Homolytic bond dissociations

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. 

Homolytic bond dissociation, the breaking of a covalent bond with two radicals resulting, generally occurs without any extra activation barrier (see also Chapter 3, Problem 1). Thus, the rate of bond dissociation (kinetics) is directly related to the stabilities of the resulting radicals (thermodynamics). 

Homolytic Bond Dissociation. A process in which a bond is broken and two radicals result. 

Homolytic bond dissociation (homolysis) electronically symmetrical bond breaking => produces radicals. 

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 ).

Local HSAB principle can also be used to calculate the relative homolytic bond dissociation energies (BDE). For the homolytic dissociation of para-substituted phenols ... 

Here, and in the following thermochemical schemes, AHs denotes solvation energy, D homolytic bond dissociation energy, I ionization potential, A electron affinity. 

F. G. Bordwell, J.-P. Cheng, J. A. Harrelson Jr. Homolytic Bond Dissociation Energies in Solution from Equilibrium Acidity and Electrochemical Data. J. Am. Chem. Soc. 1988,110, 1229-1231. 

F. G. Bordwell, X.-M. Zhang. Acidities and Homolytic Bond Dissociation Enthalpies of 4-Substituted-2,6-di-tert-Butylphenols. J. Phys. Org. Chem. 1995, 8, 529-535. 

TABLE 11. Homolytic bond dissociation energies (BDEs) of some H—N bonds133... 

TABLE 12. Acidities and homolytic bond dissociation energies of the acidic C—H bonds in 9-(dialkylamino)fluorenes 19-R1 R2NF1H)... 

Knowledge of homolytic bond dissociation energies (BDEs) is critically important for understanding radical chemistry. The bond energies of organic compounds have been reviewed extensively, but we will use recommended R-H BDE values for organic compounds given in a recent excellent... 

Heats of formation of neutral molecules, AHf(RH), can be obtained from combustion data with high accuracy. Bond dissociation energies can either be derived for homolytic bond dissociation... 

Table 2.2. Homolytic bond dissociation enthalpies, AHohom. and heats of formation, AHf(x.), of some selected bonds and radicals [kJ mol ] ...

The potential energy curves of a neutral molecule AB and the potential ionic products from processes 7.18-7.20 are compared below (Fig. 7.11). These graphs reveal that the formation of negative molecular ions, AB, is energetically much more favorable than homolytic bond dissociation of AB and that the AB " ions have internal energies close to the activation energy for dissociation. [65,73,75]... 

The pKi, values of a series of para- and meffl-substituted benzaldoximes and phenyl methyl ketoximes, ArCR=NOH (R=H, Me), have been measured in DMSO. The aldoximes exhibit pK. = 20.05 + 3.21ap. The homolytic bond dissociation energy of the O-H bond has been estimated as 88.3 (aldoximes) and 89.2kcal mol" (ketoximes) by relating the pK to the oxidation potential of the conjugate base (i.e. ox for ArCR=NO- ArCR=NO ). 

The semiempirical AMI MO method has been used to calculate heats of formation of a series of m- and p-substituted benzene and toluene derivatives ArY and ArCHaY, and their phenyl or benzyl cations, anions, and radicals heterolytic and homolytic bond dissociation energies (BDEs) and electron transfer energies for the ions have also been calculated and the relationship A//het = A//et-I-AWhomo has been confirmed (it being noted that A//homo is insensitive to ring substituents). The linear relationship found between and the appropriate HOMO or LUMO... 

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