Substituent effects on the benzylic bond dissociation enthalpy in benzyl bromides (C–Br) and tert-butylbenzenes (C–CH3): a gas phase thermolysis and liquid phase photoacoustic study

Laarhoven, Lucas J. J.; Born, J.G.P.; Arends, Isabel W. C. E.

doi:10.1039/a702724i

Cited by 30 publications

(29 citation statements)

References 26 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…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: 89%

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

1999

View full text Add to dashboard Cite

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...

show abstract

Section: Discussionsupporting

confidence: 89%

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

1999

View full text Add to dashboard Cite

show abstract

“…We have found no published data on the energy of the SCl bond in arenesulfonyl chlorides. Fu et al [11] analyzed the effect of remote substituent Y in the aromatic ring on the energy of dissociation of the XZ bond in YC 6 H 4 XZ molecules (RMP2/6-311++G**//UB3LYP/6-31G*) and found that substituent in the aromatic ring only slightly affects the energy of the CH and CF bonds in compounds YC 6 H 4 CH 2 Z where Z = H, F, which is well consistent with the experimental data [9]. However, the substituent effect on the energy of dissociation of the XZ bond in YC 6 H 4 SZ and YC 6 H 4 NHZ (Z = H, F) turned out to be fairly strong.…”

supporting

confidence: 81%

“…In addition, substituents can change the energy of the initial state due to dipoledipole interaction with the CBr bond. However, the recent experimental data on the energy of dissociation of the CBr bond in benzyl bromides showed that it almost does not depend on the substituent within experimental error (1 kcal/mol) [9]; therefore, the effect of substituent on the stability of benzyl radical via delocalization of the unpaired electron, as well as on the energy of the initial state via dipole dipole interaction, is negligible.…”

mentioning

confidence: 98%

Structural and Kinetic Relations Holding in the Halogen Abstraction from Organohalogen Compounds by Alkyl Radicals

2005

View full text Add to dashboard Cite

The relative rate constants were determined, and the absolute rate constants were estimated, for halogen transfer from N,N-dihaloarenesulfonamides, (dichloroiodo)arenes, benzyl bromides, and arenesulfonyl chlorides to cyclohexyl radical and from N,N-dihaloarenesulfonamides to benzyl radical. Polar effect of the substituent was found to be the main factor determining the rate of halogen transfer from benzyl bromides and arenesulfonyl chlorides; it increases with rise in the electrophilicity of the organohalogen substrate due mainly to charge distribution in the transition state.While considering the relations between the structure of organic molecules and the rate of radical transfer of hydrogen, the main factor is the heat effect of the hydrogen transfer stage, and its rate increases in parallel with the exothermic effect. In keeping with the Bell EvansPolanyi principle, in most cases the rate of transfer of an atom is linearly related to the heat effect. Exceptions are possible when a radical to which an atom is transferred possesses pronounced electrophilic or nucleophilic properties. In terms of the model based on the Marcus equation, increase in the electrophilicity or nucleophilicity of a radical leads to change of the internal reaction barrier as a result of charge separation in the transition state [1]. For example, the rate of abstraction of a hydrogen atom by chlorine and some other electrophilic radical species depends on polar effects of substituents in the substrate to a greater extent than on the heat effect. On the other hand, general relations holding in halogen transfer to alkyl radicals have been studied poorly. Therefore, we examined the effect of the structure of organic halogen compounds on the rate of halogen transfer therefrom to alkyl radicals.We determined the relative rate constants for abstraction of chlorine and bromine atoms from halogenating agents and arenesulfonyl chlorides by cyclohexyl and benzyl radical, following the procedure developed previDeceased.ously [2]. This procedure is based on competing abstraction of halogen from reagents XHlg 1 and YHlg 2 by alkyl radicals generated by radical chain halogenation of the corresponding alkane under conditions excluding formation of molecular halogen. Using the absolute rate constant for bromine transfer from N-bromosuccinimide (NBS) to benzyl radical, k(NBS) = (5.0 ± 0.3) 1 0 5 l mol 1 s 1 [3], we estimated the absolute rate constants of halogen transfer in the examined reactions. The relative and absolute rate constants are given in Table 1.

show abstract

“…) 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.''…”

mentioning

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

View full text Add to dashboard Cite

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.

show abstract

Substituent effects on the benzylic bond dissociation enthalpy in benzyl bromides (C–Br) and tert-butylbenzenes (C–CH3): a gas phase thermolysis and liquid phase photoacoustic study

Cited by 30 publications

References 26 publications

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

Structural and Kinetic Relations Holding in the Halogen Abstraction from Organohalogen Compounds by Alkyl Radicals

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

Contact Info

Product

Resources

About

Substituent effects on the benzylic bond dissociation enthalpy in benzyl bromides (C–Br) and tert-butylbenzenes (C–CH3): a gas phase thermolysis and liquid phase photoacoustic study

Cited by 30 publications

References 26 publications

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

Structural and Kinetic Relations Holding in the Halogen Abstraction from Organohalogen Compounds by Alkyl Radicals

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

Contact Info

Product

Resources

About

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