Oxygen-Carbon Bond Dissociation Enthalpies
J. Am. Chem. Soc., Vol. 123, No. 23, 2001 5519
4. The O-C BDE’s (∆ø ) 1.0) in 4-YC6H4O-CH3 deter-
We would have expected a much larger substituent effect that
was closely comparable to the well-established experimental
effect of substituents on O-H BDE’s in phenols (F+ ) 6.9 kca1/
mol). There is, in fact, a recent theoretical study by Wu and
Lai13 that indicates that the effects of Y on 4-YC6H4O-CH3
BDE’s and 4-YC6H4O-H BDE’s are “nearly identical”.
mined using very low-pressure pyrolysis (VLPP) were reported
to be decreased by ED Y’s with F+ ) 3.0 kca1mol-1 11
, a value
between that found for phenol (6.9-6.4) and toluene (0.5),
consistent with the trend in ∆ø along the O-H (1.4), O-C (1.0),
and C-H (0.4) bond series.
5. The N-H BDE’s in 4-YC6H4NH-H (∆ø ) 0.9) measured
Since experiment and theory disagree regarding the effect of
substituents on O-C BDE’s in phenyl ethers we decided to
examine this problem both by experiment and theory. We
reexamined the O-C bond cleavage of substituted anisoles in
the gas phase at around 1000 K and studied substituted benzyl
phenyl ether homolysis in the liquid phase at around 550 K in
the presence of a radical scavenger, 9,10-dihydroanthracene. In
benzyl phenyl ethers a substituent on the phenoxyl group,
4-YC6H4O-CH2C6H5, would be expected to have a different (and
larger) effect than the same substituent on the benzyl group,
4-YC6H4CH2-OC6H5, analogous to the known effects of sub-
stituents in phenols and in benzyl halides or toluenes, respec-
tively. We also studied phenols and phenyl ethers with the same
substituents by several density functional theory (DFT) proce-
dures for direct comparisons.
electrochemically gave F+ ) 3.0 kcal mol-1 12
, thus showing a
behavior similar to the O-C bond and intermediate between
the O-H and C-H bonds referred to above.
This well-buttressed and beautiful picture relating the direc-
tion and magnitude of the effect of Y on Z-X BDE’s in
compounds of the general formula 4-YC6H4Z-X to the differ-
ences in the electronegativities of Z and X, i.e., to ground-state
polar effects, began to unravel in 1997. In that year, Laarhoven
et al.9 reported a reexamination of 4-YC6H4CH2-Br BDE’s (item
3 above) using both PAC (that had earlier10 yielded F+ ) -5.5
kca1mol-1) and a gas-phase pyrolytic method. Both procedures
indicated that Y substituents had no experimentally detectable
effect on the benzylic C-Br BDE’s. Since the experimental
approach to 4-YC6H4CH2-Br BDE’s gave inconsistent data, we
turned to theory.4 Using density functional theory (DFT) model
calculations with the B3LYP functional and a locally dense basis
set, the BDE’s in 4-YC6H4CH2-X with X ) H, F, Cl, Br were
calculated. From this study it was found that Y has only a very
minor (<2 kca1mol-1) effect on 4-YC6H4CH2-Br BDE’s (∆ø
) -0.3). More significantly, for 4-YC6H4CH2-Cl (∆ø ) -0.5)
and 4-YC6H4CH2-F (∆ø ) -1.5), the effects of Y on these
carbon-halogen BDE’s were found to be not only very small
but also quite similar to their effects on C-Br BDE’s. For
Experimental Section
Method of Calculation. This has been described elsewhere.5c,14 In
short, geometry optimizations and vibrational frequency calculations
were performed using the semiempirical AM1 method,15 and zero-point
energies were scaled by 0.973.5c At the AM1 minima, B3LYP16,17
single-point energies were obtained for phenols and anisoles (hereafter
referred to as DFT I), and B3P8616,18 single-point energies were
calculated for anisoles and benzyl phenyl ethers (DFT II). A restricted
open-shell Hartree-Fock guess was used for the open-shell (radical)
calculations5c with a locally dense basis set:19 primary ) O-X,
6-311+G(2d,2p) (B3LYP) and 6-311G(d,p) (B3P86); secondary )
aromatic carbons, 6-311+G(d) (B3LYP) and 6-311G(d) (B3P86);
tertiary ) all remaining atoms, 6-31G(d). The electronic energy of the
hydrogen atom was set to its exact value of -0.5 hartree.5c Vibrational
frequencies for anisoles, used in the RRKM computation of the
Arrhenius parameters at the high-pressure limit (vide infra), were
obtained with (RO)B3LYP/6-31G(d,p)//B3LYP/6-31G(d,p)(DFT III).
