Radicals and homolytic

Radicals, see also Substitution, free-radical and homolytic... 

We shall postpone further discussions of reactions involving radicals and homolytic bond cleavage until we reach Chapter 10. At this point we focus our attention on reactions involving ions and heterolytic bond cleavage. 

Initiation. Free-radical initiators are produced by several processes. The high temperatures and shearing stresses required for compounding, extmsion, and molding of polymeric materials can produce alkyl radicals by homolytic chain cleavage. Oxidatively sensitive substrates can react directly with oxygen, particularly at elevated temperatures, to yield radicals. 

An interesting method for the substitution of a hydrogen atom in rr-electron deficient heterocycles was reported some years ago, in the possibility of homolytic aromatic displacement (74AHC(16)123). The nucleophilic character of radicals and the important role of polar factors in this type of substitution are the essentials for a successful reaction with six-membered nitrogen heterocycles in general. No paper has yet been published describing homolytic substitution reactions of pteridines with nucleophilic radicals such as alkyl, carbamoyl, a-oxyalkyl and a-A-alkyl radicals or with amino radical cations. 

A free-radical reaction is a chemical process which involves molecules having unpaired electrons. The radical species could be a starting compound or a product, but the most common cases are reactions that involve radicals as intermediates. Most of the reactions discussed to this point have been heterolytic processes involving polar intermediates and/or transition states in which all electrons remained paired throughout the course of the reaction. In radical reactions, homolytic bond cleavages occur. The generalized reactions shown below illustrate the formation of alkyl, vinyl, and aryl free radicals by hypothetical homolytic processes. 

The broadest classification of reactions is into the categories of heterolytic and homolytic reactions. In homolytic (free radical) reactions, bond cleavage occurs with one electron remaining with each atom, as in... 

The low bond-strength of the O—0 bond renders peroxides susceptible to homolytic fission to give oxy radicals on heating. Diacyl peroxides give rise to acyloxy radicals which then decompose to aryl radicals and carbon dioxide, Eq. (5). For example, dibenzoyl... 

The reaction is likely to proceed by a radical-chain mechanism, involving intermediate formation of carboxyl radicals, as in the related Kolbe electrolytic synthesis. Initially the bromine reacts with the silver carboxylate 1 to give an acyl hypobromite species 3 together with insoluble silver bromide, which precipitates from the reaction mixture. The unstable acyl hypobromite decomposes by homolytic cleavage of the O-Br bond, to give a bromo radical and the carboxyl radical 4. The latter decomposes further to carbon dioxide and the alkyl radical 5, which subsequently reacts with hypobromite 3 to yield the alkyl bromide 2 and the new carboxyl radical 4Z... 

Szele and Zollinger (1978 b) have found that homolytic dediazoniation is favored by an increase in the nucleophilicity of the solvent and by an increase in the elec-trophilicity of the P-nitrogen atom of the arenediazonium ion. In Table 8-2 are listed the products of dediazoniation in various solvents that have been investigated in detail. Products obtained from heterolytic and homolytic intermediates are denoted by C (cationic) and R (radical) respectively for three typical substituted benzenediazonium salts and the unsubstituted salt. A borderline case is dediazoniation in DMSO, where the 4-nitrobenzenediazonium ion follows a homolytic mechanism, but the benzenediazonium ion decomposes heterolytically, as shown by product analyses by Kuokkanen (1989) the homolytic process has an activation volume AF = + (6.4 0.4) xlO-3 m-1, whereas for the heterolytic reaction AF = +(10.4 0.4) x 10 3 m-1. Both values are similar to the corresponding activation volumes found earlier in methanol (Kuokkanen, 1984) and in water (Ishida et al., 1970). 

In conclusion, it is very likely that the influence of solvents on the change from the heterolytic mechanism of dissociation of the C —N bond in aromatic diazonium ions to homolytic dissociation can be accounted for by a mechanism in which a solvent molecule acts as a nucleophile or an electron donor to the P-nitrogen atom. This process is followed by a one- or a two-step homolytic dissociation to an aryl radical, a solvent radical, and a nitrogen molecule. In this way the unfavorable formation of a dinitrogen radical cation 8.3 as mentioned in Section 8.2, is eliminated. 

However, these mechanistic investigations show only that the reagent in the arylation proper is an aryl radical. They say nothing about the formation of this aryl radical and the homolytic substitution of an aromatic hydrogen. Experimental research on this problem started with work of Huisgen (1951). We discussed part of... 

Peroxide decomposition in aromatic and other unsaturated solvents homolytic aroniMic substitution and olefin polymerization Decomposition of peroxides in aromatic solvents leads to attack on the aromatic nucleus by radicals and hence to substitution products (for a recent summary, see Williams, 1970). In the substitution of benzene and related substrates by phenyl radicals, for example, cyclohexadienyl... 

The primary step in the photolysis of methylcobalamin is homolytic fission to give the Co(II) cobalamin and methyl radicals. Recombination can occur, i.e., the reaction is reversed, unless the radicals and/or Co(II) are removed by further reactions ... 

In broad terms, the following types of reactions are mediated by the homolytic fission products of water (formally, hydrogen, and hydroxyl radicals), and by molecular oxygen including its excited states—hydrolysis, elimination, oxidation, reduction, and cyclization. 

Only a limited number of peroxides are suitable for crosslinking purposes. The most suitable are those which form radicals through homolytic decomposition. These radicals are so aggressive that they can split hydrogen atoms from the polymer chain, producing stable peroxide decomposition products and polymer radicals. These polymer radicals can then be recombined, producing stable C-C crosslink bonds. 

On the other hand, in the polar acetonitrile diffusive separation leads to the free cation radical and NO which then undergo (homolytic) collapse. The sub-... 

M H (M = Si, Ge, Sn) bonds are reactive to undergo oxidative addition of transition metals, homolytic cleavage giving group 14 metal radicals, and so on. Some studies have been reported on the formation of M E (M = Si, Ge, Sn E = S, Se, Te) bonds by using the group 14 metal hydrides. 

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