C-H BDEs in radicals

The broken bonds (boldface = dissociated atom), Affi°(R), kcal/mol (kJ/mol) 

Pentyl-2 radical CH3CH2CH2CHCH3 32.5 1.0 136.0 4.2 Derived from in ref. 1986PED/NAY 

2-Methyl-butyl-2 radical CH3CH2C(CH3)2 35.1 1.0 146.9 4.2 Derived from AfH° in ref. 1986PED/NAY 

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BDEs in RNH radicals are much smaller than the C-H BDEs in RCH2 radicals. Two factors which contribute to this difference are (i) the difference between the hybridization of the X-H bonds that are broken in the two radicals, and (ii) the difference between the changes in hybridization that accompany formation of triplet NH and triplet CH2 from the corresponding radicals. 

The radical stabilization energy (RSE) of a substituted methyl radical CH2X is generally defined as the difference between the C-H bond dissociation energy in methane and the C-H BDE in the substituted methane CH3X ... 

Bordwell (Bordwell and Bausch, 1986) has developed a method to determine C—H BDEs from a combination of p ha values and oxidation potentials (E ) of the corresponding anions in dimethyl sulphoxide solution. These acidity-oxidation potentials (AOP) are taken as measures for BDEs and are related to the stabilization of the radicals formed. This procedure has been recently applied to the subject of captodative stabilization (Bord-well and Lynch, 1989). Values of ABZ) relative to the C—H BDE in methane are calculated according to (13). These values are set equal to the... 

The discussion shows that it is difficult and sometimes ambiguous to interpret C—H BDEs in terms of radical stabilization only. Consequently, their usefulness in the context of captodative substitution appears to be questionable. 

The C-H BDE in peptides is even lower than that of the S-H BDE in thiols as a consequence of the exceptional stability of the radical products due to captoda-tive stabilization (Viehe et al. 1985 Armstrong et al. 1996). Yet, the observed rate constants for the reaction of CH3 and CH2OH with, e.g., alanine anhydride are markedly slower than with a thiol. This behavior has been discussed in terms of the charge and spin polarization in the transition state, as determined by AIM analysis, and in terms of orbital interaction theory (Reid et al. 2003). With respect to the repair of DNA radicals by neighboring proteins, it follows that the reaction must be slow although thermodynamically favorable. 

In combination with the unchanged C-H BDE in CH4 this equates to RSE(3) values of + 67.4 2.1 and +63.2 2.9 kJ/mol for the allyl and benzyl radicals, respectively. This implies a stability difference of these two systems of just over 4 kJ/mol. 

The C-C and C-H BDEs for ethane are 377.0 and 423.0kJ/mol, respectively, and the H-0 BDE of hydroperoxyl (HO2 ) radical is only 207.5kJ/mol. The large reaction endothermicities for reactions 6.4 and 6.5 highlight the unlikeliness of their occurrence at lower temperatnres. In pyrolytic (heat, but no oxidant) or high fuel/oxidant ratio conditions, the endothermicities indicate that ethane and other alkanes will... 

The hydroxyl group of alcohol weakens the a-C—H bond. Therefore, free radicals attack preferentially the a-C—H bonds of the secondary and primary alcohols. The values of bond dissociation energy (BDE) of C—H bonds in alcohols are presented in Table 7.1. The BDE values of C—H bonds of the parent hydrocarbons are also presented. It is seen from comparison that the hydroxyl group weakens BDE of the C—H bond by 23.4 kJ mol 1 for aliphatic alcohols and by 8.0 kJ mol 1 for allyl and benzyl alcohols. 

A molecule of aliphatic ester possesses two substituents around the ester group —C(0)0—, namely, alcohol and acid residues. Ester group decreases the BDE of a-C—H bonds in both substituents alcoholic and acidic. Therefore, an ester molecule has two different types of weak C—H bonds that are attacked by peroxyl radicals a-C—H bonds of the alcohol substituent —CH20C(0)R and the a-C—H bonds of the acid substituent —CH2C(0)0R. The values of BDE of these types of C—H bonds are close but not the same. The values of BDE of the C—H bonds are collected in Table 9.10. 

In contrast, the much lower enthalpy computed for 3 lb, compared to 3lc, means that the N-H BDE of the anilinyl radical 8b is much lower than the C-H BDE of the 3-pyridylmethyl radical 8c. The results in Table 5 show that this is indeed the case, not only for R=Ph and R =3-pyridyl, but also for R=R =Ph and R=R —H.77 The data in Table 5 indicate that, not just for lb and lc but in general, triplet nitrenes are ca. 20 kcal/mol more thermodynamically stable than comparably substituted triplet carbenes. 

The reaction enthalpy and thus the RSE will be negative for all radicals, which are more stable than the methyl radical. Equation 1 describes nothing else but the difference in the bond dissociation energies (BDE) of CH3 - H and R - H, but avoids most of the technical complications involved in the determination of absolute BDEs. It can thus be expected that even moderately accurate theoretical methods give reasonable RSE values, while this is not so for the prediction of absolute BDEs. In principle, the isodesmic reaction described in Eq. 1 lends itself to all types of carbon-centered radicals. However, the error compensation responsible for the success of isodesmic equations becomes less effective with increasingly different electronic characteristics of the C - H bond in methane and the R - H bond. As a consequence the stability of a-radicals located at sp2 hybridized carbon atoms may best be described relative to the vinyl radical 3 and ethylene 4 ... 

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 ... 