Enthalpies of the species involved were differenced to give the gas-
phase bond dissociation enthalpy (BDE), i.e., for X-Y f X• + Y•,
∆H298° ) {H298°(X•) + H298°(Y•)} - H298°(X-Y).
Thermolysis of Benzyl Phenyl Ethers. The decomposition of benzyl
phenyl ethers was studied using 9,10-dihydroanthracene (9,10-DHA)
as the hydrogen atom donating solvent. For each benzyl phenyl ether
five glass ampules (ca. 2 mL in volume) were prepared. The ether (0.25
mmol) was added to a clean ampule that was then filled to its neck
with solid, finely divided 9,10-DHA and sealed under vacuum after
three or more cool-pump-heat melting cycles. The ampules were
placed in a temperature-controlled GC oven. Ampules were removed
at known times, cooled to room temperature, opened, and their contents
dissolved in acetone (50 mL) containing 0.25 mmol of anisole as
external standard. Products were identified by GC/MS (at 70 eV) and
quantified by GC/FID (average of three injections). The detailed results
of these experiments are given as Supporting Information.
+
example, comparing the strongest ED group studied, -NH2 (σp
+
) -1.30), with the strongest EW group studied, -NO2 (σp
)
0.79), the calculated ∆BDE values, BDE(4-H2NC6H4CH2-X)
- BDE(4-NO2C6H4CH2-X), were 1.6, 1.3, 1.4, and 0.5 kcal
mol-1 for X ) H, Br, Cl, and F, respectively, i.e., actually
smaller for F than for H, Br, and Cl. Obviously, polar effects
on carbon-halogen BDE’s in benzyl halides are of only minor
significance. More importantly, and in view of the large range
in Z-X electronegativity differences covered in this study4 (∆ø
from -1.5 to 0.4), the idea that the sign and magnitude of the
effects of Y on Z-X BDE’s could be correlated with ∆ø (Z-
X) would have to be discarded.
What then determines the effect of Y on 4-YC6H4Z-X BDE’s?
The obvious answer is the stabilization/destabilization by Y of
the 4-YC6H4Z• radical. If this is true, differences in BDE’s for
4-YC6H4O-X compounds caused by changes in Y should be
largely independent of X, just as we have demonstrated for
4-YC6H4CH2-X4 (see above). Clearly, therefore, the experi-
mental results regarding the effects of Y on 4-YC6H4O-CH3
BDE’s11 (item 4 above) are anomalous (F+ ) 3.0 kcal mol-1).
(7) Various estimates of the effects of Y-substituents on the benzylic
C-H BDE in toluene have been made from kinetic data for the abstraction
of the benzylic H-atom by free radicals, but it is perfectly clear that kinetic
polar effects play a role in these atom transfer reactions because the
magnitude of F or F+ depends on the nature of the attacking radical. See,
for example: Howard, J. A.; Chenier, J. H. B. J. Am. Chem. Soc. 1973, 95,
3054-3055, Pryor, W. A.; Church, D. F.; Tang, F. Y.; Tang, R. H. Frontiers
of Free Radical Chemistry; Pryor, W. A., Ed.; Academic Press: New York,
1980; p 355-379. Also: Zavitsas, A. A.; Fogel, G.; Halwagi, K. E.;
Donnaruma Legotte, P. A. J. Am. Chem. Soc. 1983, 105, 6960-6962.
(8) Suryan, M. M.; Stein, S. E. J. Phys. Chem. 1989, 93, 7362-7365.
(9) Laarhoven, L. J. J.; Born, J. G. P.; Arends, I. W. C. E.; Mulder, P.
J. Chem. Soc., Perkin Trans. 2 1997, 2307-2312.
Very Low-Pressure Pyrolysis. The VLPP instrument and the
experimental procedures have been described.20 In short, the reactants
(12) Jonsson, M.; Lind, J.; Eriksen, T. E.; Merenyi, G. J. Am. Chem.
Soc. 1994, 116, 1423-1427.
(13) Wu, Y.-D.; Lai, D. K. W. J. Org. Chem. 1996, 61, 7904-7910.
(14) DiLabio, G. A.; Pratt, D. A. J. Phys. Chem. A 2000, 104, 1938-
1943.
(15) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J.
Am. Chem. Soc. 1985, 107, 3902-3909.
(16) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
(17) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785-789.
(18) Perdew, J. P. Phys. ReV. B 1986, 33, 8822-8824.
(19) DiLabio, G. A. J. Phys. Chem. A 1999, 103, 11414-11424.
(10) Clark, K. B.; Wayner, D. D. M. J. Am. Chem. Soc. 1991, 113, 9363-
9365.
(11) (a) Suryan, M. M.; Kafafi, S. A.; Stein, S. E. J. Am. Chem. Soc.
1989, 111, 4594-4600. (b) This value of F+ was taken directly from Figure
2 in ref 11a.