Reactive intermediate generation radicals

Croft, S., Gilbert, B.C., Lindsay Smith, J.R. and Whitwood, A.C. (1992) An E.S.R. investigation of the reactive intermediate generated in the reaction between Fe and H2O2 in aqueous solution. Direct evidence for the formation of the hydroxyl radical. Free Radical Res. Commun. 17 21-39. 

If anything happens to consume some of the free-radical intermediates without generating new ones, the chain reaction will slow or stop. Such a side reaction is called a termination reaction a step that produces fewer reactive intermediates (free radicals) than it consumes. The following are some of the possible termination reactions in the chlorination of methane ... 

Mechanistic reaction kinetics of polymer thermal degradation processes show that the overall mass loss (fuel generation) rate should be first order with respect to the instantaneous mass if the reactive intermediates (free radicals) maintain a steady-state concentration, regardless of whether thermal degradation occurs by random or end-chain scission. A first-order thermal degradation scheme that accoimts for the main pyrolysis products is (4,7,10)... 

An allylic free radical can also react with oxygen to produce a hydroperoxy radical (HOO-), an even more reactive intermediate. Hydroperoxy radicals can be generated by many pathways. For example, they are sometimes by-products of drug metabolism, and they are formed as intermediates in enzyme-catalyzed reactions. 

The chemical pathways leading to acid generation for both direct irradiation and photosensitization (both electron transfer and triplet mechanisms) are complex and at present not fully characterized. Radicals, cations, and radical cations aH have been proposed as reactive intermediates, with the latter two species beHeved to be sources of the photogenerated acid (Fig. 20) (53). In the case of electron-transfer photosensitization, aromatic radical cations (generated from the photosensitizer) are beHeved to be a proton source as weU (54). 

Mechanistic Transform. A transform involving a sequence of reactive intermediates such as carbocations or carbon radicals which are generated in a stepwise mechanistic manner and which lead finally to stable predecessor structure(s). 

A chain reaction is one in which the key intermediate generated in one step then re-enters the reaction sequence in another. A chain mechanism includes at least two such regenerative reactions and a pair of intermediates. This allows the intermediates to cycle repetitively. These intermediates may be atoms or radicals, or indeed any high-energy or reactive species that can undergo suitable reactions. 

Sulfur dioxide (see above) as well as S02, SO , and SOj have been used as building blocks in three-component sulfone syntheses. It has long been known that aromatic sulfinic acids are easily available from diazonium salts and sulfur dioxide under copper catalysis . Mechanistically, aryl radicals as reactive intermediates add to sulfur dioxide generating arenesulfonyl radicals, which either take up an electron (or hydrogen) yielding a sulfinic acid or add to an olefinic double bond yielding final y -halogenated alkyl aryl sulfones (equation 78). 

Nucleophilic Trapping of Radical Cations. To investigate some of the properties of Mh radical cations these intermediates have been generated in two one-electron oxidant systems. The first contains iodine as oxidant and pyridine as nucleophile and solvent (8-10), while the second contains Mn(0Ac) in acetic acid (10,11). Studies with a number of PAH indicate that the formation of pyridinium-PAH or acetoxy-PAH by one-electron oxidation with Mn(0Ac)3 or iodine, respectively, is related to the ionization potential (IP) of the PAH. For PAH with relatively high IP, such as phenanthrene, chrysene, 5-methyl chrysene and dibenz[a,h]anthracene, no reaction occurs with these two oxidant systems. Another important factor influencing the specific reactivity of PAH radical cations with nucleophiles is localization of the positive charge at one or a few carbon atoms in the radical cation. 

Figure 2.6 Anthraquinone derivatives can photoreactively couple to substrates by means of a free radical generation process. The reactive intermediate also can be regenerated back to the initial anthraquinone by proton abstraction and oxidation, resulting in the possibility of again being photolyzed and successfully coupled to the substrate.

Phenoxyl radicals (PhO ), like the thyl radicals discussed above, are widespread reactive intermediates. The corresponding phenols (PhOH) typically have pK i values around 10, so the properties of the PhO /PhO-redox couples become highly relevant above pH 10. Standard potentials for a few of these redox couples have been determined by use of pulse radiolysis to generate the unstable phenoxyl radicals in the presence of appropriate electron donors. These conditions lead to the rapid establishment of electron-transfer equilibria as in,... 

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