Reaction mechanism nonchain

The mechanism of the reaction depicted in Scheme 4.6 differs from the Sf.,1 or Sf.,2 mechanism in that it involves the stage of one-electron oxidation-reduction. The impetus of this stage may be the easy detachment of the bromine anion followed by the formation of fluorenyl radical. The latter is unsaturated at position 9 near three benzene rings that stabilize the radical center. The radical formed is intercepted by the phenylthiolate ion. This leads to the anion-radical of the substitution product. Further electron exchange produces the substrate anion-radical and final product in its neutral state. The reaction consists of radical (R)-nucleophilic (N) monomolecular (1) substitution (S), with the combined symbol Sj j l. Reactions of Sj j l type can have both branch-chain and nonchain characters. 

The yields of reaction products from thermal nucleophilic substitution reactions in DMSO of 0- and p-nitrohalobenzenes (Zhang et al. 1993) or p-dinitrobenzene (Liu et al. 2002) with the sodium salt of ethyl a-cyanoacetate were found to be markedly diminished from the addition of small amounts of strong electron acceptors such as nitrobenzenes. At the same time, little or no diminution effects on the yields of the reaction products were observed from the addition of radical traps such as nitroxyls. These results are consistent with the conclusion that such reactions proceed via a nonchain radical nucleophilic substitution mechanism (Scheme 4.26). 

In Scheme 4.26, the nonchain mechanism explains the failure of the radical traps to affect the yield of the reaction. According to this mechanism, the reactive radical intermediates are held within the solvent cage. Evidently, the radical traps cannot penetrate the solvent cage like an electron can. In other words, the radical traps present in the solution, even in great excess, cannot intercept the radicals in the framework of the nonchain radical mechanism. Galli (1988a) provided other similar examples of this phenomenon. 

Molecular intermediates, nonchain mechanism. The general class of enzyme-catalyzed fermentation reactions... 

Mechanism III. Amines may interact with important molecule intermediates formed during the oxidation of the fuel—e.g., peroxides. If this occurred by a nonchain process, degenerate chain branching would be stopped, and there would be effective inhibition, provided that the initiation reaction between the fuel and oxygen was slow. 

It may be seen from Table III that in reactions of oxygen atoms with ethylene, formaldehyde and CO are obtained practically in equal amounts, as in the reaction involving ethane. It will be taken into account that the amount of formaldehyde obtained is practically equal to that of oxygen atoms (Table II). This is indication that CH20 is formed by a nonchain mechanism. On the basis of these facts it should be considered that the main reaction between oxygen atoms and ethylene is... 

A nonchain Sr j 1-like mechanism is suggested in which the site of methylation is dictated by radical anion charge density. Whether radical-radical (nonchain) or radical-radical anion (chain) coupling dominates the reaction depends on the oxidizability of the anion (254-256). 

Rhodium(I) or polymer supported rhodium(I) compounds catalyzed the formation ofFefCO - CNR L (x = 1 - 3 R = Bu, xylyl L = MA, citra-conic anhydride, acrylamide) (29, 30), and the dimer [CpFe(CO)2]2 catalyzed the stepwise substitution of carbonyl groups in CpFeI(CO)2 to give CpFeKCO - CNR) (x = 1,2 R = Bu, xylyl) in 60-80% yields. A nonchain free-radical mechanism was proposed for the latter reaction (28). The compounds CpFeX(CO)2 x(CNR)x (x = 1,2 X = halide, SiMe3) are known for a range of alkyl and aryl isocyanides (169-171). 

Note, the argument involving the ratio of the photochemical rate to the thermal rate as a route to the determination of k only holds because the mechanisms for both reactions are the same. This method would not work with the photolysis and thermal decomposition of propanone, see Worked Problem 6.2 and Further Problem 4. These two reactions have totally different mechanisms the photolytic reaction has a nonchain mechanism whereas the thermal decomposition, which occurs at a much higher temperature, has a chain mechanism. 

Various mechanisms for the insertion reaction are conceivable (a) ionic stepwise, (b) radical (chain or nonchain), and (c) concerted. Generally, ionic or radical mechanisms give a mixture of products with cis and trans stereochemistry. In some special cases of the ionic reaction, however, exclusive formation of a trans product has been observed (62, 66). Therefore, stereoselectivity does not necessarily imply a concerted mechanism other evidence, e.g., regioselectivity, kinetic data, solvent effects, and substituent effects, must be sought out. 

Nonchain mechanisms, though, often involve radical-radical combinations and disproportionations. Nitroxyls can be used to trap alkyl radicals that may be present in a reaction medium by a radical-radical combination reaction, giving an electron-sufficient species that is more easily studied than the free radical. The photochemical reactions of carbonyl compounds often involve radical-radical combination and disproportionation steps. 

A free-radical reaction may proceed by a chain or a nonchain mechanism. There are many experimental methods for determining whether a chain or a nonchain mechanism is operative in a reaction, but these methods don t help much in pen-cil-and-paper problems such as the ones in this book. Luckily, the reagents or reaction conditions will usually indicate which type of mechanism is operative. 

You may have noticed that hv may indicate either a nonchain or a chain mechanism. A good rule of thumb for distinguishing light-initiated nonchain and chain mechanisms is that unimolecular rearrangements or eliminations usually proceed by nonchain mechanisms, whereas addition and substitution reactions (and especially intermolecular ones) almost always proceed by chain mechanisms. However, there are a few exceptions to this rule (photochemical pinacol reaction, Barton reaction). Of course, many pericyclic reactions require light, too (Chapter 4) ... 

One could write a perfectly reasonable chain mechanism for this reaction, but a nonchain mechanism is appropriate because NO is a stable free radical, and it hangs around until it is able to combine with the alkyl radical to form a strong C-N bond. 

The present second edition of this book corrects two major errors (the mechanisms of substitution of arenediazonium ions and why Wittig reactions proceed) and some minor ones in the first edition. Free-radical reactions in Chapter 5 are reorganized into chain and nonchain processes. The separate treatment of transition-metal-mediated and -catalyzed reactions in Chapter 6 is eliminated, and more in-text problems are added. Some material has been added to various chapters. Finally, the use of italics, especially in Common Error Alerts, has been curtailed. 

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