Radicals using homolytic bond dissociation

A HOW TO Use Homolytic Bond Dissociation Energies to Determine the Relative Stabilities of Radicals... 

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... 

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... 

A very useful thermodynamic cycle links three important physical properties homolytic bond dissociation energies (BDE), electron affinities (EA), and acidities. It has been used in the gas phase and solution to determine, sometimes with high accuracy, carbon acidities (Scheme 3.6). " For example, the BDE of methane has been established as 104.9 0.1 kcahmol " " and the EA of the methyl radical, 1.8 0.7 kcal/mol, has been determined with high accuracy by photoelectron spectroscopy (PES) on the methyl anion (i.e., electron binding energy measurements). Of course, the ionization potential of the hydrogen atom is well established, 313.6 kcal/ mol, and as a result, a gas-phase acidity (A//acid) of 416.7 0.7 kcal/mol has been... 

Carbon-centered radicals play an important role in organic synthesis, biological chemistry, and polymer chemistry. The radical chemistry observed in these areas can, to a good part, be rationalized by the thermodynamic stability of the open shell species involved. Challenges associated with the experimental determination of homolytic bond dissociation energies (BDEs) have lead to the widespread use of theoretically calculated values. These can be presented either directly as the enthalpy for the C-H bond dissociation reaction described in Equation 5.1, the gas-phase thermodynamic values at the standard state of 298.15K and 1 bar pressure being the most commonly reported values. 

In QSAR 1.113, 62% of the variance is accounted for by and 28% is explained by log P. It appears that free-radical-mediated toxicity is responsible for the growth-inhibitory effects of the phenols. Homolytic bond dissociation energies related to the homolytic cleavage of the OH bond in the following reac-tion (X—C6H4OH + CeHsO. X—CgH O. + CeHgOH) have been used in lieu of values. The net result is similar, as seen in QSAR 1.114(242). 

In a landmark paper, Breslow and coworkers described the determination of pA), values of weak hydrocarbon acids by use of thermochemical cycles involving electrochemical reduction data for triarylmethyl, cycloheptatrienyl, and triphenyl- and trialkylcyclopropenyl cations and radicals [9aj. Later, they derived pATa data from standard oxidation potentials and bond-dissociation energies [9b, c]. The methodology was further developed by Nicholas and Arnold [10a] for the determination of cation radical acidities, and later modified and extensively used by Bordwell and coworkers [10b, c] so that homolytic bond-dissociation energies and cation radical... 

This inability of HF calculations to model correctly homolytic bond dissociation is commonly illustrated by curves of the change in energy as a bond is stretched, e.g. Fig. 5.19. The phenomenon is discussed in detail in numerous expositions of electron correlation [62]. Suffice it to say here that representing the waveflmction as one determinant (or a few), as is done in Hartree-Fock theory, does not permit correct homolytic dissociation to two radicals because while the reactant (e.g. H2) is a closed-shell species that can (usually) be represented well by one determinant made up of paired electrons in the occupied MOs, the products are two radicals, each with an unpaired electron. Ways of obtaining satisfactory energies, with and without the use of electron correlation methods, for processes involving homolytic cleavage, are discussed further in section 5.5.2. 

Equation 2.77 was later used to calculate the ENs for radicals of various composition the averaged values are listed in Table S2.16. It is evident that the presence of multiple bonds in radicals substantially affects the atomic ENs. Equation 2.77 was shown to give the ENs which describe quite accurately the homolytic bond dissociation enthalpies of common covalent bonds (including highly polar ones) with an average... 

Empirical linear correlation analysis has also produced results that may be used in predicting the rates of FeS-mediated pollutant transformation based on pollutant thermodynamic or molecular properties. One study (77) performed linear correlation analysis of log kohs values for 8 halogenated aliphatic molecules with five molecular properties one-electron reduction potentials, lowest unoccupied molecular orbital (LUMO) energies, fi ee energies of formation of aqueous phase radicals formed upon one-electron reduction, gas-phase homo-[ytic bond dissociation enthalpies, and aqueous solubilities. Of these parameters, homolytic bond dissociation enthalpies (7)r.x values) were best correlated with log obs values for FeS reductive dechlorination (R =0.82). The correlation between log obs and 7)r x is illustrated in Figure 4. Another parameter shown... 

Instead of alkyl nitrite, other alkoxyl radical precursors such as ROOH, ROOR, ROI, ROC1, etc. can also be used for the same type of reaction. The high reactivity of these compounds comes from the weak bond dissociation energies in O-O, 0-1, and O-Cl bonds. Another simple method is as follows. Photolytical treatment of alcohol (5) with NIS (AModosuccinimide) provides the tetrahydrofuran skeleton (6), through the formation of alkyl hypoiodite (ROI), homolytic cleavage of the 0-1 bond to form an alkoxyl radical, 1,5-H shift to form a carbon-centered radical, reaction with ROI to form 8-iodoalcohol, and finally ionic cyclization to form a tetrahydrofuran skeleton, together... 

The homolytic thermal dissociation of R—M, previously used by Paneth in generating and studying free alkyl radicals, occurs with the more electronegative alkyls, such as those of mercury, lead, and the metalloids. Currently there is much interest in evaluating carbon-metal bond dissociation energies from such pyrolyscs (Section III.B). 

Consequently, the homolytic gas-phase X-H bond dissociation enthalpy, Dx h, for the corresp>onding neutreil molecule can be calculated using Hess law and the one-electron reduction potential of tiie radical cation (X-H ) in combination with the pl< of the radical cation or the one-electron reduction potential of the radical (X ) in combination with the pK, of the neutral molecule (X-H). This results in equation 5 where C is a constant that depends on the family of compounds and the solvent. [25]... 

---