Radical reactions, homolytic bond dissociation energies

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. 

For the first two peptides, CysS radicals abstract hydrogen atoms from the or-carbon of glycine with 7 = (1.0 to 1.1) x 10 s , while the reverse reaction proceeds with = (8.0 to 8.9) x 10 s . For the latter peptide, CysS radicals abstract hydrogen atoms from the ce-carbon of alanine with = (0.9 to 1.0) x 10 s while the reverse reaction proceeds with k -j = 1.0 x 10 s" The order of reactivity, Gly > Ala, is in accordance with previous studies on intermolecular reactions of thiyl radicals with these amino acids. The fact that < k y suggests that some secondary structure prevents the adoption of extended conformations for which calculations of homolytic bond dissociation energies would have predicted k j > k y. 

H-shift (Reaction (3.15)) was considered feasible based on (i) the analogy to the well-known, solvent-assisted 1,2-H-shift within alkoxy radicals [19] and (ii) the exo-thermicity based on the homolytic bond dissociation energies (BDEs) of the N-H (406 kj mol ) and the C-H bond (363 kj mol ) (representative values for the Gly anion [47]). However, both pulse radiolysis and y-radiolysis experiments concluded that the 1,2-H-shift in aminyl and amidyl radicals derived from amino acids and peptides must be rather slow (kis 1.2 x 10 s ) [37, 40]. 

A more detailed analysis of the radical mechanisms has been presented . Generally, all three processes show first-order kinetics but Ej reactions do not exhibit an induction period and are unaffected by radical inhibitors such as nitric oxide, propene, cyclohexene or toluene. For the non-chain mechanism, the activation energy should be equivalent to the homolytic bond dissociation energy of the C-X bond and within experimental error this requirement is satisfied for the thermolysis of allyl bromide For the chain mechanism, a lower activation energy is postulated, hence its more frequent occurrence, as the observed rate coefficient is now a function of the rate coefficients for the individual steps. Most alkyl halides react by a mixture of chain and E, mechanisms, but the former can be suppressed by increasing the addition of an inhibitor until a constant rate is observed. Under these conditions a mass of reliable reproducible data has been compiled for Ej processes. Necessary conditions for this unimolecular mechanism are (a) first-order kinetics at high pressures, (b) Lindemann fall-off at low pressures, (c) the absence of induction periods and the lack of effect of inhibitors and d) the absence of stimulation of the reaction in the presence of atoms or radicals. 

A characteristic reactivity of coenzyme B12 (2) in organometallic reactions is the ease of its homolytic decomposition to a 5 -deoxyadenosyl radical and cob(II)alamin (5) (Fig. 6) (26,28). From kinetic analyses of thermal decomposition reactions in solution, the homolytic bond dissociation energies of the (Co-C)-bond of 2 and 3 have been estimated as about 30 and 37 kcal mol" respectively (26,28). Accordingly, the (Co-C)-bond of 3 is considerably more resistant against homolysis than that of 2. At 110°, the thermolysis of adenosyl-cobinamide (8+) in aqueous solution occurs at only a 20 times lower rate than that of the coenzyme 2 (28). Thus, the nucleotide coordination contributes little in activating the... 

The results summarized in Table 11 reveal markedly different structural effects on the heats of homolytic bond dissociation energies (for the hydrogen-transfer reaction 6) and heterolytic bond dissociation energies (for the proton-transfer reaction 2) of ammonium [18] and phosphonium ions [50]. Relative to ammonium ions, nitrogen cation ion radicals (c/. series 1) are stabilized by methyl groups, for example, by roughly twice as much as the ammonium ions are... 

Since homolytic or radical processes are largely governed by the effects of bond dissociation energies, a knowledge of BDE is required for the evaluation of chemical reactivity in such reactions. However, we have found, as we mention later, that BDE s are also an important factor influencing other types of reactions involving bond heterolyses. 

The functionalization reaction as shown in Scheme 1(A) clearly requires the breaking of a C-H bond at some point in the reaction sequence. This step is most difficult to achieve for R = alkyl as both the heterolytic and homolytic C-H bond dissociation energies are high. For example, the pKa of methane is estimated to be ca. 48 (6,7). Bond heterolysis, thus, hardly appears feasible. C-H bond homolysis also appears difficult, since the C-H bonds of alkanes are among the strongest single bonds in nature. This is particularly true for primary carbons and for methane, where the radicals which would result from homolysis are not stabilized. The bond energy (homolytic dissociation enthalpy at 25 °C) of methane is 105 kcal/mol (8). 

This is related to reaction (X) for propene, but for isobutene this process is unlikely because it involves formation of a 2-methylallyl ion and destruction of a tertiary ion in the gas phase this reaction would be highly endothermic [113] because the ionisation potential of the 2-methylallyl radical [114] is appreciably greater than that of the tertiary butyl radical [115], and the difference in the homolytic C—H bond dissociation energies is in the same... 

Bond dissociation energies (BDEs) provide a measure of both the reactivity of a compound (with respect to homolytic bond rupture) and the stability of the corresponding radical. There have been many theoretical investigations of BDEs for a wide variety of species [36], In particular, the C-H BDE for a substituted methane is given by the enthalpy change for the reaction ... 

Cobalt-Carbon Bond Homolysis Studies with coenzyme Bi2 model compounds and TEMPO as a radical trap have allowed the determination of rate constants and activation parameters, which in turn led to estimates of the Co-C bond dissociation energies (BDEs). The key role of the homolytic Co-C cleaving step in these reactions was established by the following observations (Co11) is produced addition of external... 

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 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. 

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