Characteristics of cationic phase-transfer catalysts in the oxidation of
hydrocarbons by O
2
Laszlo J. Csanyi* and Karoly Jaky
Department of Inorganic and Analytical Chemistry, University of Szeged, P.O. Box 440,
H-6701 Szeged, Hungary. E-mail: ljcsanyi=chem.u-szeged.hu; Fax: 36 62 420505;
T el: 36 62 544000 ext. 3574
Received 14th November 2000, Accepted 3rd April 2001
First published as an Advance Article on the web 3rd May 2001
The oxidation of tetralin and cyclohexene by O was investigated in the presence of cationic phase-transfer
2
catalysts (PTCs). It was found that the oxidation takes place analogously to the recently investigated catalysed
decompositions of hydroperoxide initiator molecules. The natures of both the onium cation and the
counteranion are determining factors. The catalytic activity of the onium salt is determined by the e†ective
charge on the onium ion and by the size and polarizability of the anion. For both hydrocarbons, the primary
product of oxidation is the corresponding hydroperoxide, which may undergo further oxidation. For tetralin,
tetralyl hydroperoxide underwent disproportionation into O and tetralol, and reuse of the O thus produced
2
2
resulted in a considerable ““overoxidationÏÏ in the closed reactor. Tetralone was formed in a smaller amount.
The main products in the oxidation of cyclohexene were its hydroperoxide, cyclohexene oxide and
2-cyclohexen-1-ol. In contrast to the oxidation of tetralin, ““overoxidationÏÏ did not occur here, and the
formation of 2-cyclohexen-1-one was never observed. The oxidations of these hydrocarbons in the presence of
cationic PTCs proved to be strongly inÑuenced by the concentration of homogeneously dissolved water. The
oxidation products of these hydrocarbons also exerted considerable inÑuence on the progress of the oxidation.
For many experiments (Tables 1È3), stock solutions of the
Introduction
given PTCs were prepared in chlorobenzene and stored at
room temperature in the dark. In others (Tables 5 and 6), ali-
quots of these catalyst stock solutions were mixed with an
equal volume of water adjusted to the given pH, and the
mixture was stirred efficiently at 3000 rpm for at least 10 min.
Then, after separation of the phases, the organic phase was
Ðltered on a Whatman 1PS silicone-treated phase separator
and stored similarly at room temperature and in the dark.
These catalyst solutions di†er in their homogeneously dis-
solved water contents. Analogous data can be found in Table
1 in ref. 3.
The oxidation was carried out in the dark in a ther-
mostatted, magnetically well-stirred batch reactor (Ðtted with
a reÑux condenser cooled with water to 10 ¡C). A reaction
mixture containing 2.00 or 3.00 cm3 hydrocarbon and an
appropriate volume of catalyst solution was made up to 8.00
It was recently reported1 that cationic PTCs interact with the
nucleophilic inner O atom of tert-butyl hydroperoxide (t-
BHP), or other hydroperoxide initiator molecules. This results
in the decomposition into radicals when water is also present
in the system, and the latter simultaneously attacks the outer
O atom of the hydroperoxide by H-bond formation. The yield
of radicals depends on the nature of the quaternary centre of
the cationic PTC, the charge density of which is always con-
trolled by the given counteranion. The present paper discusses
how the oxidation of tetralin (T) and cyclohexene (Ch) takes
place in the presence of either di†erent cationic PTCs with a
common anion, or a common onium ion with di†erent anions.
It is also demonstrated how the progress of oxidation is inÑu-
enced by the oxidation products of these substances and by
the presence of water.
cm3 with chlorobenzene, and saturated with dried O for 3
2
min at room temperature. The reaction vessel was next con-
nected to the thermostat preheated to 70 ¡C and, after a
warming up period, of exactly 3 min, the reactor was attached
Experimental
Materials: The PTC reagents and t-BHP were Fluka pro-
ducts and were used without any puriÐcation. Cyclohexene
and 1,2,3,4-tetrahydronaphthalene (T) were also Fluka pro-
ducts; for removal of their peroxidic impurities, they were
passed over an activated Al O column, then distilled twice
to a gas burette of syringe type Ðlled with O , Ðtted with a
2
temperature control, and recording of the O uptake was
2
simultaneously started. The gas measuring device automati-
cally regulates the inside pressure at the atmospheric level.
After a net conversion time of 120 min (without the time lag),
2
3
under a N blanket and stored in the dark, in a refrigerator.
the reaction products were estimated by iodometry (O , the
2
act
Chlorobenzene was applied as non-polar solvent. Its puriÐ-
hydroperoxide content) and by gas chromatograpy (the -ol,
cation is described in ref. 2. Cyclohexene oxide (Ch-O), 2-
cyclohexen-1-ol (Ch-ol) and 2-cyclohexen-1-one (Ch-one) were
Fluka products; and puriÐed by distillation. Fluka 3,4-
dihydro-1(2H)-naphthalenone (T-one) was distilled at 138 ¡C,
at 15 mm Hg. 1,2,3,4-Tetrahydro-1-naphthol (T-ol) was rec-
rystallized four times from petroleum ether (30È60 ¡C).
-one and -epoxide contents). The column for Ch was 2 m ] 4
mm id Ðlled with Chromosorb W-AW-DMCS coated with
Carbowax 20 M; carrier gas: N at 40 cm3 min~1, detector:
2
FID. The column for T was 1 m ] 4 mm id Ðlled with
Chromosorb W coated with 20% LAC IR 296; carrier gas:
N
at 40 cm3 min~1; detector: FID. Reaction products were
2
2018
Phys. Chem. Chem. Phys., 2001, 3, 2018È2024
This journal is ( The Owner Societies 2001
DOI: 10.1039/b009145f