Radical combination

Closs G L 1969 A mechanism explaining nuclear spin polarizations In radical combination reactions J. Am. Chem. Soc. 91 4552-4... 

Table 6.5 Some Free Radical Combination Reactions Which Yield n-mers and Their Rate Laws...

The latter undergo radical combination to form the dianion, which subse quently polymerizes ... 

Table 7.2 Values of the Cross-Propagation Constants k 12 for Four Monomer-Radical Combinations...

Bulk Polymerization. The bulk polymerization of acryUc monomers is characterized by a rapid acceleration in the rate and the formation of a cross-linked insoluble network polymer at low conversion (90,91). Such network polymers are thought to form by a chain-transfer mechanism involving abstraction of the hydrogen alpha to the ester carbonyl in a polymer chain followed by growth of a branch radical. Ultimately, two of these branch radicals combine (91). Commercially, the bulk polymerization of acryUc monomers is of limited importance. 

Tertiary peroxyl radicals also produce chemiluminescence although with lower efficiencies. For example, the intensity from cumene autooxidation, where the peroxyl radical is tertiary, is a factor of 10 less than that from ethylbenzene (132). The chemiluminescent mechanism for cumene may be the same as for secondary hydrocarbons because methylperoxy radical combination is involved in the termination step. The primary methylperoxyl radical terminates according to the chemiluminescent reaction just shown for (36), ie, R = H. 

Joining two heteroatoms to a ring by radical combination is not presently a common route to heterocycles. It might become more important if the art of metal-catalyzed redox reactions keeps advancing at the present pace. Current examples are the conversion of 1,5-dithiols to 1,2-dithiepanes by oxidants such as FeCla, and the oxidation of 1,3-propane-bis-hydrazines to 1,2,3,4-tetrazepines (Sections 5.18.4.1 and 5.18.10.1). 

Making just one C—C bond by radical combination is not a prominent method for preparing heterocyclics, except for those cases in which the radical pair arises from a cleavage or extrusion reaction of a heterocyclic starting material. Such cases are treated below (Section 5.03.5.2.2). 

An alternative reaction mechanism has been suggested for nitroarylation of enolates. An impetus for considering other mechanisms is the fact that the by-products which might be expected from aryl radicals, such as reduction products from hydrogen abstraction from the solvent or biaryls from coupling, are not observed. One alternative is that, rather than being a chain process, the reaction may involve recombination whereby the radicals combine more rapidly than they separate. 

Oxidation causes rubbers and resins to harden, and also produces colour darkening in resins. The radicals formed in the above chain reaction are highly reactive and very unstable, so they will be rapidly converted to more stable products. For most rubbers and resins, these radicals combine to form cross-links which cause hardening (Fig. 33). 

Once radicals are generated in the presence of air the reaction becomes autocatalytic (see Fig. 15). Ignoring the relatively infrequent radical combination processes (Fig. 15, Eqs. 8-10) the net reaction (Fig. 15, Eqs. 1-7) shows that for every new radical generated three more are created. Oxidation produces water, and alcohols. 

Step through the sequence of structures corresponding to combination of two methyl radicals to give ethane (methyl radical combination). 

Both of the reactions, radical combination and Diels-Alder cycloaddition, cause new bonds to be made. Bond making normally releases energy. Why then are the barriers for the two reactions so different (Hint Consider the nel bond making/bond breaking in the two reactions.)... 

However, when MAIs are thermolyzed in solution, the role of the cage effect has to be taken into account. The thermolytically formed macroradicals can, due to their size, diffuse only slowly apart from each other. Therefore, the number of combination events will be much higher for MAIs than for low-molecular weight AIBN derivatives. As was shown by Smith [16], the tendency toward radical combination depends significantly on the rigidity and the bulkiness of the chain. Species such as cyclohexyl or diphenylmethyl incorporated into the MAI s main chain lead to the almost quantitative combination of the radicals formed upon thermolysis. In addition, combination chain transfer reactions may... 

B Termination The chain process is eventually ended by a reaction that consumes the radical. Combination of two growing chains is one possible chainterminating reaction. 

In dibenzothiophene-S,S-dioxide the S atom is in a ring, and hence more constrained. The yield of SOz in the radiolysis is linear with the dose to about 13 Mrad after which it levels off as in p,p -ditolyI sulfone. However, the yield of S02 in this case is much lower (a factor of 25) than in the case of p,p -ditolyl sulfone (G = 0.002 compared to G = 0.05). This stability of the dibenzothiophene sulfone could be partially due to back reaction to reform the parent sulfone and partially due to more efficient energy delocalization. The expected biphenylene product was not detected due to limitations of the analytical method. Bowmer and O Donnell70 studied the volatile products in y-radiolysis of dialkyl, alkyl aryl and diaryl sulfones. Table 2 gives the radiolytic yields of S02 and of the hydrocarbon products of the alkyl or aryl radicals. The hydrocarbon products are those obtained either by H atom abstraction or by radical combination. The authors69 suggested the mechanism... 

Scheme 2 summarizes the mechanism for the formation of thiosulfonate from sulfinyl radicals it is shown that the sulfinyl radicals combine to give both yic-disulfoxides and O, S-sulfenyl sulfinates, although they may rearrange to thiosulfonates either via a free radical route or via a concerted mechanism. The reader is referred to the recent review of Freeman22, who has collected and discussed the vast amount of information published on yic-disulfoxides and O, S-sulfenyl sulfinates. 

Chain reactions begin with the initiation of a reactive intermediate that propagates the chain and concludes with termination when radicals combine. Branching chain reactions can be explosively fast. 

A termination step is reached when two radicals combine to form a nonradical product, as in... 

Since the benzene emission in the thermal decomposition of benzoyl peroxide results from radical transfer by the phenyl component of a benzoyloxy-phenyl radical pair, phenyl benzoate produced by radical combination within the same pair should appear in absorption. A weak transient absorption has been tentatively ascribed to the ester (Lehnig and Fischer, 1970) but the complexity of the spectrum and short relaxation time (Fischer, personal communication) makes unambiguous assignment difficult. Using 4-chlorobenzoyl peroxide in hexachloro-acetone as solvent, however, the simpler spectrum of 4-chlorophenyl 4-chlorobenzoate is clearly seen as enhanced absorption, together with... 

Cross coupling of two different carboxylates (= mixed Kolbe electrolysis) is a method for synthesizing unsymmetrical compounds (Eq. 8). As, however, the intermediate radicals combine statistically, the mixed coupling product... 

The mechanism shown in Scheme 6 is, for the most part, consistent with points (1) to (9). Thus, initially formed is a o--complex that is stable only at low temperatures. Upon matrix warm-up, a caged, radical pair forms and, if the R portion possesses a sufficient excess of vibrational energy, decomposition processes may occur. The radicals combine to form RPdX, which may, or may not, be isolated. 

R8 is the simplest of a large suite of peroxyl radical combination reactions, generalized as R02 + H02 and R02 + R02 that generate poorly characterized radical and non-radical reaction products. Such reactions are of greatest significance in air with low nitric oxide concentration where the R02 species can reach elevated concentrations (95). The dependence of [H02 ] upon the tropospheric NO concentration is discussed below. 

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