Addition reactions of radicals

Absolute rates have been measured for some carbene reactions. The rate of addition of phenylchlorocarbene shows a small dependence on alkene substituents, but as expected for a very reactive species, the range of reactivity is quite narrow.119 The rates are comparable to moderately fast bimolecular addition reactions of radicals (see Part A, Table 11.3). 

Addition Reactions of Radicals with Substituted Alkenes... 

Chapter 10 considers the role of reactive intermediates—carbocations, carbenes, and radicals—in synthesis. The carbocation reactions covered include the carbonyl-ene reaction, polyolefin cyclization, and carbocation rearrangements. In the carbene section, addition (cyclopropanation) and insertion reactions are emphasized. Recent development of catalysts that provide both selectivity and enantioselectivity are discussed, and both intermolecular and intramolecular (cyclization) addition reactions of radicals are dealt with. The use of atom transfer steps and tandem sequences in synthesis is also illustrated. 

Scheme 4 Election-transfer and addition reactions of radicals and radical ions.

Scheme 10.10 illustrates the addition reaction of radicals with alkenes using a variety of methods for radical generation. 

A study of die addition reactions of radicals to fiillerenes (C60/C70) by EPR has appeared and die dynamic effects in the EPR spectra of fidlerenyl radicals due to hindered rotation and the multi-addition of radicals to fidlerenes are described.9 Other review articles which have appeared this year include recent advances in the radical substitution reactions of alkyl, aryl, and vinyl halides10 and the substitution and photochemical reactions of heterocyclic A -oxidcs.11 The mechanisms for the oxidation of hydrocarbons, lipids, and low-density lipoproteins have been reviewed.12... 

Anionic polymerizations are generally much faster than free-radical reactions although the A p values are of the same order of magnitude for addition reactions of radicals and solvated anionic ion pairs (free macroanions react much faster). The concentration of radicals in free-radical polymerizations is usually about 10 -10 M while that of propagating ion pairs is 10 -10 M. As a result, anionic polymerizations are lO -lO times as fast as free-radical reactions at the same temperature. 

Problem 8.3 Account for the fact that anionic polymerizations are generally much faster than free-radical reactions although the kp values are of the same order of magnitude for addition reactions of radicals and solvated ion pairs (free macroanions react much faster). 

Unsaturated bonds in polyethylene are not the product of the photofragmentation of macroradicals alone but also of their disproportionation. The disappearance of C=C bonds corresponds to addition reactions of radicals with multiple carbon-carbon bonds. Addition reactions increase the quantum yield of crosslinks when related to that of formation of free radicals. 

For irradiation at a higher dose rate, the radical-radical combination reactions (R6) wiU efficiently occur and compete with the addition reactions of radicals and solute molecules to initiate the polymerization (R2), while the addition reactions (R2) effectively occur during irradiation at a lower dose rate, because of the reduction of radical loss (Nakagawa 2010). This will lead to an increased polymer yield with a decreasing dose rate. As solvent radicals work not only as an initiator (R2) but also the terminator (R4) of polymerization, the probability for polymer radicals to terminate with solvent radicals (R4) will be less by irradiation at a lower dose rate. This will make it easy for polymer radicals (R5) to produce a polymer with a higher molecular weight. 

Sources of Radical Intermediates Introduction of Functionality by Radical Reactions Addition Reactions of Radicals with Substituted Alkenes Cyclization of Free-Radical Intermediates Fragmentation and Rearrangement Reactions... 

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