Homolytic cleavage processes

The two silylene-generating processes are believed to occur in the singlet excited state in pericyclic fashion, while the radical-pair-forming homolytic cleavage process is believed to occur in the triplet state (Michl and Balaji,... 

Radicals can be formed in several ways, but all involve homolytic cleavage (one electron is transferred to each adjacent atom from the bond), as depicted by X—Y, leaving two radical products. The equilibrium constant for this homolytic cleavage process depends on both the relative bond strength of X—Y and also on the relative stabilities of X and Y. Increasing the temperature of the reaction will generally shift the equilibrium toward a higher concentration of free radicals, This equilibrium makes it convenient to... 

RADICALS GENERATED BY HOMOLYTIC CLEAVAGE PROCESSES THERMOLYSIS AND PHOTOLYSIS... 

So, it is harder to remove the first H than the second. The bond energy for H2O is the average of these two, i.e. 459kJmol". Note that in these expressions we have separated the bonds into neutral radicals, hence the symbol. The bond energies refer to these homolytic cleavage processes, rather than to the heterolyic splitting of H2O to produce an anionic hydroxyl and proton. 

Since the quantum yield of the Norrish type I reaction is generally low, it has been assumed that the initial homolytic cleavage is a reversible process. Evidence came from an investigation by Barltrop et al. which has shown that erythro-2,3-dimethylcyclohexanone 12 isomerizes to t/zreo-2,3-dimethylcyclohexanone 13 upon irradiation ... 

A free-radical process consists of at least two steps. The first step involves the formation of free radicals, usually by homolytic cleavage of bond, that is, a cleavage in which each fragment retains one electron ... 

The reaction endothermicity establishes a minimum for the activation energy whereas abstraction of a hydrogen atom from carbon is a feasible step in a chain process, abstraction of a hydrogen atom from a hydroxyl group is unlikely. Homolytic cleavage of an O-H bond is likely only if the resulting oxygen radical is stabilized, such as in phenoxy radicals formed from phenols. 

The cage effect described above is also referred to as the Franck-Rabinowitch effect (5). It has one other major influence on reaction rates that is particularly noteworthy. In many photochemical reactions there is often an initiatioh step in which the absorption of a photon leads to homolytic cleavage of a reactant molecule with concomitant production of two free radicals. In gas phase systems these radicals are readily able to diffuse away from one another. In liquid solutions, however, the pair of radicals formed initially are caged in by surrounding solvent molecules and often will recombine before they can diffuse away from one another. This phenomenon is referred to as primary recombination, as opposed to secondary recombination, which occurs when free radicals combine after having previously been separated from one another. The net effect of primary recombination processes is to reduce the photochemical yield of radicals formed in the initiation step for the reaction. 

Hydroxy radicals are intermediates in the reaction of Ti3+ and H2O2 (175). This system is also capable of hydroxylation of aromatics and alkanes but, in contrast to reactions with Fenton s reagent (Fe2+ + H202, reductive, homolytic cleavage, Eq. (11)), only non-chain processes are possible, because Ti4+ is not usually an oxidant. Hence, relatively high selectivities are feasible. 

Several, oxidatively coupled xanthates (64-66), compounds (also called xanthides) containing the photochemically reactive, sulfur-sulfur bond, have been studied.130 Homolytic cleavage of this reactive bond is the primary reaction for these compounds, although this process is normally masked by recombination of the radicals produced. This primary, light-initiated process becomes apparent when a mixture of the xanthide 64 and ethyl xanthide (67) is irradiated in cyclohexane, because an equilibrium between 64, 67, and the mixed xanthide 68 is rapidly established. 

Figure 16.2 Thermochemical cycles involving the heterolytic and homolytic cleavages of the R-X bond, and reduction or oxidation processes. The reaction numbers are the same as in figure 16.1.

Homolytic cleavage of most a bonds may be achieved if the compound is subjected to a sufficiently high temperature, typically about 200 °C. However, some weak bonds will undergo homolysis at temperatures little above room temperature. Bonds of peroxy and azo compounds fall in this category, and such compounds may be used to initiate a radical process. Di-tert-butyl peroxide, dibenzoyl peroxide... 

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