Chain carrying radical

In this scheme, CHO appears irrelevant we return to it later. The rate law can be derived by making the steady-state approximation for each of the chain-carrying radical intermediates ... 

Clearly kinetics alone will not distinguish the two schemes. To gain this distinction one can deliberately add a reagent that, judging from its independent chemistry, will react with one of the possible chain-carrying radicals. If the suspected radical is indeed an intermediate, and it reacts with the addend, the overall reaction will be slowed or halted. The added substance is a chain-breaker. In this case Fe2+ and Cu2+ (separately) were added. The first of these would very likely react with either of the peroxyl radicals, ROO or MOO. Indeed, Fe2+ dramatically inhibits the reaction. This evidence confirms the chain nature of the process, but does not distinguish between the mechanisms since both ROO and MOO would be scavenged by Fe2+. 

Esters of AMiydroxypyridine-2-thione are another versatile source of radicals,286 where the radical is formed by decarboxylation of an adduct formed by attack at sulfur by the chain-carrying radical.287 The generalized chain sequence is as follows. 

These reactions presumably involve terminal addition of the chain-carrying radical, followed by fragmentation and recyclization. 

Allenylcobaloximes, e.g. 26, react with bromotrichloromethane, carbon tetrachloride, trichloroacetonitrile, methyl trichloroacetate and bromoform to afford functionalized terminal alkynes in synthetically useful yields (Scheme 11.10). The nature of the products formed in this transformation points to a y-specific attack of polyhaloethyl radicals to the allenyl group, with either a concerted or a stepwise formation of coba-loxime(II) 27 and the substituted alkyne [62, 63]. Cobalt(II) radical 27 abstracts a bromine atom (from BrCCl3) or a chlorine atom (e.g. from C13CCN), which leads to a regeneration of the chain-carrying radical. It is worth mentioning that the reverse reaction, i.e. the addition of alkyl radicals to stannylmethyl-substituted alkynes, has been applied in the synthesis of, e.g., allenyl-substituted thymidine derivatives [64],... 

Evidence supports this contention as well as the suggestion that reaction (8.122) would be more important than reaction (8.121) in controlling the S03 concentration with reaction (8.119). Furthermore, one must recognize that reactions (8.119) and (8.122) are effective means of reducing the O radical concentration. Since reaction (8.116) has been shown to be an effective means of reducing H radical concentrations, one can draw the important general conclusion that S02 and S03 compounds reduce the extent of superequilibrium concentration of the characteristic chain-carrying radicals that exist in hydrocarbon flames. 

The anomeric radical 11 adds to the olefin 12 to give the intermediate 13. Interception of this adduct radical 13 by tin hydride 14 yields the saturated product 16 and the organometallic radical 15. Eventually, the latter reacts with the precursor 17 to produce the chain-carrying radical 11 and the tin compound 18. 

Fig. 5.55. Mechanistic aspects III of nucleophilic aromatic substitution reactions of aryldiazonium salts via radicals introduction of Nu = I through reaction of aryldiazonium salts with KI. In this (chain) reaction the radical I2 —apart from its role as chain-carrying radical—plays the important role of initiating radical. The scheme shows how this radical is regenerated the initial reaction by which it presumably forms remains to be provided, namely (1) Ar-N =N + I -> Ar- N=N + h ...

Fig. 5.45. Mechanism of the nucleophilic aromatic substitution reaction of Figure 5.44. The radical I2 plays the role of the chain-carrying radical and also the important role of the initiating radical in this chain reaction. The scheme shows how this radical is regenerated. It remains to be added how it is presumably formed initially (1) Ar—N+=N + I- ->...

An early version was the Hofmann-Loffler-Freytag reaction (Scheme 1). Irradiation of a chloramine in acid leads to formation of the aminium radical, which can abstract a hydrogen to generate a caibon radical. Then the resulting carbon radical abstracts chlorine from another protonated chloramine, producing a chlorinated carbon and regenerating the chain-carrying radical. On treatment with base, the product... 

