Radicals homolytic

Early attempts to fathom organic reactions were based on their classification into ionic (heterolytic) or free-radical (homolytic) types.1 These were later subclassified in terms of either electrophilic or nucleophilic reactivity of both ionic and paramagnetic intermediates - but none of these classifications carries with it any quantitative mechanistic information. Alternatively, organic reactions have been described in terms of acids and bases in the restricted Bronsted sense, or more generally in terms of Lewis acids and bases to generate cations and anions. However, organic cations are subject to one-electron reduction (and anions to oxidation) to produce radicals, i.e.,... 

For reactions of radicals (homolytic reactions) arrows are used to indicate motion of single electrons rather than of electron pairs. It is desirable to use arrows with half-heads. For example, reaction of a superoxide radical... 

Free radical (homolytic) reactions involve species with an unpaired electron. The ultraviolet-light-initiated reaction of methane with chlorine is an example (Scheme 4). 

As one might suspect, the different alkyl groups in the two classes of coenzymes lead to both different biochemical roles and different chemistry. In essence, coenzyme B 12-dependent enzymatic processes are radical (homolytic) processes—the chemistry of Co(II) and R —whereas methyl B -dependent... 

A detailed discussion on the properties of Mils is given in Section V.D.l and its calculated structure is shown in Figure 9. In this section we will discuss the role of pentacoordinated MRs in free-radical homolytic substitution reactions. 

The carbon-carbon single bond can be broken down into either two ions (heterolytic cleavage) or into two radicals (homolytic cleavage). A bond in a molecule exists because its formation is energetically favorable. Consequently, the assemblage from either pair of precursors should be a thermodynamically allowed process. The charged species shown in Scheme 2.13 contain a large... 

Dialkyl peroxides (1), R-O-O-R (R and R are = or primary, secondary, tertiary alkyl, cycloalkyl, aralkyl and heterocyclic radicals Homolytic decompn when heated or irradiated with prodn of free radicals for org synthesis difficult to hydrolyze and reduce rearrangement crosslinking and polymerization polymeric peroxides are thick liqs or amorph wh powds used as polymerization catalysts Primary radicals are unstable, lowest members such as dimet peroxide are shock sens and dangerous expls sensitivity lessens with increasing mw polymeric peroxides (copolymers of olefins and Oj) explode on heating... 

This mechanism can be contrasted with that proposed for the photolysis and thermal decomposition of the uranium compounds. In their case, spin trapping and other data support a free radical homolytic scission mechanism [(367-369) see also (437)]. Possibly both mechanisms are operative with uranium compounds. 

The method FIND-ALL-PATHWAYS begins by calling the composite operators that constitute the domain of free-radical operations on the substrates specified by the keyword argument -.substrates. These operators loop over the substrate list and evaluate properties specified by Kfree-radical- Homolytic dissociation of ethane is initiated when FEASIBLE-P, a compound predicate method of (Kmitiation... 

Selectivities to certain products may likewise be adjusted using pressure or temperature. In SCF water, reaction chemistry is governed by fiee-radical (homolytic) mechanisms for an ion product of water, Kw, less than 10 and by ionic (heterolytic) mechanisms for larger values of Ku/(16). This describes the observation that decomposition reactions shift from pyrolysis to hydrolysis as the density of water and thus Kw is increasedQl). In the near-critical region, Kw can be manipulated by small changes in temperature and pressure to control the reaction selectivity. 

Gardini, G. P., Minisci, F. Nucleophilic character of acyl radicals. Homolytic acylation of quinoxaline. J. Chem. Soc., C 1970, 929. 

One of the more unusual examples of hydrocarbon activation was reported by Wayland involving an example of radical homolytic cleavage of the C-H bond of methane (Fig. lb). In this reaction, the methyl group is transferred to one porphyrin metal center and the hydrogen to a second metal center. The reaction follows termolecular kinetics, which suggests a linear transition state for the cleavage (Eq. 21) [88]. In addition to methane, only the benzylic C-H bonds of tolu-... 

In this chapter, we discuss reactions that either add adjacent (vicinal) groups to a carbon-carbon double bond (addition) or remove two adjacent groups to form a new double bond (elimination). The discussion focuses on addition reactions that proceed by electrophilic polar (heterolytic) mechanisms. In subsequent chapters we discuss addition reactions that proceed by radical (homolytic), nucleophilic, and concerted mechanisms. The electrophiles discussed include protic acids, halogens, sulfenyl and selenenyl reagents, epoxidation reagents, and mercuric and related metal cations, as well as diborane and alkylboranes. We emphasize the relationship between the regio-and stereoselectivity of addition reactions and the reaction mechanism. 

Fluorinated selenides were also used as the source of polyfluoroalkyl radicals. Homolytic decomposition of 22 and 23 under UV irradiation in the presence of furan gives 2-polyfluoroalkyl furans. 

Selenopenams have been synthesized via free radical homolytic substitution of aryl or alkyl radicals at the selenium atom. Refluxing the Barton ester 119 R = Me in benzene gave the selenocycle 120 R = Me in low yield. Alternatively, if 119, R = H, is irradiated (250W, tungsten lamp) the selenopenam 120 R = H is produced in moderate yield. 

From these early studies, it was considered that thermal degradation of PET does not involve a radical (homolytic) pathway, at least at the temperatures generally encountered by this polymer. The initial... 

As noted in the discussion on TVA analysis of PET, the authors believe that thermal degradation can be adequately explained by a free-radical (homolytic) breakdown mechanism. 

Alkylation and Acylation by Free-radical Reactions. Heteroaromatic bases, being electron-deficient compounds, readily react with nucleophilic reagents, particularly when protonation enhances their electron-deficient nature. Because of the nucleophilic character of the wide range of available alkyl radicals, homolytic alkylation of heteroaromatics is of interest comparable to that of electrophilic alkylation in the homocyclic series. Homolytic acylation is of no less importance because of the wide applicability of this general reaction in the heterocyclic field. 

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