Homolytic relative

State correlation diagram for heteronuclear case in which both heterolytic bond cleavage products are energetically favored relative to homolytic cleavage. 

TABLE 111-35. ISOMER RATIOS ANP RELATIVES RATES FOR HOMOLYTIC 2-THlAZOLYLATTON OF ALKYLBENZENES (207)... 

We assess the relative stability of alkyl radicals by measuring the enthalpy change (AH°) for the homolytic cleavage of a C—H bond m an alkane... 

Inorg anic Compounds. Hydrogen chloride reacts with inorganic compounds by either heterolytic or homolytic fission of the H—Cl bond. However, anhydrous HCl has high kinetic barriers to either type of fission and hence, this material is relatively inert. 

Table 7 Relative Rates and Partial Rate Factors for the Homolytic Phenylation of Five-membered...

Most of the free-radical mechanisms discussed thus far have involved some combination of homolytic bond dissociation, atom abstraction, and addition steps. In this section, we will discuss reactions that include discrete electron-transfer steps. Addition to or removal of one electron fi om a diamagnetic organic molecule generates a radical. Organic reactions that involve electron-transfer steps are often mediated by transition-metal ions. Many transition-metal ions have two or more relatively stable oxidation states differing by one electron. Transition-metal ions therefore firequently participate in electron-transfer processes. 

The competitive method employed for determining relative rates of substitution in homolytic phenylation cannot be applied for methylation because of the high reactivity of the primary reaction products toward free methyl radicals. Szwarc and his co-workers, however, developed a technique for measuring the relative rates of addition of methyl radicals to aromatic and heteroaromatic systems. - In the decomposition of acetyl peroxide in isooctane the most important reaction is the formation of methane by the abstraction of hydrogen atoms from the solvent by methyl radicals. When an aromatic compound is added to this system it competes with the solvent for methyl radicals, Eqs, (28) and (29). Reaction (28) results in a decrease in the amount... 

Hepuzer et al. [91] have used the photoinduced homolytical bond scission of ACPB to produce styrene-based MAIs. These compounds were in a second thermally induced polymerization transferred into styrene-methacrylate block copolymers. However, as Scheme 24 implies, benzoin radicals are formed upon photolysis. In the subsequent polymerization they will react with monomer yielding nonazofunctionalized polymer. The relatively high amount of homopolymer has to be separated from the block copolymer formed after the second, thermally induced polymerization step. 

Homolytic scission of the 0-0 bond of hydrogen peroxide may be effected by heat or UV irradiation.245 The thermal reaction requires relatively high temperatures (>90 Photolytic initiation generally employs 254 nm light. Reactions in organic media require a polar cosolvent (e.g. an alcohol). 

The conductometric results of Meerwein et al. (1957 b) mentioned above demonstrate that, in contrast to other products of the coupling of nucleophiles to arenediazonium ions, the diazosulfones are characterized by a relatively weak and polarized covalent bond between the p-nitrogen and the nucleophilic atom of the nucleophile. This also becomes evident in the ambidentate solvent effects found in the thermal decomposition of methyl benzenediazosulfone by Kice and Gabrielson (1970). In apolar solvents such as benzene or diphenylmethane, they were able to isolate decomposition products arising via a mechanism involving homolytic dissociation of the N — S bond. In a polar, aprotic solvent (acetonitrile), however, the primary product was acetanilide. The latter is thought to arise via an initial hetero-lytic dissociation and reaction of the diazonium ion with the solvent (Scheme 6-11). 

The results of dediazoniations in aqueous acid without copper evidently show very little selectivity between sites with quite different electron densities. This is seen, for example, in the products from the reactions of the triarylmethanol derivatives 10.50 with R = C1 and R = CH3 (R" = R" = H). The yield ratios for ring closure to rings B and C (products 10.53 and 10.52 respectively) are 35/39 for R =C1 and 43/45 for R = CH3. The significantly higher yield of phenols (10.54) in the case of R = C1 (39%) relative to that of R = CH3 (10%) indicates that, as expected for a significant contribution by a heterolytic mechanism, the compound 10.50 (R = CH3) has a lower heterolytic, but not a lower homolytic, reactivity. For the same two reagents, but in the presence of copper powder, the ratios of 10.53 to 10.52 are 26/41 for R = C1 and 40/57 for R = CH3, with very little formation of phenols (3%). 