The measured reaction orders support the proposed mechanism. Path A is certainly first order in reactant. Paths B and C may be 1/2, first, or 3/2 order, depending on the termination reaction (15). Most likely, termination involves combination or disproportionation of small chain carrying radicals (CH3, C2H5 ). With first-order initiation, this would result in 3/2-order kinetics. The overall reaction order would then be somewhere between first (Mechanism A) and 3/2 (Mechanisms B and C). The measured order of 1.33 for dodecene agrees with this prediction. The fact that propylene and nonene are formed with reaction orders of 1.10 and 1.16 with respect to dodecene (Table III) supports the hypothesis that they are formed largely by a first-order decomposition. 

It is thus probable that every photon which causes the dissociation of an aldehyde molecule into free radicals, ultimately gives rise to two chain-carrying radicals. 

In continuing with the mechanism, consider t-BuO to be the chain-carrying radical. This means that t-BuO will be used up in the first propagation step but regenerated in the last propagation step. In this case, the last propagation step is the second step. 

Is there another possible reaction pathway Could the chlorine radical be the chain-carrying radical If so, equation 3, an exothermic reaction, could represent one of the propagation steps. 

The aryl radical adds to the carbonyl group, a bond cleaves, and the resulting radical abstracts hydrogen from tri- -butyltin hydride to form the product and a new chain-carrying radical. 

Cases in which allyl radicals display sufficient reactivity to participate successfully in radical chain reactions include the addition of bromotrichloromethane to butadiene the reaction of cyclopentadiene with tosyl cyanide, the addition of thiols , stannanes " and hydrogen halides . All these reactions follow the simple two-step radical chain mechanism depicted in Scheme 1, and the low reactivity of the intermediate allyl radicals can be compensated by using the trapping agent in excess or even as the solvent. In chain reactions with three or more chain-carrying radicals, this compensation is not possible anymore, because the concentration of the reaction partners has to be chosen such that the selectivity requirements for all intermediate radicals are satisfied. Complex radical chain reactions with polyenes as one of the reactants are therefore not known. 

Use of other halogenating agents, such as A -bromosuccinimide, under conditions where the chain-carrying radical was succinimidyl rather than bromine, resulted in a majority of attack in the side chain. However, the final abstraction of a bromine atom from 7V-bromosuccinimide was so rapid that j8-scission could not compete and unrearranged products accumulated. Similarly, in the reaction of A -bromo-3,3-dimethylglutarimide, or of 7V-bromophthaIimide (10), with methylcyclopropane (11), only unrearranged (bromomethyl)cyclopropane (12) could be isolated. ... 

An interesting result was the way in which the viscosity, r/, had another effect, namely to control the diffusion of the free radicals out of the initial cage in which they were formed. Geminate recombination of radicals is therefore important and the formation of chain-carrying radicals was dependent on Also of interest is the observation that the decomposition... 

An interesting stabilizer system that may involve a redox couple is the use of copper iodide in the stabilization of polyamides. This is often the melt stabilizer of choice, and the reaction (Scheme 1.68) to remove the main chain-carrying radical is proposed, followed by a redox reaction with peroxy radicals to regenerate the cuprous ion. 

Polymer stabilizers are very reactive systems. This is expressed not only in scavenging chain-carrying radicals or deactivation of ROOH but also in interactions between various stabilizers or their transformation products [250], This chemical cooperation may have a positive effect accounting for additivity or even synergism. Unfortunatelly, some interactions are negative, i.e. antagonistic, and should be avoided. 

This example represents the earliest report of a radical fragmentation reaction, a reaction that is now commonly used in organic synthesis for the allylation or vinylation of carbon-centered radicals [2], These radical chain processes proceed by the addition of a radical to a suitably substituted allyl or vinyl derivative (Scheme 2). The reactions rely on the facile, unimolecular )9-scission of the relatively weak C-Z bond in radical intermediate A or B to form radical Z and the allylated or vinylated product. Radical Z may be the chain-carrying radical or it may undergo further reaction to generate a chain-carrying radical. 

Suess [64] and Schulz [65] observed that the presence of solvents during the polymerization of styrene lowered the molecular weight In the case of CCLt, Mayo suggested that the chain-carrying radical abstracted Cl from CCh (1.20), terminating the chain but leaving CCls, which added to monomer and initiated a new chain (1.21). 

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