Homolytic reactions, definition

The material is arranged as follows. Photochemical reactions are discussed first (Section VI,A) as they represent the most thoroughly studied and only definitely established examples of the simplest type of reaction, viz., the homolytic fission of the Co—C bond. Thermal (i.e., nonphotochemical)... 

In the last ten years arylation has been tbe most studied homolytic aromatic substitution, also in the heteroaromatic series. Numerous data concerning a large variety of heterocycles have permitted the definition of many details for the individual substrates, without adding, however, anything particularly new as regards the general characteristics of the reaction, already outlined in the previous review of Norman and Radda. These characteristics are substantially the same as those observed in the homocyclic aromatic series, for which comprehensive reviews are available. There is therefore a sharp difference in behavior between arylation and other homolytic substitutions described in the previous sections. These latter have quite different characteristics, and sometimes they are not known, in the homocyclic series. 

Photolysis of allyl iodide in thiophene gives a mixture of 2-allyl- (63.8%) and 3-allyl-(36.2%) thiophenes (77JOC1570). This is in contrast to homolytic phenylation, where almost exclusive 2-phenylation takes place (Section 3.14.2.9). It has been suggested that the rate-determining step in the allyl substitution reaction has a small but definite charge-transfer character. 

In this chapter, the basic principle of arrow pushing was introduced in the context of organic reactions driven by homolytic cleavage, heterolytic cleavage, or concerted mechanisms. Furthermore, the concept of polarity was introduced using heteroatoms and common organic functional groups. This discussion led to the definitions of nucleophiles and... 

The separate and quantitative evaluation of accelerating steric effects on the homolytic cleavage of C—C-bonds allowed the definition of intrinsic barriers for the bond dissociation reaction, i.e. AG or AH at Hs = 0 in Fig. 2 or 3 or in Eq. (1-11) in general. AH is equal to or slightly than the corresponding bond dissociation energy (BDE) 6a-811 as pointed out earlier. 

The RSE is calculated here as the difference between the homolytic C-C bond dissociation energy in ethane (5) and a symmetric hydrocarbon 6 resulting from dimerization of the substituted radical 2. By definition the C-C bonds cleaved in this process are unpolarized and, baring some strongly repulsive steric effects in symmetric dimer 6, the complications in the interpretation of substituent effects are thus avoided. Since two substituted radicals are formed in the process, the reaction enthalpy for the process shown in Equation 5.5 contains the substituent effect on radical stability twice. The actual RSE value is therefore only half of the reaction enthalpy for reaction 5.5 as expressed in Equation 5.6. 

Besides oxidative addition, there is also another type of homolytic addition in transition metal chemistry [11]. By definition, in this type of reaction, a substrate XY adds to two metal centers in such a way that the formal oxidation state of each metal increases or decreases by one unit (Eq. 2-48). 

Energy of a homolytic cleavage of communication OH (Z) of hindered phenols is an efl cacy qirantity indicator in reactions inhibiting oxidations. Experimental results of definition D [1-3] are known. In... 

The foregoing criteria, though useful, are not definitive. In many of the reactions the yields are poor in others, products characteristic of both polar and radical reactions are obtained. These studies demonstrate that both heterolytic and homolytic paths are available for the decomposition of diazonium and iodonium salts. However, they do not, per se, permit one to formulate the details for either reaction course. 

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