Baechler and coworkers204, have also studied the kinetics of the thermal isomerization of allylic sulfoxides and suggested a dissociative free radical mechanism. This process, depicted in equation 58, would account for the positive activation entropy, dramatic rate acceleration upon substitution at the a-allylic position, and relative insensitivity to changes in solvent polarity. Such a homolytic dissociative recombination process is also compatible with a similar study by Kwart and Benko204b employing heavy-atom kinetic isotope effects. 

Figure 14 shows the relative rates of various reactions for the decomposition of ArCOaH in acetic acid at 25 °C. Thermal homolytic decompostion is negligible under these conditions. The relative rates of reaction of ArCOaH with Co, Br" and Mn are 3900 4.7 1 (ref. 9), which is not what one would expect from the decreasing order of reduction potentials Br" > Mn > Co What this means in practice is that in a mixture containing roughly equal amounts of Co Mn and Br" together with ArCOaH more than 99% of the latter will preferentially react with the Co Similarly, replacement of 5% of Mn with Co resulted in a nine-fold increase in rate (ref. 9). 

However, the significant key difference for rhodium arises from the chemistry of the Rh(ll) dimer, [Rh(Por)]2, which exhibits a relatively low Rh—Rh bond strength. It undergoes homolytic dissociation and exists in equilibrium with the monomer, Rh(Por)- (Eq, (15)). The rhodium dimer can also exist in equilibrium with the hydride Rh(Por)H (Eq. (16)), and thus the hydride complex can exhibit the chemistry of the dimer, driven by formation of the Rh(Por)- monomer formed as in Eqs. (15) and (16). 

The migratory aptitude for p-nitrophenyl relative to phenyl is 4.4+0.3 which was interpreted as indicating an exclusively homolytic mechanism. The following chain scheme was proposed... 

To conclusively disprove the involvement of the chromanol methide radical, the reaction of a-tocopherol with dibenzoyl peroxide was conducted in the presence of a large excess of ethyl vinyl ether used as a solvent component. If 5a-a-tocopheryl benzoate (11) was formed homolytically according to Fig. 6.6, the presence of ethyl vinyl ether should have no large influence on the product distribution. However, if (11) was formed heterolytically according to Fig. 6.9, the intermediate o-QM 3 would be readily trapped by ethyl vinyl ether in a hetero-Diels-Alder process with inverse electron demand,27 thus drastically reducing the amount of 11 formed. Exactly the latter outcome was observed experimentally. In fact, using a 10-fold excess of ethyl vinyl ether relative to a-tocopherol and azobis(isobutyronitrile) (AIBN) as radical... 

This reflects the relative ease with which the C—H bond in the alkane precursor will undergo homolytic fission, and more particularly, decreasing stabilisation, by hyperconjugation or other means, as the series is traversed. There will also be decreasing relief of strain (when R is large) on going from sp3 hybridised precursor to essentially sp2 hybridised radical, as the series is traversed. The relative difference in stability is, however, very much less than with the corresponding carbocations. 

So far as the overall substitution reaction (— 107) is concerned, marked differences from electrophilic and nucleophilic attack become apparent as soon as the behaviour of substituted benzene derivatives (C6HjY) is considered. Thus homolytic attack on C6H5Y is found to be faster than on C6H6, no matter whether Y is electron-attracting or -withdrawing, as shown by the relative rate data for attack by Ph ... 

There is, however, no very satisfactory explanation of why such m-attack as does take place on QH5Y should also be faster than attack on QHg or of why attack on the o-position in C6H5Y is commonly faster than attack on the p-position. The relatively small spread of the partial rate factors for a particular QH5Y means that homolytic aromatic substitution normally leads to a more complex mixture of products than does electrophilic attack on the same species. 

B Homolytic Bond Dissociation Energies and the Relative Stabilities of Radicals ... 

A study518 of the mechanism of oxidation of alcohols by the reagent suggested that a reversible, oriented adsorption of the alcohol onto the surface of the oxidant occurs, with the oxygen atom of the alcohol forming a coordinate bond to a silver ion, followed by a concerted, irreversible, homolytic shift of electrons to generate silver atoms, carbon dioxide, water, and the carbonyl compound. The reactivity of a polyhydroxy compound may not, it appears, be deduced from the relative reactivity of its component functions, as the geometry of the adsorbed state, itself affected by solvent polarity, exerts an important influence on the selectivity observed.519... 

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. 

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