Solvent Effects on the HOOC + HOOC Reaction
FULL PAPER
ried out on a Hewlett-Packard 5890 instrument interfaced to a Hewlett-
Packard 5971 A Mass Selective Detector (DB-5 capillary column, 30 mꢀ
i d d
[15] The rate of initiation is R =2ek [AIBN], where k is the rate con-
stant for thermal decomposition of AIBN and e represents the frac-
0
.25 mm, film thickness 0.25 mm). Finally, HPLC-UV-DAD analyses of
solutions containing g-terpinene and CF COOH were done on a Waters
Instrument model 1525 connected with Waters model 996 PDA detector
column: Phenomenex Luna, C18 250ꢀ4.6 mm (5 mm) at 208C using as
eluent system H O/CH CN).
tion of geminate radical pairs that diffuse out of the solvent cage in
which they are produced. Values of 2ek (in s ) at 508C are: 1.3ꢀ
d
ꢀ
1
3
ꢀ
6
[16]
ꢀ6
[16]
10 in cyclohexane, 2.0ꢀ10 in acetonitrile, and about 2.5 ꢀ
ꢀ
6
[17]
(
10 in tert-butyl alcohol.
[16] M. C. Foti, G. Ruberto, J. Agric. Food Chem. 2001, 49, 342–348.
2
3
ꢀ
6
ꢀ1
Peroxidation of g-terpinene: Solutions of g-terpinene and AIBN at vari-
[17] Although not directly reported, the mean value 2ek
d
=2.5 ꢀ10
s
ous concentrations in the solvent in use were mixed 1:1 (v/v) in a UV
in tert-butyl alcohol at 508C can be calculated from data in: a) M.
Lucarini, G. F. Pedulli, L. Valgimigli, J. Org. Chem. 1998, 63, 4497–
4499; b) P. Pedrielli, L. M. Holkeri, L. H. Skibsted, Eur. Food Res.
Technol. 2001, 213, 405–408.
cuvette saturated with
2 2
O /N mixtures (prepared with a gas mixer
system), hermetically sealed, and quickly heated to 508C. Thereafter, the
cell compartment was maintained at 508C and the absorbance at 272 nm
[
5]
was monitored over time as previously described. Excellent straight
[18] J. L. Bolland, Q. Rev. Chem. Soc. 1949, 3, 1–21; L. Bateman, Q.
Rev. Chem. Soc. 1954, 8, 147–167.
2
lines (R =0.97–0.99) of absorbance versus time were usually obtained,
whose slopes (divided by the value of e272) gave the initial rate of reac-
[19] Because of the fast reaction of dioxygen with TC [reaction (5)], it is
obvious that the steady-state concentration of TC will be much lower
than that of HOOC.
ꢀ
1
tion, d[Cy]/dt in ms units. UV cells of 1, 0.5, and 0.1 cm optical path
[
12]
were used according to the initial absorbance of the solution. The reac-
tion orders for AIBN, g-terpinene, and O were obtained by changing the
2
[
20] a) D. V. Avila, C. E. Brown, K. U. Ingold, J. Lusztyk, J. Am. Chem.
Soc. 1993, 115, 466–470; b) L. Valgimigli, K. U. Ingold, J. Lusztyk, J.
Org. Chem. 1996, 61, 7947–7950; c) D. W. Snelgrove, J. Lusztyk,
J. T. Banks, P. Mulder, K. U. Ingold, J. Am. Chem. Soc. 2001, 123,
concentration of the pertinent species while keeping the others constant.
4
69–477; d) L. Zhang, J. Cradlebaugh, G. Litwinienko, B. E. Smart,
K. U. Ingold, W. R.Dolbier Jr., Org. Biomol. Chem. 2004, 2, 689–
94.
Acknowledgements
6
We thank the MIUR (PRIN research program) for financial support. We
also thank Dr. Carmelo Daquino (ICB-CNR Sez. Catania) for his valua-
ble help in the HPLC-MS analyses.
[21] M. H. Abraham, P. L. Grellier, D. V. Prior, J. J. Morris, P. J. Taylor, J.
Chem. Soc. Perkin Trans. 2 1990, 521–529.
[
22] K. Adamic, J. A. Howard, K. U. Ingold, Can. J. Chem. 1969, 47,
803–3808, and references therein.
23] D. J. McKay, J. S. Wright, J. Am. Chem. Soc. 1998, 120, 1003–1013.
3
[
[
1] V. Formacek, K.-H. Kubeczka in Essential Oils Analysis by Capillary
Gas Chromatography and Carbon-13 Spectroscopy, Wiley, New
York, 1985; F. Senatore in Oli Essenziali, EMSI, Rome, 2000.
2] G. Ruberto, M. T. Baratta, Food Chem. 2000, 69, 167–174.
3] J. Grassmann, S. Hippeli, E. F. Elstner, Plant Physiol. Biochem.
[24] J. A. Howard, K. U. Ingold, Can. J. Chem. 1967, 45, 785–792.
[25] B. H. J. Bielski, Photochem. Photobiol. A 1978, 28, 645–649.
[
26] Assuming that the Arrhenius preexponential factor for these radi-
[
[
9
ꢀ1 ꢀ1
cal–radical reactions is about 10 m
constants imply that E
s
, the room-temperature rate
ꢀ1
a
is about 5 kcalmol , which would produce a
2
002, 40, 471–478.
roughly twofold increase in the rate constants at 508C. This 5 kcal
[
4] G. A. Russell, J. Am. Chem. Soc. 1955, 77, 4583—4590; G. A. Rus-
sell, J. Am. Chem. Soc. 1956, 78, 1047—1054; D. G. Hendry, G. A.
