Absolute Rates of Intermolecular C-H Abstraction Reactions
J. Am. Chem. Soc., Vol. 121, No. 32, 1999 7341
The data for this study can be found in Table 9 in the Supporting
Information.
(100 µL), C-H source (400-500 µL), and dimethylphenylsilane-d (150
µL) were introduced by syringe into a nitrogen-flushed NMR quartz
tube sealed with rubber septa and secured with Parafilm tape. The tube
was frozen in liquid nitrogen and subjected to three freeze-pump-
thaw cycles followed by pressurization with nitrogen. The tube was
subjected to UV photolysis in a Rayonet reactor at room temperature
overnight. The reaction mixture was distilled to give fraction with bp
< 90 °C free of silane-d. It was dissolved in CCl4 for 2H NMR analysis.
The selectivity of hydrogen abstraction was determined by integration
of the deuterium resonances corresponding to the different sites of
deuterated product.
Reduction of Bromomethyl Methyl Ether. Bromomethyl methyl
ether (0.12 g), dimethylphenylsilane-d (0.24 g), benzene-d6 (0.4 g), and
tert-butyl peroxide (0.03 g) were delivered in a nitrogen-flushed NMR
Pyrex tube sealed with rubber septa and secured with Parafilm tape.
The tube was frozen in liquid nitrogen and subjected to three freeze-
pump-thaw cycles followed by pressurization with nitrogen. The tube
under vacuum was sealed with a burner and heated to 115 °C in an oil
bath overnight to give complete conversion of the starting ether.
Dimethyl ether-d, 1H NMR δ 3.09 (t, 2H, J ) 1.5 Hz, CH2D), 3.11 (s,
3H, CH3), was formed in 82% yield by 1H NMR. In a similar reaction
in a Pyrex tube using UV light and tert-butyl peroxide (6 mol %) at
room temperature for 3 days, 17% conversion and 80% yield of
dimethyl ether-d were observed.
Competition of H versus D Abstraction from tert-Butyldimeth-
ylsilane and tert-Butyldimethylsilane-d17 for i-C3F7 Radical. The
reactions were initiated with di-tert-butylhyponitrite38 at 37 °C. The
overall concentration of the silanes was kept constant, and only their
relative concentration was changed over the set of six tubes. Ratios of
i-C3F7H/i-C3F7D were determined by the ratio of integrals of CFH and
CFD resonances in the 19F NMR at -215.0 and -215.6 ppm,
respectively. The kD ) 2.4 ((0.4) × 106 M-1 s-1 was then calculated
from knowledge of the rate of H-abstraction from t-BuMe2SiH which
was determined in the previous experiment.
The data for this study can be found in Table 10 in the Supporting
Information.
General Procedure for Competition Experiment of Hydrogen
Abstraction from C-H Source versus Deuterium Abstraction from
tert-Butyldimethylsilane-d for Nonafluorobutyl Radical. Each of six
NMR tubes provided with the capillary glass tube containing solution
of CFCl3 in C6D6 as an external standard for measuring 19F NMR was
flushed with nitrogen and sealed with rubber septa. 1-Iodononafluoro-
butane, tert-butyldimethylsilane-d, and a hydrogen containing source
were inserted by syringe. Each tube was filled with 1,3-bis(trifluoro-
methyl)benzene to the constant volume. Di-tert-butylhyponitrite38 was
added last. Concentration of the reactants was determined according
to the weight ((0.001 g). The volume of the silane was kept constant
through the set of six tubes, while the concentration of the C-H source
was different for every sample. Each tube was secured with Parafilm
tape, frozen in liquid nitrogen, and subjected to three successive freeze-
pump-thaw cycles followed by pressurization with nitrogen and
warming up to room temperature. The reaction was initiated with di-
tert-butylhyponitrite at 34-38 °C until sufficient consumption of
starting material was monitored by 19F NMR (usually ∼24 h). Product
Computational Methods
Density functional theory calculations were performed using the
Gaussian98W program package.39 All substances and the radicals
formed by H atom abstraction were optimized at Becke-style 3-Param-
eter (B3LYP)40 density functional theory (DFT) level of theory using
the 6-31G(d) basis set. Vibration frequencies were calculated at the
same level of theory; all thermal energies were correct by factor 0.9804.
