Radical chain carriers

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. 

Since a radical is consumed and formed in reaction (3.3) and since R represents any radical chain carrier, it is written on both sides of this reaction step. Reaction (3.4) is a gas-phase termination step forming an intermediate stable molecule I, which can react further, much as M does. Reaction (3.5), which is not considered particularly important, is essentially a chain terminating step at high pressures. In step (5), R is generally an H radical and R02 is H02, a radical much less effective in reacting with stable (reactant) molecules. Thus reaction (3.5) is considered to be a third-order chain termination step. Reaction (3.6) is a surface termination step that forms minor intermediates (T) not crucial to the system. For example, tetraethyllead forms lead oxide particles during automotive combustion if these particles act as a surface sink for radicals, reaction (3.6) would represent the effect of tetraethyllead. The automotive cylinder wall would produce an effect similar to that of tetraethyllead. 

Besides new insight into the reactivity of free radicals, methods for die production of carbon-centered free radicals have also seen major improvements in die last several years. One very common new mediod is to use tin-based reagents as radical chain carriers. Trialkyltin radicals readily abstract bromine or iodine from carbon to produce a carbon-centered free radical. Placement of a bromide or iodide substituent on a substrate dius permits formation of a carbon-centered free radical at diat position using tin-based mediodology. This process was initially developed for die reduction of alkyl halides, and it remains an excellent synthetic method for diat purpose. The complete chain mechanism for die reduction is shown. 

The third class of stabilisers are included only for the sake of completeness but the mechanism by which they operate are probably the best understood. They may be described as antioxidants and are effective because they intercept the radical chain carriers or decompose the peroxides or hydroperoxides which are potential radical initiators. 

It was established by Bolland and Gee that organic hydroperoxides appear as one of the first products of oxidation (10, 11). Subsequent oxidation of the polymer is autocatalysed by the decomposition of hydroperoxides which produce free radical chain carriers for the chain reaction. 

Selected examples of such oxidations of straight-chain aliphatic compounds are listed in Table 1. The remarkable monohalogenation and co-1 selectivities in these reactions have been attributed to polar, steric, and conformational effects. The yields of monohalogenated derivatives, based on converted (60-80"/)) substrate, are generally quantitative. Comparison of these selectivities with a number of oxidations using bromine, chlorine or rerf-butoxy radical chain carriers shows that aminium radical-mediated oxidation is far superior to others for synthetic, indeed industrial, applications [28]. The high selectivity and clean monooxidation displayed by the aminium radical chain process has been referred as an enzyme-mimetic reaction ... 

Studied, and the relatively slow autoxidation of triphenylphosphine was ascribed to the stability of the radical chain carrier (19). Trifluoroacetyl-diphenylphosphine is known to oxidize to the phosphine oxide (20), and a rather unusual explanation of this reaction has been suggested by the same workers (Scheme 4). The formation of 2,2,2-trifluoroethanol by... 

Because of their commercial importance, we still need to do more work on thermal cracking reactions, since their scope and complexity extend considerably beyond the world of Rice-Herzfeld mechanisms. For example, consider the pyrolysis of butane (K.J. Laidler, Chemical Kinetics, McGraw-Hill, New York, 1965). This molecule affords the formation of a number of radical chain-carrier species, and the number of elementary steps increases accordingly... 

Another very important elass of ehain reaetions, perhaps the most important from a commereial viewpoint, ineludes those involved in polymerization. Materials such as polyethylene and polystyrene are formed in chain reactions with free radical chain carriers. These addition polymerization ehains are similar in substance to those we have been discussing, but differ in three important respects. First, the monomer, particularly when purified, is often quite unreaetive and it is necessary to use small quantities of separate substanees (initiators) that essentially trick the monomer into... 

The mechanism of hydroquinone autoxidation likely proceeds by a radical chain pathway. Kinetic studies carried out under relevant reaction conditions support a second-order rate law for the reaction, rate =Ar[QH2] [Oj], with an apparent activation energy of = 15 kcal/mol [21]. Based on these kinetic findings, as well as DFT studies [22], anthrahydroquinone autoxidation has been proposed to occur through initial, rate-limiting, direct H-atom abstraction from the hydroquinone species by O2 (Eq. (14.2)). The semiquinone species then react readily with triplet O2 (Eq. (14.3)), and hydroperoxy radical, HO2, has been proposed to act as a radical chain carrier (Eq. (14.4)). 

RTC mechanism is similar to that of the conventional TC with the essential difference that radical chain carriers are generated not only thermally but also by ionizing radiation. There are two main conditions necessary for a chain cracking reaction ... 

Gg is radiation-chemical yield of radical chain carriers... 

In the calculations provided hereafter, the following values of radiation-chemical yields of radical chain carriers and constants of their recombination were used G=5 radicals/100 eV 3 x 10" cm /... 

This mechanism involves a closed sequence, or chain, consisting of reactions (52) and (53). The two free radical chain carriers are the neopentyl radical and the... 

It includes a unimolecular initiation reaction (ui) leading, by transfer reactions, which have not been written, to the free radical chain carriers 6 or p. The chain is propagated by the reactions (bs) and (me) and gives rise to products m and 6H. The free radical p decomposes by a monomolecular reaction, whereas the free radical 6 reacts in a bimolecular reaction the initial letters of these two adjectives have given their Greek symbols to the two types of radical. The free radicals 6 and 11 are called chain carriers. The termination reactions include the two by two recombination or... 

Both a base-catalyzed 200) and a radical process 201) for redistribution have been reported, but not investigated the latter involves a silyl radical chain carrier. 

PhS)2Sb provides a source of phenylthiyl radicals (PhS ), the radical chain carrier. 

Consistent with a radical chain mechanism, the rate of O2 insertion was found to be sensitive to light, and the addition of radical initiator AIBN was required in order to observe reproducible reaction rates. Based on analysis of the kinetics of O2 insertion into the Pd-C bond of 24, a mechanism involving mononuclear Pd(III) intermediates was proposed (Fig. 16). Palladium(III) intermediate 27, formed by the combination of dimethyl Pd(II) complex 24 with peroxy radical 26 [84], generates the observed Pd(II) peroxide 25 by homolytic Pd-C cleavage to reduce Pd(III) complex 27 and generate radical chain carrier Me. ... 

In this scheme, yH represents the organic compoxmd submitted to pyrolysis, y. stands for the free radical chain carrier which decomposes according to a monomolecular reaction (propagation process 2) and 3. stands for the free radical (or atom) chain carrier which reacts with y H by a bimolecular process, abstracting an H atom and recreating the free radical y. (propagation process 3). Lastly, m and 3H are the major primary products of yH pyrolysis ... 

Termination reaction involving a radical chain carrier and a primary radical. 

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