Reactions Between Radicals

The fact that reactions between radicals are in most cases very fast could lead to the conclusion that direct radical combination is the most synthetically useful reaction mode.3 This, however, is not the case because direct radical-radical reactions have several disadvantages  

RSC Green Chemistry No. 6 Radical Reactions in Aqueous Media By V. Tamara Perchyonok V. Tamara Perchyonok 2010 

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The value of ,<)// ,- 80°C) in the cross-reaction between radicals 4 and 8 has been examined.175 This system is a model for cross-termination in MMA-BMA eopolymeri/alion. The value of kjkw (1.22) is similar to that found for the self-reaction of 8(1.17) and much larger than that for the self-reaction of 4 (0.78). There is a small preference (m 1.4 fold) for the transfer of hydrogen from the butyl ester (8) to the methyl ester (4). 

Redox reactions between radicals and metal ions... 

REDOX REACTIONS BETWEEN RADICALS AND METAL IONS... 

Device quality a-Si H made by HWCVD (as they termed it) was first reported by Mahan et al. [19, 527], They obtained n-Si H with hydrogen concentrations as low as 1%. Deposition rates as high as 5 nm/s [528] and 7 nm/s [529] have been achieved for n-Si H of high quality. In order to obtain device quality material it was shown by Doyle et al. [525] that the radicals that are generated at the filament (atomic Si and atomic H) must react in the gas phase to yield a precursor with high surface mobility. Hence, the mean free path of silane molecules should be smaller than the distance between filament and substrate, d(s- Too many reactions between radicals and silane molecules, however, result in worse material. In fact, optimal film properties are found for values of pdf of about 0.06 mbar-cm [530, 531]. 

Mortensen, A. and L. H. Skibsted. 1997b. Real time detection of reactions between radicals of lycopene and tocopherol homologues. Free Rad. Res. 27 229-234. 

Steenken, S. One Electron Redox Reactions between Radicals and Organic Molecules. An Addition/Elimination (Inner-Sphere) Path. 177, 125-146 (1996). 

But even in the case of electron-transfer reactions between radical ions of the same hydrocarbon, the mechanism leading to emission is simple only in the case of direct formation of the respective hydrocarbon... 

Reactions between radicals resulting in radical combination or disproportionation ... 

In the past few years, however, very efficient new methods of cyclisation proceeding via radical intermediates have been developed and several reviews [19a] and a comprehensive book by Giese [19b] have been published. Rather than reactions involving the dimerisation of two radicals -as in the Kolbe electrochemical synthesis [20] or the radical induced dehydrodimerisation developed by Viehe [21]-more important are the reactions between a radical with a non-radical species. The advantage of this type of reaction is that the radical character is not destroyed during the reaction and a chain-reaction may be induced by working with catalytic amounts of a radical initiator. However, in order to be successful two conditions must be met i) The selectivities of the radicals involved in the chain-reaction must differ from each other, and ii) the reaction between radicals and non-radicals must be faster than radical combination reactions. 

One-Electron Redox Reactions between Radicals and Organic Molecules... 

The competition between ET and 5n2 processes in the reaction between radical anions of various aromatic compounds, e.g. anthracene, pyrene, (E)-stilbene, and m- and / -cyanotoluene, and substrates such as RHal (where R = Me, Et, Bu, 2-Bu, neopentyl, and 1-adamantyl) or various methanesulfonates has been studied in DMF as solvent. The reaction mechanism could be characterized electrochemically in many of the systems indicated above. The presence of an 5n2 component is related not only to the steric requirements of the substrate, but also to the magnitude of the driving force for the ET process. 

At some point, the propagating polymer chain stops growing and terminates. Termination with the annihilation of the radical centers occurs by bimolecular reaction between radicals. Two radicals react with each other by combination (coupling) or, more rarely, by... 

The actual species responsible for cationic polymerizations initiated by ionizing radiation is not established. The most frequently described mechanism postulates reaction between radical-cation and monomer to form separate cationic and radical species subsequently, the cationic species propagates rapidly while the radical species propagates very slowly. The proposed mechanism for isobutylene involves transfer of a hydrogen radical from monomer to the radical-cation to form the r-butyl carbocation and an unreactive allyl-type radical ... 

Reactions with Radicai ions of Like Charge. In the final section, we briefly mention reactions between radical ions of like charge. One of the longstanding problems of radical ion chemistry involves the actual stmcture of ketyls. Following extensive conductivity and magnetic susceptibility studies in the 1930s detailed ESR and optical studies have demonstrated the existence and interconversion of at least four distinct species a free ketyl anion a monomer ion pair a... 

As described above, spectroelectrochemical methods are useful in studying the reactivity of radical anions and cations in non-aqueous solutions. Related to this, electrochemiluminescence (ECL), which is often caused by the reaction between radical... 

Products arising from mutual reactions between radicals are unlikely because of the low level of concentrations involved and the rapid alternative reactions with H2 and 02. CH4, CO, HCHO, and C02 are undoubtedly secondary products and are probably formed by the following reactions. 

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