-400 °C, Rice et al investigated the decomposition of acetone sensitized by dimethyl mercury. The a-mount of acetonyl acetone formed was equal to that of dimethyl mercury decomposed, indicating the absence of chains. At higher temperatures, however, sensitized chain decomposition has been observed. According to Kodama and Takezaki , 

Less detailed investigations were carried out with other added substances such as for instance oxygen, ethylene oxide, trimethyl amine, ethers, acetyl acetone, acetonyl acetone, etc.. The results are, however, contradictory and inconclusive. 

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Antidegmdents. This group of chemicals is added to prevent undesirable chemical reactions with the polymer network. The most important are the antioxidants, which trap free radicals and prevent chain scission and cross-linking. Antiozonants are added to prevent ozone attack on the rubber, which can lead to the formation and growth of cracks. Antiozonants function by diffusion of the material to the surface of the rubber, thereby providing a protective film. Certain antioxidants have this characteristic, and waxes also are used for this purpose. 

The fact that the pyrolysis reaction proceeds by a free radical and a chain mechanism were pointed out by F.O. Rice. Initiation takes place. by the homolytic sdssioa of a C -C bond with the production of free ladicaK These give rise to the reaction diam by extracting a hydrogen atom horn the hydrocarbon and by fonning a new free ij caL... 

Sometimes the induction period of the chain reaction (for example, oxidation or polymerization) is measured rather than the consumption of the radical acceptor. In this case, known concentrations of the initiator and inhibitor (radical acceptor) are introduced into the system. If during the induction process the initiator remains the only source of free radicals and the inhibitor is consumed only in the reaction with free radicals and all chains terminate in the reaction of the radicals with inhibitor, then... 

The ultraviolet lamps used in the photochlorination process serve to dissociate the chlorine into free radicals and start the radical-chain reaction. Other radical sources, such as 2,2 -a2obisisobutyronitrile, have been used (63,64). Primary by-products of the photochlorination process include 1,1,2-trichloroethane (15—20%), tetrachloroethanes, and pentachloroethane. Selectivity to 1,1,1-trichloroethane is higher in vapor-phase chlorination. Various additives, most containing iodine or an aromatic ring in the molecule, have been used to increase the selectivity of the reaction to... 

Arenesulfonyl chlorides77 as well as alkenesulfonyl chlorides78 react with vinylarenes in the presence of RuCl2(PPh3)3 and 1 molar equiv. of Et3N to form a,/ -unsaturated sulfones in 70-90% yields. The reaction mechanism for the ruthenium(II) catalyzed reaction involves a free-radical redox-transfer chain process as outlined below77 ... 

Still another, and chains, long or short, may be built up. This is the mechanism of free-radical polymerization. Short polymeric molecules (called telomers), formed in this manner, are often troublesome side products in free-radical addition reactions. 

Interaction of acceptors of reactive free radicals and compounds that suppress the transfer reaction of an inhibitor radical with the substrate as it occurs in a system comprising antioxidants and polymer chain with conjugated system of double C = C bonds. 

A term describing certain combinations of mechanical action and chemical reactions exemplified by, but not confined to, the mastication of elastomers. In this process it is considered that the deforming forces break the molecular chains into two pieces, with formation of free radicals at the chain ends. Such radicals may recombine, or combine with oxygen or other... 

Force-field methods, calculation of molecular structure and energy by, 13,1 Free radical chain processes in aliphatic systems involving an electron-transfer reaction, 23, 271 Free radicals, and their reactions at low temperature using a rotating cryostat, study of, 8. I Free radicals, identification by electron spin resonance, 1, 284... 

When free radicals appear in a system, two basically different types of reactions are possible reactions with conservation of free valence and reactions in which radicals (or atoms) interact with each other without conservation of free valence. For example, the peroxyl radical propagates the chain by the reaction... 

In the initial period the oxidation of hydrocarbon RH proceeds as a chain reaction with one limiting step of chain propagation, namely reaction R02 + RH. The rate of the reaction is determined only by the activity and the concentration of peroxyl radicals. As soon as the oxidation products (hydroperoxide, alcohol, ketone, etc.) accumulate, the peroxyl radicals react with these products. As a result, the peroxyl radicals formed from RH (R02 ) are replaced by other free radicals. Thus, the oxidation of hydrocarbon in the presence of produced and oxidized intermediates is performed in co-oxidation with complex composition of free radicals propagating the chain [4], A few examples are given below. 

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. 

The experimental data are in agreement with this equation. In the presence of dioxygen, the alkyl radicals formed from enol rapidly react with dioxygen and thus the formed peroxyl radicals react with Fe2+ with the formation of hydroperoxide. The formed hydroperoxide is decomposed catalytically to molecular products (AcOH and AcH) as well as to free radicals. The free radicals initiate the chain reaction resulting in the increase of the oxidation rate. 

Radicals are also formed in solution by the decomposition of other radicals, which are not always carbon free radicals, and by removal of hydrogen atoms from solvent molecules. Because radicals are usually uncharged, the rates and equilibria of radical reactions are usually less affected by changes in solvent than are those of polar reactions. If new radicals are being made from the solvent by hydrogen abstraction, and if the new radicals participate in chain reactions, this may not be true of course. But even in cases of non-chain radical reactions in which no radicals actually derived from the solvent take part in a rate-determining step, the indifference of the solvent has perhaps been overemphasized. This will be discussed more fully when radical and polar reactions are compared in Chapter XII. 

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