Homolytic cleavage homolysis

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... 

Initiation normally requires molecules with weak bonds to undergo homolytic cleavage to produce free radicals. Since bond homolysis even of weak bonds is endothermic, energy in the form of heat (A) or light (hv) is usually required in die initiation phase. However, some type of initiation is required to get any free-radical reaction to proceed. That is, you must first produce free radicals from closed-shell molecules in order to get free-radical reactions to occur. Benzoyl peroxide contains a weak 0-0 bond that undergoes thermal cleavage and decarboxylation (probably a concerted process) to produce phenyl radicals which can initiate free-radical chain reactions. 

In polar reactions, heterolytic (unsymmetrical) bond cleavage (heterolysis) and bond formation occur, while homolytic (symmetrical) bond cleavage (homolysis) and bond formation occur in radical reactions as shown below (Scheme a). 

A widely accepted mechanism of coenzyme Bir dependent rearrangements encompasses, as the initial step, the homolytic cleavage of the carbon-cobalt bond to generate the 5 -deoxyadenosyl radical. The thermodynamic and kinetic aspects of this and related processes involving the homolysis of transition metal-carbon bonds are discussed. 

The known coenzyme Bi2-dependent enzymes all perform chemical transformations in enzymatic radical reactions that are difficult to achieve by typical organic reactions. Homolytic cleavage of the Co bond of the protein-bound coenzyme B12 (3) to a 5 -deoxy-5 -adenosyl radical (9) and cob(n)alamin (5) is the entry to reversible H-abstraction reactions involving the 5 -position of the radical (9). Indeed, homolysis of the Co bond is the thermally most easily achieved transformation of coenzyme B12 (3) in neutral aqueous solution (with a homolytic (Co-C)-BDE of about 30 kcal mol ). However, to be relevant for the observed rates of catalysis by the coenzyme B12-dependent enzymes, the homolysis of the Co-C bond of the protein-bound coenzyme (3) needs to be accelerated by a factor of about 10 , in the presence of a substrate. Coenzyme B12 might then be considered, first of aU, to be a structurally sophisticated, reversible source for an alkyl radical, whose Co bond is labihzed in the protein-bound state (Figure 8), and the first major task of the... 

Breaking a bond by equally dividing the electrons between the two atoms in the bond is called homolysis or homolytic cleavage. 

A fourth class of radical-forming reaction is homolytic cleavage. For an example, we can go back to dibenzoyl peroxide, the unstable compound we considered earlier in the chapter because it readily undergoes homolysis. 

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