Radical-chain reactions, inhibition steps

Most important, the existence of an induction or inhibition period suggests a free-radical step in the decomposition of the thiophene ring. Further evidence for the free-radical nature of the reaction was obtained from experiments conducted under less severe conditions in order to isolate the initial ring-opened intermediate before subsequent loss of the one-carbon fragment. Efforts to isolate the initial decomposition product were unsuccessful. Apparently, the loss of the one-carbon fragment occurs rapidly, consistent with a free-radical chain reaction of some type. 

The combustion of methane can be used as a model for the combustion inhibition mechanism in the gaseous phase. This is a radical chain reaction proceeding in a series of step reactions as follows [4] ... 

The kinetics of the zinc diisopropyl dithiophosphate-in-hibited oxidation of cumene at 60°C. and Tetralin at 70°C. have been investigated. The results cannot be accounted for solely in terms of chain-breaking inhibition by a simple electrow-transfer mechanism. No complete explanation of the Tetralin kinetics has been found, but the cumene kinetics can be explained in terms of additional reactions involving radical-initiated oxidation of the zinc salt and a chain-transfer step. Proposed mechanisms by which zinc dialkyl dithiophosphates act as peroxide-decomposing antioxidants are discussed. 

We see that the rate of production of products is determined by two quantities, the first a quasi-thermodynamic quantity, the equilibrium concentration of free radicals, and the second a kinetic quantity, namely, the rate at which each radical can go through a chain cycle. When the cycle is made up of two steps of disproportionate speed as in the present case, it is the slower step (in this case Br + H2) which is of importance in determining the over-all rate. It is this feature which explains in this case the specific inhibition by HBr even though the over-all reaction is essentially not reversible. The slow step in a chain will in general (though not always) be endothermic. This implies that its reverse is exothermic and hence of lower activation energy, and so faster. We can thus always expect inhibition by products in chain reactions except in those cases in which the fast steps are of unusual speed. 

Phenoxyl radicals formed in the inhibition step (eqnation 10) are normally terminated by rapid reaction with peroxyl radicals (eqnation 11). However, phenoxyl radicals, particnlarly nnhindered ones, are also able to participate in chain transfer reactions by hydrogen atom abstraction from hydroperoxides which bnild np (eqnation 21), which is the reverse of eqnation 10, or initiate new reaction chains by hydrogen atom abstraction from substrate (RjH) (eqnation 20). 

As noted, the transient nature of most free radical species is a major consideration in ESR studies of free radicals. Free radical chemistry [77] involves an initiation step in which the free radicals are formed, often followed by one or more propagation (chain) reactions before termination. Because most radical-radical termination reactions are fast, the majority of free radicals decay rapidly by self reaction, i.e., they are transient even in the absence of another species. (In non-transient, i.e., persistent, radicals the radical center is sterically hindered, thereby inhibiting self-reaction.) A comment on terminology may be appropriate at this point many transient radicals are frequently described as stable or unreactive, which can lead to some confusion. The source of this confusion is that reactivity and stability are often used to denote... 

The observations that these reactions are inhibited by nitrobenzene (a free radical inhibitor), no hydrogenated by-products are formed and that CF BrCl gives only a-CFjCl carbonyl compounds, led the authors to propose a radical chain mechanism for these reactions (Scheme 1). The chain initiation step is the formation of XFiC radical and enamine radical cation by electron transfer from the enamine to BrCFjX. The addition of this perhaloalkyl radical to the enamine generates a RjNC R R" type radical which is known to have an unusually low oxidation potential with 1/2 in the range of — 1 V (sce). An electron transfer from this radical to another molecule of perhaloalkane then takes place to form the iminium salt and another perhaloalkyl radical which continues the chain. A similar mechanism operates in the case of Rp. ... 

The oxidation induced by ozone is often controlled by a preceding chain reaction that leads to the decomposition of ozone to a more reactive secondary oxidant, OH. This chain reaction, in which radicals act as chain carriers, is promoted by certain types of solutes but inhibited by others. Therefore, the overall oxidation rale often increases with the ratio of the concentration of the promoter relative to I hat of the inhibitor. However, a more generally useful treatment would involve I reating each reaction step separately and relating it to individual and known reaction steps of OH (Staehelin and Hoigne, 1985). 

In such cases, a chain initiation step is required hence, if the reaction can be initiated with light, AIBN, or benzoyl peroxide, a radical chain process is indicated. Since the chain involves the continual presence of a radical after initiation, inhibition of the reaction by a radical trap, such as galvanoxyl or duroquinone, also provides evidence for a radical chain mechanism. Because some radical traps react with organometallics, however, lack of inhibition may be the result of deactivation of the trap. 

A complex sequence of chemical reactions has been postulated for the oxidation of hydrocarbon polymers. Most of the evidence supports a free-radical chain mechanism with typical steps of initiation, propagation, chain branching, termination and inhibition, parallel to the similar mechanisms postulated for volatile hydrocarbons [9-11]. 

Maleic acid and its esters are isomerized by irradiation of their solutions in the presence of various alkyl bromides, sometimes bromine itself. The C-Br (or Br-Br) bond is sonolyzed to bromine atoms, which add reversibly to the double bond.56 Geometric stabilization of the adduct radical and then elimination of bromine produces fumaric compounds (Fig. 13). When the initiator is bromine, oxygen inhibits the reaction (R = H or CH3), an argument in favor of a radical chain process with the initiation step, the cleavage of the Br2 molecule, occurring in the cavitation bubble. The lifetime of the bromine atoms is s, which at the... 

The oxidation of benzaldehyde in the presence of cobaltous acetate has been studied in detail by Marta, Boga and co-workers [226-230]. A radical chain mechanism was involved and inhibition of this reaction both by jS-naphthol and by cobaltous ion at high concentration, have been observed. The initiation step was found to involve the decomposition of a Co CPhCOaH) complex to give Co species which were the reactive intermediates. The rate constant and heat of formation of the Co (PhC03H) complex were determined. Bawn and Jolley [225] have shown that at low Co concentration, the oxidation of benzaldehyde follows rate law, equation (177). 

The relatively stable radicals (A) produced (e.g. phenoxyl from phenols and amin-oxyl from aromatic amines) cannot continue the kinetic chain and disappear from the system by coupling with other or the same free radicals. It should be noted that this process is stoichiometric and hydroperoxides are produced in each inhibiting step (reaction 10). 

This is called the SrnI mechanism," and many other examples are known (see 13-3, 13-4,13-6,13-12). The lUPAC designation is T+Dn+An." Note that the last step of the mechanism produces ArT radical ions, so the process is a chain mechanism (see p. 895)." An electron donor is required to initiate the reaction. In the case above it was solvated electrons from KNH2 in NH3. Evidence was that the addition of potassium metal (a good producer of solvated electrons in ammonia) completely suppressed the cine substitution. Further evidence for the SrnI mechanism was that addition of radical scavengers (which would suppress a free-radical mechanism) led to 8 9 ratios much closer to 1.46 1. Numerous other observations of SrnI mechanisms that were stimulated by solvated electrons and inhibited by radical scavengers have also been recorded." Further evidence for the SrnI mechanism in the case above was that some 1,2,4-trimethylbenzene was found among the products. This could easily be formed by abstraction by Ar- of Ft from the solvent NH3. Besides initiation by solvated electrons," " SrnI reactions have been initiated photochemically," electrochemically," and even thermally." ... 

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