Radical ions homolytic cleavage

It should, however, be emphasized that the relaxation from (X ) -l-i to X (+) does not always require a negligible energy. Counterexamples where the homolytic cleavage of an ion radical appears to be endowed with a sizable internal reorganization energy can be found in the electrochemical reduction of perbenzoates,60 where the observation of a transition between a concerted and a stepwise mechanism fits with an exothermic cleavage of the anion radicals. 

In view of the increasing interest in the radical ion chemiluminescence 71 has also been investigated to see whether radical ions play an essential role here, especially by E. G. Janzen et al. 136,137,138) The reaction pathway to N-methylacridone 72 was thought to proceed perhaps via a homolytic cleavage of the lucigenin carbinol base 74, yielding N-methylacridone radical anion 75. 

FIGURE3.24. Orbital and state correlations for the homolytic cleavage of radicals or ion radicals. Adapted from Figure 2 of reference 36, with permission from the American Chemical Society. 

The gaseous dichlorocarbene radical cation reacted with alkyl halides via a fast electrophilic addition to form a covalently bonded intermediate (CI2C—X—R)+ in a Fourier transform ion cyclotron resonance mass spectrometer. This intermediate fragments either homolytically or heterolytically to produce net halogen atom or halogen ion transfer product. Addition of carbonyls to the carbene ion is followed by homolytic cleavage of the C-O bond to yield a new carbene radical cation. 

Some chemical additives can induce ion-radical formation and direct the reaction along the ion-radical route. The effect was discovered and studied in cases of nucleophilic substitutions of cumene derivatives (Kornblum 1975, 1982). Cumyl radicals are formed at the first step of substitution irrespective of whether a dissociative or homolytic cleavage takes place as a result of electron transfer to the cumene derivatives (Zheng et al. 1999). 

The persulfate ion undergoes homolytic cleavage producing two sulfate ion radicals. 

Fragmentation of the odd-electron molecular ion (radical-cation, M +) may occur by homolytic or heter-olytic cleavage of a single bond. In homolytic cleavage, each electron moves independently as shown by a (single-barbed) fishhook the fragments here are an even-electron cation and a free radical (odd electron). 

Homolytic cleavage of covalent bonds is an alternative means of generating free radicals. This may be assisted by the addition of an electron as in the case of carbon tetrachloride activation. The electron may be donated by cytochrome P-450, allowing the loss of chloride ion and the production of a trichloromethyl radical (Fig. 4.7). This can initiate other radical reactions by reacting with oxygen or unsaturated lipids. 

Cation radicals of weak acids may react either by heterolytic cleavage (loss of a proton to produce a radical) or by homolytic cleavage (loss of a hydrogen to form a carboca-tion). Alkane cation radicals in liquid hydrocarbon solution undergo ion-molecule reactions, such as proton transfer on a submillisecond scale (Werst et al. 1990). 

The first step is an homolytic cleavage of the Co-OH bond with the formation of the hydroxyl radical which is then scavenged by the carbonate ion. 

When an organic co-solvent, such as acetonitrile or f-butyl alcohol, is added the reaction rate increases. This effect has been attributed to a variation in the solvation status of the reacting complex a lower solvation of the hydroxo complex may facilitate the formation of a neutral hydroxo radical. A radical scavenging of these organic solvents has been excluded because the presence of an excess of carbonate ions assures that the rate-determining step is the homolytic cleavage of the Co-OH bond. 

A different redox behavior has been observed in strongly alkaline aqueous media ([OH-] > 1 M) [85]. Co(A2ODC)2+ is reported to be reduced to the well characterized Co + species [86]. This reduction is strongly dependent on the hydroxide concentration and it is not influenced by the presence of carbonate ions or other radical scavengers. A homolytic cleavage of the Co-OH bond has... 

The commonest reactions involve the displacement of halide by hydroxide or cyanide ion to yield co-ordinated phenols or nitriles. Once again, the metal may play a variety of different functions. The polarisation of the C-Cl bond is the most obvious, but stabilisation of the product may be of equal importance, as could the involvement of a metal coordinated nucleophile. The availability of a one-electron redox inter-conversion between copper(n) and copper(i) also opens up the possibilities of radical mechanisms involving homolytic cleavage of the C-Cl bond. All of these different processes are known to be operative in various reaction conditions. In other cases, organocopper intermediates are thought to be involved. 

The photoreduction of 2- and 3-chloroanisole in alcoholic solvents has been studied and is considered to be best accounted for by invoking methoxyphenyl radicals which abstract hydrogen atoms from the solvent378.4-Chloroanisole probably reacts partly via a homolytic cleavage, but in view of the results of quenching and sensitization experiments, another pathway consists of electron transfer from the solvent (ROH) to excited aryl halide, followed by dissociation of the radical anion into chloride ion and aryl radical. 

Even though an initial homolytic cleavage, as outlined in Scheme 1, is well established during some of the B12-coenzyme catalyzed rearrangements, the ease with which electron transfers may occur always leaves open the possibility that subsequent electron transfer between the substrate radical and Bi2 could generate either a carbonium ion or... 

EPR experiments have shown that the redox ability of WZ catalysts is sufficient to initiate a homolytic cleavage of C-H bonds in alkanes. Exposure of a WZ catalyst to n-pentane at 523 K led to the formation of W5+ species and organic radicals on the surface.27 The formation of organic radicals also occurred when WZ catalysts interacted with other hydrocarbons, including benzene.31 We therefore infer that one-electron transfer, although it is not regarded as a step in the catalytic cycle, can initiate catalysis by a process that leads to the formation of the carbenium ion chain carriers,27 as also occurs in acidic solutions.32 We emphasize that a strong redox reactivity is necessary but not sufficient for the catalytic activity of WZ. 

The production of a radical cation from an even-electron, or closed shell, ion is necessarily accompanied by that of a radical corresponding to the homolytic cleavage of a bond ... 

---