For the primary and secondary a-alkoxy radicals 24 and 29, the rate constants for reaction with Bu3SnH are about an order of magnitude smaller than those for reactions of the tin hydride with alkyl radicals, whereas for the secondary a-ester radical 30 and a-amide radicals 28 and 31, the tin hydride reaction rate constants are similar to those of alkyl radicals. Because the reductions in C-H BDE due to alkoxy, ester, and amide groups are comparable, the exothermicities of the H-atom transfer reactions will be similar for these types of radicals and cannot be the major factor resulting in the difference in rates. Alternatively, some polarization in the transition states for the H-atom transfer reactions would explain the kinetic results. The electron-rich tin hydride reacts more rapidly with the electron-deficient a-ester and a-amide radicals than with the electron-rich a-alkoxy radicals. 

The determination of thermodynamic stability of a radical from C—H bond-dissociation energies (BDE) in suitable precursors has a long tradition. As in other schemes, stabilization has to be determined with respect to a reference system and cannot be given on an absolute basis. The reference BDE used first and still used is that in methane (Szwarc, 1948). Another more refined approach for the evaluation of substituent effects by this procedure uses more than one reference compound. The C—H BDE under study is approximated by a C—H bond in an unsubstituted molecule which resembles most closely the substituted system (Benson, 1965). Thus, distinctions are made between primary, secondary and tertiary C—H bonds. It is important to be aware of the different reference systems if stabilization energies are to be compared. 

In connection with the captodative effect, Riichardt (Zamkanei et al., 1983) has determined the BDE of the tertiary C—H bond in [20] and compared it with the tertiary bond in isobutane. He concludes that the stabilization of 12.8 kcal mol which he derives from this comparison falls 4kcal mol short of the value of 16.5 kcal mol which he calculates for the sum of the substituent effects for phenyl (9 kcal mol ), cyano- (5.5 kcal moP ) and methoxyl (1.5kcal mol ) groups. The latter values were derived from studies on C—C BDEs. Not even additivity of the substituent effects is observed. The existence of a captodative stabilization of radical [21] is denied (see, however, the studies on the thermolysis of [24]). 

Bordwell compares the alkoxy effect with the C—H BDE for dimethyl ether as reported by McMillen and Golden (1982). The latter is lower by 12 kcal mol" than that of methane, and from the comparable magnitude of the effect in the a-alkoxy acetophenonyl radical it is concluded that an additive substituent effect exists. Similar arguments hold for the dimethylamino-substituted radical. It is stated that additivity is more than expected for bis-... 

Delocalization of the odd electron into extended n systems results in considerable radical stabilization. The C—H BDE at C3 of propene is reduced by 13 kcal/mol relative to that of ethane. That the stabilization effect in the allyl radical is due primarily to delocalization in the n system is shown by the fact that the rotational barrier for allyl is 9 kcal/mol greater than that for ethyl. Extending the conjugated system has a nearly additive effect, and the C—H BDE at C3 of 1,4-pentadiene is 10 kcal/mol smaller than that of propene. Delocalization of the odd electron in the benzyl radical results in about one-half of the electron density residing at the benzylic carbon, and the C—H BDE of the methyl group in toluene is the same as that in propene. 

Phenylnitrene is intrinsically more stable than the isomeric pyridylcarbene. Kamey and Borden explained that the reason why the N—H BDEs in RNH radicals are smaller than the C—H BDEs of RCH radicals, that is, rehybridization, is also responsible for the fact that nitrenes are thermodynamically more stable than carbenes.The lone pair of electrons of a nitrene reside in a low-energy sp hybrid orbital. This effect dramatically stabilizes nitrenes relative to carbenes in which the nonbonding electrons reside in either pure p or pseudo sp orbitals. 

Aromatic amines are known as to be efficient inhibitors of hydrocarbon and polymer oxidation (see Chapters 15 and 19). Aliphatic amines are oxidized by dioxygen via the chain mechanism under mild conditions [1,2]. Peroxyl and hydroperoxyl radicals participate as chain propagating species in the chain oxidation of amines. The weakest C—H bonds in aliphatic amines are adjacent to the amine group. The bond dissociation energy (BDE) of C—H and N—H bonds of amines are collected in Table 9.1. One can see that the BDE of the N—H bond of the NH2 group is higher than the BDE of the a-C—H bond in the amine molecule. For example, DN = 418.4 kJ mol 1 and DC H = 400 kJmol-1 in methaneamine. However, the BDE of N—H bond of dialkylamine is lower than that of the C—H bond of... 

Cubyl anion has been prepared by reacting (trimethylsilyl)cubane with fluoride ion in a Fourier transform mass spectrometer and its reactions with acids such as H2O, Me2NH, EtNH2, MeNH2, and NH3 have been monitored. The results suggest that cubane is thermodynamically more acidic than cyclopropane. The electron affinity of the cubyl radical and the C—H BDE for cubane have also been estimated.126... 

The HO-H bond dissociation energy (BDE) is 499 kj mol-1, while the C-H bonds in saturated hydrocarbons are much weaker (BDE = 376-410 kj mol-1 Berkowitz et al. 1994 for a compilation, see Chap. 6). Thus, there is a considerable driving force for H-abstraction reactions by -OH. On the other hand, vinylic hydrogens are relatively tightly bound, and an addition to the C-C double bond is always favored over an H-abstraction of vinylic or aromatic hydrogens. Hence, in the case of ethene, no vinylic radicals are formed (Soylemez and von Sonntag 1980), and with benzene and its derivatives the formation of phenyl-type radicals has never been conclusively established. 

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