Methyl radicals hydrogen atom transfer

The last three entries in Table 13 reflect a marked preference for a-methylene versus methyl proton transfer for electron-withdrawing a substituents. These amines are also unusual in that they react with t in nonpolar solvents and do not display exciplex fluorescence. While this unusual behavior was initially attributed to a free radical hydrogen atom transfer mechanism leading to the formation of exceptionally stable "merostabilized" a-aminoallyl radicals (115), our current view is that the high kinetic acidity of the a-C-H bond of these amines when complexed with t is responsible for their behavior. 

In ionic polymerization a hydride (H-) transfer or a proton transfer are the analogues of the hydrogen atom transfer in radical polymerization. A hydride (H-) ion transfer is observed in many isomerizations and dimerizations of hydrocarbons which proceed via carbonium-ion mechanism. A similar process is responsible for chain transfer ip some carbonium-ion polymerizations. The transfer of negative ions like Cl- is also common, e.g. triphenyl methyl chloride is an efficient transfer agent in such a polymerization. Transfer of a proton is, on the other hand, a very common mode of termination of anionic polymerization. Indeed, this mode of termination was discussed previously in connection with branching reactions, and it was postulated in the earliest studies of anionic poly-... 

Table 4-1 compares two different reactions, namely, anode oxidation and oxidation with cerium ammonium nitrate (which are bona fide electron-transfer processes) and bromination by /V-bromosuccinimide in the presence of azobis(iso-butyro)nitrile (which is bona fide hydrogen-atom-transfer process). Both electron-transfer and hydrogen-atom-transfer processes have the benzylic radical as a common intermediate, but positional selectivity is stronger for electron-transfer processes. Another important point is the preference of the 2-positioned methyl group over the 1-positioned group, in terms of selectivity. For 1,2,3-tetramethylbenzene, such a preference reaches values from 16 to 55, and it is over 200 for 5-methoxy-1,2,3-tctramcthylbcnzcnc. 

Finally, some termination step occurs, two of which are shown in the scheme. The most common is coupling, in which two radicals combine, leading to one larger macromolecule. Polystyrene radicals typically undergo termination by coupling. Another reaction that is common with some monomers (e.g., methyl methacrylate) is called disproportionation in which on the reaction of two radicals, a hydrogen atom transfers from one species to the other. 

Alkyl radicals higher than methyl can undergo two competing bi-molecular termination reactions (a) combination to give a saturated dimer (reaction 35) and (b) disproportionation to give an alkene and an alkane by a hydrogen atom transfer from one radical to another (reaction 36). 

According to this picture, the elimination reaction occurs from a distonic radical cation in which the positive charge is localized on the nitrogen atom and is maintained at this site throughout the reaction. Thus, the formal deprotonation step can be described as an hydrogen-atom transfer from the 9-methyl group to an incipient 2,6-lutidine radical cation. 

The kinetics of the photolysis is much more complex at lower temperatures than at around 300 °C. The role of rate-determining step, i.e. the hydrogen atom transfer reaction (20) at high temperatures, is taken over by the decomposition of the acetyl radical as the temperature decreases. At the highest temperatures, the chains are terminated almost exclusively by the recombination of the methyl radicals, while at medium and low temperatures the disproportionation step (26) as well as self combination of the formyl and acetyl radicals are dominant. The first-order wall reaction of the radicals, such as reactions (22) and (31), may also play an important role, especially at low light intensities and pressures. On account of the aforesaid, it seems almost impossible to attempt a general discussion of the kinetics of the reaction. Instead, only selected questions will be dealt with in detail. 

The thermal initiator is benzopinacol (4), which is stable at room temperature (shelf-life of the uncured liquid conformal coating is more than 6 months at room temperature), but which undergoes a homolytic cleavage upon heating to form diphenylhydroxy-methyl radicals (Equation 2), which are believed to initiate polymerization by hydrogen atom transfer to the double bond of the acrylate (Equation 3) (9). 

Progress with evaluations of kinetic data has been much slower to date the following evaluations of transfer reactions have been made (i) gas phase reactions of hydroxyl radicals including some transfer reactions [6, 7], (ii) gas phase reactions of hydroperoxyl radicals [8], (iii) various metathetical reactions of atoms and inorganic radicals involved in high temperature studies [9], (iv) various metathetical reactions of atoms and inorganic radicals involved in atmospheric chemistry [10, 11], Hydrogen atom transfer reactions of methyl radicals in the gas phase have recently been evaluated [12]. 

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