Mechanism of the Aerobic Oxidation of a-Pinene
thermostat, equipped with an immersion heater and thermocouple
(standard run at 363ꢂ2 K). Samples (ꢂ250 mL) were withdrawn
from the reactor and analyzed by gas chromatography (GC)
(HP6890; HP-5 column, 30 m/0.32 mm/0.25 mm; flame ionization
detector). n-Nonane (1 mol%, Sigma Aldrich, >99%) was added to
the a-pinene substrate (Sigma Aldrich, 98%, devoid of stabilizers)
and used as an inert internal standard. The hydroperoxide yields
were determined via a double injection, with and without reduc-
tion of the reaction mixture by trimethylphosphine (1m in toluene,
Sigma Aldrich). From the obtained augmentation in alcohol con-
tent, the corresponding hydroperoxide yield was determined.
Product identification was done with GC–MS using both split injec-
tion (Tinject =2508C) and cool-on-column injection (Tinject =508C) to
verify the thermal stability of the products. No difference in prod-
uct distribution could be observed.
Quantum chemical calculations were performed with Gaussian03
software[38] at the UB3LYP/6-311++G(df,pd)//UB3LYP/6-31G(d,p)
level of theory.[39] Earlier, this method was validated against several
benchmark levels of theory (G2M, G3 and CBS-QB3) for H abstrac-
tion reactions by peroxyl radicals.[6] The reported relative energies
of the stationary points on the potential energy surfaces (PESs),
(the energy barriers Eb and reaction energies DE) were corrected
for zero point energy (ZPE) differences. Rate constants of elemen-
tary reactions were estimated by TST, in terms of the complete par-
tition functions of the transition state (TS) and the reactant(s) and
product(s) as well as their relative energy difference, Eb. For certain
reactions, featuring loose TSs with hindered internal rotations,
known prefactors were combined with the computed barriers. In
those cases, this procedure results in a more accurate estimation
of the rate constant than relying on conventional TST calculations
where all internal motions were treated as harmonic oscillations to
compute the pre-factor.[6]
[15] C. A. Tolman, J. D. Druliner, M. J. Nappa, N. Herron, in Activation and
Functionalization of Alkanes (Ed.: C. L. Hill), Wiley-VCH, Weinheim, 1989,
pp. 303.
[16] K. G. Fahlbusch, F. J. Hammerschmidt, J. Panten, W. Pickenhagen, D.
Schatkowski, Flavors and Fragrances, in Ullmann’s Encyclopedia of Indus-
trial Chemistry, Wiley-VCH, Weinheim, 2005.
[17] J. P. Vitꢂ, W. Francke, Chem. Unserer Zeit 1985, 19, 11.
[19] a) J. E. Ancel, N. V. Maksimchuk, I. L. Simakova, V. A. Semikolenov, Appl.
slitsyn, I. N. Klabukova, A. N. Trofimov, Chem. Plant Raw Mater. 2004, 3,
109–116.
[20] S. G. Bell, X. Chen, R. J. Sowden, F. Xu, J. N. Williams, L. L. Wong, Z. Rao,
[21] M. J. da Silva, P. Robles–Dutenhefner, L. Menini, E. V. Gusevskaya, J. Mol.
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[26] N. V. Maksimchuk, M. S. Melgunov, J. Mrowiec–Białon, A. B. Jarze˛bski,
[27] A comparison of mass spectrometry data with commercially available
(S)-cis-verbenol (Sigma Aldrich, 95%) reveals that the synthesized ver-
benol shows slightly different fragmentation intensities for the follow-
ing m/z values: 55, 59, 79, 81, 91, and 94. The synthesized verbenol can
therefore not purely consist of the cis-diastereomer. This observation is
in agreement with the B3LYP/6-31G(d,p) calculated energy difference
for the diastereo-determining intermediate radicals: E(cis-R(a)OO·)-
E(trans-R(a)OO·)=0.4 kcalmolꢁ1, which is small and could generate a
small diastereomeric excess of the trans oxidation products.
[29] W. Tsang, J. Phys. Chem. Ref. Data 1991, 20, 221.
Acknowledgements
The authors kindly acknowledge financial support from the Swiss
National Science foundation and the ETH Zurich.
[31] The value for the Henry coefficient of O2 in a-pinene used in the text
(35 mmbarꢁ1) is the arithmetic mean of the value we measured for N2
in a-pinene (30 mmbarꢁ1; 0–100 bar) and the National Institute of
Standards and Technology (NIST) recommended value for O2 in b-
pinene (40 mmbarꢁ1). See R. Sander, “Henry’s Law Constants”, in NIST
Chemistry WebBook, NIST Standard Reference Database Number 69
(Eds. P. J. Linstrom, W. G. Mallard), National Institute of Standards and
Technology, Gaithersburg, 2008 . See also http://webbook.nist.gov (ac-
cessed November 2009).
Keywords: autoxidation · epoxidation · oxidation · quantum
chemistry · radical reactions
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[32] Z. Alfassi, The Chemistry of Free Radicals (Ed.: Z. Alfassi), Wiley, West
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[4] N. M. Emanuel, E. T. Denisov, Z. K. Maizus, Liquid Phase Oxidation of Hy-
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[5] The characteristic lifetime t of ROO· radicals is given by 1/{2kterm[ROO·]}.
Specifically for the case of a-pinene oxidation at 363 K, [ROO·] is already
as high as 1.5ꢀ10ꢁ7 m [derived experimentally from Equation (9)] at
[34] L. Zhang, K. A. Kitney, M. A. Ferenac, W. Deng, T. S. Dibble, J. Phys.
0.5% conversion; hence
t
is as low as 1–5 s, given 2kterm ꢆ6ꢀ
[35] Pseudo-first-order rate constant=3ꢀ2.0ꢀ108 mꢁ1
s
ꢁ1 ꢀexp(ꢁ3.5 kcal
106 mꢁ1 ꢁ1. This ROO· lifetime is much shorter than the timescale of sev-
s
molꢁ1)ꢀ[RH], given the reaction path degeneracy of three and [RH]=
6.3m.
eral minutes over which [ROO·] changes significantly, such that a [ROO·]
quasi-steady-state will be established immediately and maintained
throughout the reaction.
[38] Gaussian 03, Revision B.03, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E.
Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven,
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ChemSusChem 2010, 3, 75 – 84
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