Bond dissociation energies carbon-hydrogen radicals

Resonance theory can also account for the stability of the allyl radical. For example, to form an ethylene radical from ethylene requites a bond dissociation energy of 410 kj/mol (98 kcal/mol), whereas the bond dissociation energy to form an allyl radical from propylene requites 368 kj/mol (88 kcal/mol). This difference results entirely from resonance stabilization. The electron spin resonance spectmm of the allyl radical shows three, not four, types of hydrogen signals. The infrared spectmm shows one type, not two, of carbon—carbon bonds. These data imply the existence, at least on the time scale probed, of a symmetric molecule. The two equivalent resonance stmctures for the allyl radical are as follows ... 

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... 

A comparison of the stabilities of different carbon radicals is provided by the bond dissociation energies of the bond between the carbon and a hydrogen. This is the energy that must be added when the reaction shown in the following equation occurs ... 

Alternatively, the BDE values may be reported relative to the C-H bond dissociation energy in methane (3) as the reference. This is quantitatively described in Equation 5.3 as a formal hydrogen transfer process between methane (3) and a substituted carbon-centered radical 2. The reaction enthalpy for this process is often interpreted as the stabilizing influence of substituents Rj, R2, and R3 on the radical center and thus referred to as the radical stabihzation energy (RSE). When defined as in Equation 5.3, positive values imply a stabilizing influence of the substituents on the radical center. The RSE energies are connected to the BDE values in Equations 5.land 5.2 as described in Equation 5.4. 

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. 

This reactivity pattern follows the X-H bond dissociation energies. Even though nitrogen is more electronegative than carbon, it forms weaker bonds to hydrogen. Similarly, for dialkylaminyl radical, the addition rate constants of 5-exo-trig cycli-... 

Direct hydrogen atom abstraction occurs less frequently from the nucle-obases, despite the expected modest carbon—hydrogen bond dissociation energy of the carbon—hydrogen bonds in the methyl groups of thymidine and 5-methyl-2 -deoxycytidine due to resonance stabilization of the incipient radicals. The respective radicals are also formed by deprotonation of the nucleobase radical cations, intermediates involved in electron transfer that are produced via one-electron oxidation. Amine radicals are also postulated as intermediates produced from the spontaneous decomposition of chloramines that arise from reactions of nucleosides with hypochlorous acid." " However, the majority of nucleobase radical intermediates arise from the... 

Here AHf (A ) is the heat of formation of radical A, AHf (B ) is the heat of formation of radical B, and AHf (A-B) is the heat of formation of A-B. DH° (A-B) is also called the bond dissociation energy of A-B. Table 1.10 gives a list of standard bond dissociation enthalpies for bonds involving hydrogen atoms, and Table 1.11 gives a list of DH° values for bonds between carbon atoms in various alkyl groups and a number of common organic substituents. ... 

The presence of an electron withdrawing group increases the SH bond dissociation energy (BDE) relative to that of a simple alkanethiol. For this reason, MTG is a good candidate to serve as a polarity reversal catalyst (PRC) that promotes the overall hydrogen atom transfer from a substrate R-H to a carbon centered radical. This reactivity has been applied to the addition of aldehydes to alkenes (eq A)] to the alkylation of electron-rich alkenes in the presence of silane (eq 5), to the preparation of j8-lactams via aminoacyl radical generation (eq 6), and to hydroamination of double bonds (eq 7). ... 

PROBLEM 8.3 The bond dissociation energies for the carbon-hydrogen bonds in Table 8.2 show a steady decline as the breaking carbon-hydrogen bond becomes more substituted. What can you infer from these data about the stability of the neutral species, R% called free radicals ... 

Table 4.8 shows that replacing a hydrogen with a carbon increases the stability of a radical. Thus, the radical produced from methane is less substituted than the primary radical generated from ethane. The C—H bond dissociation energy of ethane is 422 kJ mole". ... 

In addition to depolymerization through p-scission, intra-and intermolecular hydrogen transfer occur. So, primary carbon-centered radicals are expected to isomerize by intramolecular hydrogen abstraction (backbiting) and to form secondary radicals, which are more stable. On the basis of activation and bond dissociation energies, Kuroki et al claimed that backbiting reactions and intermolecular radical transfer reactions are much more likely to occur than depolymerization reactions. ... 

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