Are relative bond energies a measure of radical stabilization energies?

Clark, K. B.; Wayner, Danial D. M.

doi:10.1021/ja00024a049

Cited by 96 publications

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“…Thus, our results are more consistent with those of Leigh et al 10 and Laarhoven et al 9 (i.e., no significant substituent effects on C-Br BDEs) than with the results of Clark and Wayner 11 or Andrieux et al 12 (i.e., substantial effects, see the introduction). We therefore The trends in the calculated enthalpies for isodesmic reaction 4 (∆H 4 ) should correspond to the trends in the calculated enthalpies for isodesmic reaction 2 (and 3), i.e., to ∆∆BDE-(X-H).…”

Section: Discussionsupporting

confidence: 91%

Theoretical Study of Carbon−Halogen Bond Dissociation Enthalpies of Substituted Benzyl Halides. How Important Are Polar Effects?¹

1999

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It has been suggested (Clark, K. B.; Wayner, D. D. M. J. Am. Chem. Soc. 1991, 113, 9363-9365) that C-Br bond dissociation enthalpies (BDEs) in 4-YC 6 H 4 CH 2 -Br decrease as Y becomes more electronwithdrawing because of increasing destabilization of the C δ+ -Br δ-dipole and, furthermore, that the direction and magnitude of the effect of Y on 4-YC 6 H 4 Z-X BDEs could be correlated with the sign and magnitude, respectively, of the electronegativity difference between Z and X. The results of density functional theory (DFT) calculations using a locally dense basis set approach with the B3LYP functional and 6-311+G(2d,2p) as the primary basis set on 4-YC 6 H 4 CH 2 -X, for X ) H, Br, Cl, and F with Y ) NH 2 , HO, CH 3 O, CH 3 , H, CF 3 , CN, NO 2 and BH 2 , show that the effects of Y on CH 2 -X BDEs are small (e2.0 kcal/mol) for all four classes of compounds and are roughly equal for each Y for the three halides. Furthermore, almost all Y's reduce CH 2 -X BDEs relative to Y ) H. Clark and Wayner's intriguing hypothesis that the magnitude of the effects of Y on 4-YC 6 H 4 Z-X BDEs depends on the magnitude of the differences in the electronegativities of Z and X should be discarded.For the past half-century, the influence, or lack of influence, of para-substituents, Y, on the Z-X bond dissociation enthalpies (BDEs) of compounds having the general formula 4-YC 6 H 4 Z-X has been a subject of intense experimental and theoretical interest. For phenols (Z ) O, X ) H), 3 anisoles (Z ) O, X ) CH 3 ), 4 and anilines (Z ) NH, X ) H), 5 experiment and theory agree that electron-donating Y groups decrease the Z-X BDEs and that electron-withdrawing Y groups increase the Z-X BDEs. These substituent-induced changes in BDEs are most substantial for phenols. The 4-YC 6 H 4 O-H (and 3-YC 6 H 4 O-H) BDEs can be correlated with the σ + substituent constant of Y, 3a,c and the resulting straight line has a slope F + ) 7.3 kcal/mol. 3a The values of ∆BDE (4-YC 6 H 4 O-H -C 6 H 5 O-H) are, for example, -5.6 and 4.7 kcal/mol for Y ) CH 3 O and CN, respectively. 3c Similar correlations can be made for anisoles, F + ) 2.9 kcal/mol, 4a and anilines, F + ) 3 kcal/mol. 5a For toluenes, 4-YC 6 H 4 CH 2 -H 6 (and the related 10-substituted 9-methylanthracenes 7 ), experiment and theory agree that the Ysubstituents have little or no effect on C-H BDEs, e.g., F + ) 0.5 kcal/mol. 6e The Y-substituents also have little or no effect on C-C BDEs in YC 6 H 4 CH 2 -CH 3 8 and 4-YC 6 H 4 C(CH 3 ) 2 -CH 3 . 9 In contrast to the foregoing, there is no agreement regarding the effect of substituents on benzyl bromide C-Br BDEs. Early studies on the unimolecular decomposition rates of benzyl bromides in the gas phase failed to reveal any substantial substituent effects on their C-Br BDEs. 10 Forty years later, in a ground-breaking study, Clark and Wayner 11 measured some 4-YC 6 H 4 CH 2 -Br BDEs in the liquid phase using the photoacoustic calorimetric (PAC) technique and found a rather large substituent effect. The most exciting part of their discovery was...

