Nonchain mechanisms

The border conditions between chain and nonchain mechanisms of oxidation depends not only on temperature but also on the hydrocarbon concentration and the rate of chain initiation. The following equation describes this dependence ... 

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... 

Transition complex, nonchain mechanism. The spontaneous decomposition of azomethane... 

To find out whether the given primary or secondary product, or the sum of these, is formed by a single step, or by a chain of conversions, the limiting primary product concentration should be compared with the initial oxygen atom concentration determined from CO. If these are equal, this is an indication that the primary product is formed by a nonchain mechanism. 

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... 

Type of process. Detection of CH, CH3COCH and CH3CO demonstrates a free radical process, while the quantum yield of the product CO indicates a nonchain mechanism (see Section 6.8). 

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. 

Fig. 13. One-electron nonchain mechanisms proposed by TAMREAC for the allyl—alkyl cross-coupling.

Transfer of an Ar-alkyl group to certain nucleophiles can also occur by a radicaloid nonchain mechanism, e.g., 985 (R=CH2Ph, R=Ph) + CMe2N02 986 987 + 988. 

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. 

Another mechanistic possibility involving radical intermediates is the nonchain mechanism as outlined in equations 7.46.9S... 

In the Barton reaction, an alkyl nitrite is converted into an alcohol-oxime. The nitrite is photoexcited to a 1,2-diradical. It fiagments to NO and an alkoxy radical (RO-), and the latter abstracts H- from the nearest C—bond. The resulting alkyl radical then combines with NO to give a nitroso compound, which then tautomerizes to the oxime, probably by a polar stepwise mechanism. 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 Barton reaction has been used for the remote functionalization of hydrocarbons, especially steroids. 

The SrnI mechanism for substitution (Chapter 2) is initiated by electron transfer to the substrate undergoing substitution. A nonchain mechanism for substitution that is initiated by electron transfer can also be envisioned. Electron transfer from the nucleophile to the electrophile occurs to give a radical and a radical anion. The leaving group departs the radical anion to give a new radical. The two radicals then combine to give the product. The overall result is the same as if a simple Sn2 reaction had occurred. 

Tessier and Forst followed the decomposition of hydrogen peroxide mass spectrometrically, and observed that the steady-state concentration of H02 was below the detectable limit, from which it was concluded that the simplest nonchain mechanism, namely reactions (13), (31), and (14), is entirely satisfactory. 

The oxidative addition of f-PrI to [PtMe2(phen)] appears to proceed by both the radical-chain and the nonchain mechanisms. In addition to the expected [PtMe2(phen)(I)(/-Pr)], [PtMe2(phen)(I)2] and [PtMe2(phen)(I)-(OO-i-Pr)] are also formed in air. " The yield of the last depends on the... 

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