Russell, J. Am. Chem. Soc. 1964, 86, 2371—2374; G. W. Burton,
K. U. Ingold, Science 1984, 224, 569–573.
ꢀ1
mol activation enthalpy can be assigned to the enthalpy of forma-
tion of the hydrogen bond between the HOOC radical and these
HBA solvents.
[
[
27] G. Litwinienko, K. U. Ingold, J. Org. Chem. 2003, 68, 3433–3438.
28] M. C. Foti, C. Daquino, C. Geraci, J. Org. Chem. 2004, 69, 2309–
[
[
[
5] M. C. Foti, K. U. Ingold, J. Agric. Food Chem. 2003, 51, 2758–2765.
6] S. Sortino, S. Petralia, M. C. Foti, New J. Chem. 2003, 27, 1563–1567.
7] Two different TC radicals are formed by hydrogen-atom abstraction
2
314.
[
[
29] G. Litwinienko, K. U. Ingold, J. Org. Chem. 2004, 69, 5888–5896.
2
from g-terpinene CH groups and these can form, four different per-
30] The 120-fold increase in the termination rate constant may be a
oxyl radicals with conjugated double bonds and two with non-conju-
gated double bonds. These last two radicals probably undergo ex-
ꢀ
slight overestimate since more O
2
C
means less HOOC and hence less
ꢀ
chain propagation because O
2
C
cannot abstract a hydrogen atom
[
6,8]
tremely rapid reversion to TC and O
8] D. A. Pratt, J. H. Mills, N. A. Porter, J. Am. Chem. Soc. 2003, 125,
801–5810 and references therein.
9] The similarity between reaction (1) and the autoxidation of 1,4-cy-
2
.
from g-terpinene.
[
[
[
31] High concentrations of acetic acid produced a small decrease in the
5
rate of g-terpinene autoxidation. This reduction was shown not to
[
10]
be due to a change in the rate of initiation, R
i
, since the rates of dis-
clohexadiene to form benzene and hydrogen peroxide, the ability
ꢀ
4
[
5]
appearance of hydroquinone (6.45ꢀ10 m) in oxygenated aceto-
of g-terpinene to retard the autoxidation of linoleic acid, and the
ꢀ
2
[
11]
nitrile at 508C containing AIBN (1.31ꢀ10 m) were the same in the
presence of 0.9m AcOH as in the absence of acid, namely, 2.46ꢀ
abilities of FeCl
3 2
and CuCl to retard g-terpinene autoxidation
also support the conclusion that HOOC is the chain carrier.
ꢀ
8
ꢀ8
ꢀ1
1
0
and 2.40ꢀ10 ms , respectively.
[
[
[
10] J. A. Howard, K. U. Ingold, Can. J. Chem. 1967, 45, 785–792.
11] M. C. Foti, K. U. Ingold, J. Org. Chem. 2003, 68, 9162–9165 .
12] Although g-terpinene was carefully distilled, it contained significant
quantities of p-cymene which precluded the spectrophotometric
method for monitoring g-terpinene autoxidation at [TH]>1m.
[
32] Addition of 150 mL of CF CO H ([CF CO H]=0.94m) to 2.0 mL of
3
2
3 2
a 60 mm g-terpinene solution at 508C caused an increase of the ab-
sorbance at 272 nm of about 0.006 units per second. When the quan-
3 2 3 2
tity of CF CO H was reduced to 20 mL ([CF CO H]=0.125m), the
ꢀ
4
absorbance increased to a rate of about 3ꢀ10 units per second.
[
13] For p-cymene in tert-butyl alcohol at 508C,
e
(272 nm)=
ꢀ
1
ꢀ1
After adding AIBN to a final concentration of 3.2 mm, this rate fur-
5
00m cm . The e values in acetonitrile and cyclohexane and fur-
ꢀ3
ther increased to 1.3ꢀ10 units per second. In the absence of
ther experimental details have been reported in reference [5].
CF
3
CO
2
H, the rate of AIBN-induced peroxidation of g-terpinene
[
14] “Spontaneous”, oxygen-induced autoxidation of neat g-terpinene at
ꢀ
8
ꢀ1
ꢀ3
5
08C under air occurs with a rate of about 10 ms which is more
was about 1.0ꢀ10 absorbance units per second.
than 100 times slower than the rates of the AIBN-initiated autoxida-
tion at much lower g-terpinene concentrations. Interestingly, the
spontaneous autoxidation gave a very minor amount (~3% of the
yield of p-cymene) of 1,2,4,5-tetrahydroxy-1-isopropyl-4-methyl-
cyclohexane. The properties of this unusual product and the mecha-
nism by which it is formed will be reported later.
[33] 1,3-Cyclohexadiene has an absorption band at 259 nm and emax =
ꢀ
1
ꢀ1
3200m cm . Exocyclic conjugated dienes are characterized by
3
ꢀ1
ꢀ1 [16]
l
max <259 nm and emax >3ꢀ10 m cm .
[34] P. D. Lightfoot, B. Veyret, R. Lesclaux, Chem. Phys. Lett. 1988, 150,
120–126; T. J. Wallington, P. Dagaut, M. J. Kurylo, Chem. Rev. 1992,
92, 667–710.
Chem. Eur. J. 2005, 11, 1942 – 1948
ꢃ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1947