Bond energies homolytic

Apart from the selectivity with respect to the degree of oxidation, a second selectivity issue arises for C3 and higher alkanes the selectivity with respect to the particular C-H bond that is functionalized. Again, since the homolytic bond energies decrease in the order primary C-H > secondary C-H > tertiary C-H bonds, radical pathways involving C-H bond homolysis almost always show a marked preference for the functionalization of tertiary C-H bonds. This is in contrast to many commodity chemicals that are terminally functionalized [5]. 

The crosses refer to anions where the donor atom is from the third, fourth, or fifth row. These data points fall regularly below the line I > Br Cl- SeH- > SH- AsH2- > PH2- and GeH3- > SiH3-. These nucleophiles are all weaker bases than their ease of oxidation would suggest. This behavior is easily traced to low values of the homolytic bond energy of HX. 

For these neutral bases where the donor atom is a second-row element, again a strong correlation between the basicity and the ease of losing an electron exists. The larger scatter in Figure 1 is due to random variations in the homolytic bond energy. 

If molecules in which the donor atom is in the third, fourth or fifth row of the periodic table are added to Figure 2, then similar results to those of Figure 1 are found. The heavier donor atoms lie below the line, corresponding to weak homolytic bond energies. Exceptions occur for alkyl phosphines and phosphites, which lie above the line. These exceptions are related to the fact that the ionization potentials of these molecules are anomalously large (23). 

For neutral MLn bases, the situation is not so clear. A number of gas-phase proton affinities have been measured for neutral complexes where ionization potentials are also known (27). A larger range of homolytic bond energies is found, between 53 and 87 kcal/mol. A plot of PA versus IP resembles Figure 2, but the correlation coefficient is only 0.694. However, in some cases gas-phase protonation occurs on the ligand and not on the metal. In such cases, the homolytic bond energy is not that of M-H bond. 

Finally, the proton affinities of several atomic metal anions, M" (M -V, Cr, Fe, Co, Mo, and W), have been determined by bracketing methods (47). Combining these data with measured electron affinities of the metals yielded homolytic bond energies for the neutral hydrides, D (M-H). The monohydride bond energies compare favorably with other experimental and theoretical data in the literature and were used to derive additional thermodynamic properties for metal hydride ions and neutrals. 

With these results, it is possible to rank order a large number of anionic bases (13). The most convenient way is to take differences in D°, the usual homolytic bond energies. 

For simple aliphatic amines, like the methylamines, there is a linear inverse correlation between proton affinities and vertical IPs ". A low IP value should therefore indicate high proton affinity and vice versa. The proton affinity PA(B) of a molecule B is related to the homolytic bond energy D(B+—H) in the conjugate acid, as indicated by equation 8. If the homolytic bond dissociation energy is assumed to be constant for a particular functional group, e.g. N—H, the proton affinity wiU exhibit a linear correlation with the quantity IP(H) — IP(B), and such correlations have been reported for the proton affinities and the nitrogen lone-pair ionizations . ... 

The difference in electron affinity between OH and F or SH and Cl is much larger than the corresponding homolytic bond energies. HF in H2O is more acidic than H2O itself, because F is more stable than OH . The same holds for the difference in acidity of H2S and HCl. 

Some of the evidence indicating that alkyl substituents stabilize free radicals comes from bond energies The strength of a bond is measured by the energy required to break It A covalent bond can be broken m two ways In a homolytic cleavage a bond between two atoms is broken so that each of them retains one of the electrons m the bond... 

The energy required for homolytic bond cleavage is called the bond dissociation energy (BDE) A list of some bond dissociation energies is given m Table 4 3... 

In the absence of solvation mechanisms, the process of homolytic bond scission in organic compounds requires much less energy than heterolytic bond scission... 

Bordwell et al., 1988, 1989) and Amett (Amett et al., 1990a,b, 1992 Venimadhavan et al., 1992) have employed thermodynamic cycles consisting of heterolysis of a molecule and redox processes of the resulting ions to evaluate homolytic dissociation energies of C—H, C—C, C—N, C—O and C—S bonds in solution. In a similar way, knowledge of the A//het(R-R ) values allows determination of the heat of homolysis of carbon-carbon bonds [A/fhomo(R"R )] using (27). The results are summarized in Table 4. 

Table 10.1 Single-Bond Homolytic Dissociation Energies AH° at 25°C... 

A Homolytic Bond Dissociation Energies and Heats of Reaction ... 

Fig. 9 Deprotonation of cations radicals of synthetic analogs of NADH by oxygen or nitrogen bases in acetonitrile. Correlation between the intrinsic barrier and the homolytic bond dissociation energy of the cation radical (AH + —> A+ + H ).

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