Other radical abstractions

As will be seen in subsequent sections, the HO2 radical is an important combustion intermediate, however, due to its low reactivity, it plays little part in propagating the combustion process via abstractions from fuel molecules. As discussed in Chapter 1, abstraction by HO2 from intermediates such as aldehydes, generates H2O2 and provides a route to branching in the intermediate temperature region. The reaction with formaldehyde has been studied directly as a function of temperature by Jemi-Alade etal. [19]. HO2 radicals were generated by the photolysis of H2CO/O2 mixtures 

An unweighted Arrhenius plot for 22, incorporating the work of Jemi-Alade et al. [19] and other low temperature measurements yields the following expression 

High-temperature shock tube studies [20] show significant curvature in the 

Arrhenius plot and the resulting modified expression as shown in Fig. 2.11 is 

CH3 abstraction reactions can play a significant role in combustion chemistry, for although there is no net change in either hydrocarbon or alkyl 

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In H abstraction, a hydrogen radical reacts with a molecule (primarily a paraffin) and produces a hydrogen molecule and a radical. In the same way, a methyl radical reacts to produce a radical and methane. Similar reactions with other radicals (ethyl and propyl) can also occur. In addition, some radicals like H, CH, etc, are added to olefins to form heavier radicals. 

Nevertheless, many free-radical processes respond to introduction of polar substituents, just as do heterolytic processes that involve polar or ionic intermediates. The substituent effects on toluene bromination, for example, are correlated by the Hammett equation, which gives a p value of — 1.4, indicating that the benzene ring acts as an electron donor in the transition state. Other radicals, for example the t-butyl radical, show a positive p for hydrogen abstraction reactions involving toluene. ... 

The hydrogen abstraction addition ratio is generally greater in reactions of heteroatom-centered radicals than it is with carbon-centered radicals. One factor is the relative strengths of the bonds being formed and broken in the two reactions (Table 1.6). The difference in exothermicity (A) between abstraction and addition reactions is much greater for heteroatom-centered radicals than it is for carbon-centered radicals. For example, for an alkoxy as opposed to an alkyl radical, abstraction is favored over addition by ca 30 kJ mol"1. The extent to which this is reflected in the rates of addition and abstraction will, however, depend on the particular substrate and the other influences discussed above. 

Correlated or geminate radical pairs are produced in unimolecular decomposition processes (e.g. peroxide decomposition) or bimolecular reactions of reactive precursors (e.g., carbene abstraction reactions). Radical pairs formed by the random encounter of freely diffusing radicals are referred to as uncorrelated or encounter (P) pairs. Once formed, the radical pairs can either collapse, to give combination or disproportionation products, or diffuse apart into free radicals (doublet states). The free radicals escaping may then either form new radical pairs with other radicals or react with some diamagnetic scavenger... 

That this mechanism can take place under suitable conditions has been demonstrated by isotopic labeling and by other means. However, the formation of disproportionation and dimerization products does not always mean that the free-radical abstraction process takes place. In some cases these products arise in a different manner.We have seen that the product of the reaction between a carbene and a molecule may have excess energy (p. 247). Therefore it is possible for the substrate and the carbene to react by mechanism 1 (the direct-insertion process) and for the excess energy to cause the compound thus formed to cleave to free radicals. When this pathway is in operation, the free radicals are formed after the actual insertion reaction. 

Alkenes. When the substrate molecule contains a double bond, treatment with chlorine or bromine usually leads to addition rather than substitution. However, for other radicals (and even for chlorine or bromine atoms when they do abstract a hydrogen) the position of attack is perfectly clear. Vinylic hydrogens are practically never abstracted, and allylic hydrogens are greatly preferred to other positions of the moleeule. Allylic hydrogen abstraction from a cyclic alkenes is usually faster than abstraction from an acyclic alkene. ... 

The generation of radicals from lipids appear to be dependent on the abstraction of hydrogen by other radicals. Consistent with this idea is the observation that either lipid peroxidation or anoxia can cause a release of free arachidonic acid fix>m culture cells, and this release can be blocked by antioxidants (Braughler et al., 1985, 1988). 

Chemical combustion is initiated by the oxidation or thermal decomposition of a fuel molecule, thereby producing reactive radical species by a chain-initiating mechanism. Radical initiation for a particular fuel/oxygen mixture can result from high-energy collisions with other molecules (M) in the system or from hydrogen-atom abstraction by 02or other radicals, as expressed in reactions 6.1-6.3 ... 

In order to document the radical disproportionation reaction, we have used FT-IR spectroscopy to characterize the irradiation products. Upon irradiation of 1 in pentane, the formation of the characteristic peak near 2100 cm-1 due to Si-H stretching vibrations was readily apparent. The IR spectrum obtained in perdeuterated pentane was identical, suggesting that radical processes other than abstraction from the solvent are involved. Furthermore the ESR spectrum obtained in this solvent is identical to that already described. This raises the question whether the initially formed silyl radicals really abstract hydrogen from carbon with the formation of carbon-based radicals as suggested (13), particularly in light of the endothermicity of such a process. 

Asa radical, oxygen can abstract hydrogen atoms just like other radicals. 

Radicals are also formed in solution by the decomposition of other radicals, which are not always carbon free radicals, and by removal of hydrogen atoms from solvent molecules. Because radicals are usually uncharged, the rates and equilibria of radical reactions are usually less affected by changes in solvent than are those of polar reactions. If new radicals are being made from the solvent by hydrogen abstraction, and if the new radicals participate in chain reactions, this may not be true of course. But even in cases of non-chain radical reactions in which no radicals actually derived from the solvent take part in a rate-determining step, the indifference of the solvent has perhaps been overemphasized. This will be discussed more fully when radical and polar reactions are compared in Chapter XII. 

Electron transfer. The conversion of silylhydrides to silyl radicals is through hydrogen abstraction by various other radicals. 

Radicals formed in one of these initiation reactions may themselves be the means of producing other radicals, by reacting with another molecular species. Abstraction of a hydrogen atom is a particularly common reaction leading to a new radical. 

Difluoroamino radical acetylene addition reactions, 33 185-189 addition reactions, 33 183-189 hydrogen abstraction reactions, 33 182-183 reactions with olefins, 33 183-185 reactions with other radicals, 33 181... 

Abstraction/recombination reactions are not limited to radical- radical recombination. An intriguing alternative has been demonstrated19, zo, as illustrated by the intramolecular addition of the free radical 2, generated by remote abstraction, to an alkene. As previously observed for other radical cyclizations, there is a significant selectivity for the thermodynamically less preferred entfo-product21 from this cyclization. 

This technique was quickly adopted by others and it was soon found by F.O. Rice and co-workers that the pyrolysis of many organic compounds at 800 to 1000°C removed metallic mirrors, implicating the formation of free radicals. The cleavage of larger free radicals into smaller radicals and olefins under these conditions, was also proposed (equation 22), as well as chain reactions in which radicals abstract hydrogen from alkanes. Reactions of alkyl halides with metal atoms in the gas phase were also found by M. Polanyi and co-workers to yield alkyl radicals (equation 23). 

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