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Section: Discussionsupporting

confidence: 91%

Theoretical Study of Carbon−Halogen Bond Dissociation Enthalpies of Substituted Benzyl Halides. How Important Are Polar Effects?¹

1999

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“…The conclusion that the O-H bond dissociation enthalpies are only determined by radical stabilization were questioned by Clark and Wayner. 95 Their photoacoustic calorimetry studies led to the conclusion that C-Br bond dissociation enthalpies in a series of para-substituted benzyl bromides, 4-XC 6 H 4 CH 2 Br, decreases with electron withdrawing substituents, whereas it was known that the C-H bond in para-substituted toluenes, 4-XC 6 H 4 Me, varies in the opposite direction. Given the fact that in both homolytic processes the same radicals (4-XC 6 H 4 CH 2…”

Section: In the Present Context Is To Stress That The Trend Defined Bmentioning

confidence: 99%

“…) are formed, the contrasting variation in the bond dissociation enthalpies must be due to a ͑de͒stabilization of the parent The results by Clark and Wayner mentioned above 95 have been recently questioned by Mulder and co-workers, 96 who found no change in C-Br bond dissociation enthalpies in several para-substituted benzyl bromides. We believe, however, that the dipole interaction hypothesis remains a useful reasoning, although it is not entirely clear, after the above discussion on stability, what authors mean by ''stabilization of the compound.''…”

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confidence: 94%

Energetics of the O–H Bond in Phenol and Substituted Phenols: A Critical Evaluation of Literature Data

Santos

Simões

1998

Journal of Physical and Chemical Reference Data

199

208

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This review presents a critical assessment of the available experimental information ͑contained in ϳ90 literature references͒ on the thermochemistry of the O-H bond in phenol and substituted phenols. The analysis led to a set of recommended values for the O-H bond dissociation enthalpies, which in turn allowed us to discuss several empirical and theoretical methodologies used to estimate these data.

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“…Earlier studies on C-H BDEs concluded that the bond homolysis process did not exhibit the Hammett relationship because of the involvement of the spin delocalization effect [7,8]. In 1991, Wayner et al found that the para-substituent effect on C-Br BDEs of benzyl bromides was very different from the *Corresponding authors (email: fuyao@ustc.edu.cn; lliu@mail.tsinghua.edu.cn) para-substituent effect on C-H BDEs of toluenes [9]. However, Ingold and co-workers reported in 2001 and 2002 that the para-substituent effect on C-Y BDEs of PhCH 2 -Y (Y = H, F, Cl, Br) was roughly equal for each Y [10,11].…”

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confidence: 99%

Theoretical estimation of Hammett σ p constants of organic radical groups

Wang

Liu

2010

Chin. Sci. Bull.

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A quantum chemistry method was developed to calculate the Hammett substituent constants of various organic radicals. These newly obtained constants allow, for the first time, the quantitative analysis of the electron demand of organic radicals. Calculations reveal that the electron demand of organic radicals varies dramatically. It was demonstrated that the Hammett relationship of bond dissociation energies is determined only by the change of electron demand during the homolysis process. organic radicals, homolysis, bond dissociation energy, Hammett relationship, electron demandCitation:Wang C, Fu Y, Liu L. Theoretical estimation of Hammett σ p constants of organic radical groups.

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Are relative bond energies a measure of radical stabilization energies?

Cited by 96 publications

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Theoretical Study of Carbon−Halogen Bond Dissociation Enthalpies of Substituted Benzyl Halides. How Important Are Polar Effects?¹

Theoretical Study of Carbon−Halogen Bond Dissociation Enthalpies of Substituted Benzyl Halides. How Important Are Polar Effects?¹

Energetics of the O–H Bond in Phenol and Substituted Phenols: A Critical Evaluation of Literature Data

Theoretical estimation of Hammett σ p constants of organic radical groups

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Are relative bond energies a measure of radical stabilization energies?

Cited by 96 publications

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Theoretical Study of Carbon−Halogen Bond Dissociation Enthalpies of Substituted Benzyl Halides. How Important Are Polar Effects?1

Theoretical Study of Carbon−Halogen Bond Dissociation Enthalpies of Substituted Benzyl Halides. How Important Are Polar Effects?1

Energetics of the O–H Bond in Phenol and Substituted Phenols: A Critical Evaluation of Literature Data

Theoretical estimation of Hammett σ p constants of organic radical groups

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Theoretical Study of Carbon−Halogen Bond Dissociation Enthalpies of Substituted Benzyl Halides. How Important Are Polar Effects?¹

Theoretical Study of Carbon−Halogen Bond Dissociation Enthalpies of Substituted Benzyl Halides. How Important Are Polar Effects?¹