Single point energy calculations were performed at the B3LYP level
of theory using the 6-311+G(3df,2p) basis set (B3LYP/6-311+G(3df,
2p)//B3LYP/6-31G(d)).
Bond dissociation energy is defined as the enthalpy (∆H°) of eq 2.
The enthalpy is obtained by the thermal correction of electronic energy
of this equation to 298.15 °C, 1 atm, and then further corrected by
∆PV which is RT in this case.
ratios for each reaction mixture were determined by integration of 19
F
NMR resonances at -138.7 (dm, J ) 54 Hz, CF2H) and -139.4 (m,
CF2D) ppm. Total yields of products n-C4F9H and n-C4F9D were
generally 80-95% and were determined by the integration of fluori-
nated products and starting materials versus the CFCl3 resonance before
and after the reaction. Since loss of either of the two products is
unlikely, even in cases where yields were lower, such ratios were still
considered an accurate measure of the relative rates of H- versus
D-abstraction.
R-H f R• + H•
(2)
The slopes obtained from plotting [CF2H]/[CF2D] versus [C-H
source]/[silane] provided the kH/kD ratio. The known kD value was then
used to determine the values of kH given in Table 2.
Kinetic data for these studies can be found in Tables 14-36 in the
Supporting Information.
Acknowledgment. Support of this research in part by the
National Science Foundation, including a GOALI grant, is
acknowledged with thanks. This paper is dedicated with pleasure
in commemoration of the 70th birthday of Keith U. Ingold, a
consummate scientist and pioneering contributor to the field of
radical chemistry.
Procedure for n-C4H9CF2CF2• Radical. The general procedure was
the same as that for the n-C4F9• radical. The ratios of n-C4H9CF2CF2H/
n-C4H9CF2CF2D for hydrogen abstraction from substrates versus
deuterium abstraction from t-BuMe2SiD were determined by integration
of the CF2H δF -137 (d, J ) 54 Hz) and CF2D δF -137.7 (t, J ) 8
Hz) resonances in the 19F NMR spectra of the 1,1,2,2-tetrafluorohexane
product.
Supporting Information Available: Tables of kinetic data
(PDF). This material is available free of charge via the Internet
JA990521Q
Kinetic data for these studies can be found in Tables 37-39 in the
Supporting Information.
(38) Mendenhall, G. D. Tetrahedron Lett. 1983, 24, 451-454.
(39) Gaussian 98, Revision A.3, Frisch, M. J.; Trucks, G. W.; Schlegel,
H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.;
Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam,
J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;
Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman,
J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith,
T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.;
Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.;
Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J.
A. Gaussian, Inc., Pittsburgh, PA, 1998.
•
Procedure for the i-C3F7 Radical. The procedure is the same as
that used for the n-C4F9• radical. The ratios of i-C3F7H/i-C3F7D obtained
for hydrogen abstraction from substrates versus deuterium abstraction
from t-BuMe2SiD were determined by integration of the product CFH
and CFD resonances at -215 and -215.6 ppm, respectively, in the
19F NMR spectra of the 1,1,1,2,3,3,3-heptafluoropropane products.
The kinetic data for these studies can be found in Tables 11-13 in
the Supporting Information.
•
Regioselectivity of Hydrogen Abstraction by n-C4F9 from Dif-
ferent C-H Sources. In a typical experiment 1-iodononafluorobutane
(37) Avila, D. V.; Ingold, K. U.; Lusztyk J.; Dolbier, W. R., Jr.; Pan, H.
Q. Tetrahedron 1996, 52, 12351-12356
(40) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.