A Kinetic Study of 2-Ethyl-1-hexanol Oxidation by Dichromate Using Clay-Supported 1-Butyl 4-aza-1-azonia Bicyclo[2.2.2]octane Chloride as the Phase-Transfer Catalyst

Article abstract of DOI:10.1021/op034120d

Selective oxidation of primary alcohols to aldehydes is a long-standing problem of organic chemistry (Bueler, C. A.; Pearson, D. E. Survey of Organic Synthesis; Wiley-Interscience: New York, 1977; Vol. 2, p 480; House, H. O. Modern Synthetic Reactions, 2nd. ed.; W. A. Benjamin: Menlo Park, California, 1972; p 257; Epstein, W. W.; Sweet, F. W. Chem. Rev. 1967, 67, 247; Landini, D. ; Montanari, F.; Rolla, F. Synthesis 1979, 134). The use of potassium dichromate as a synthetically useful oxidizing agent is reported for the oxidation of an industrially important lipophilic alcohol, employing modified clay as the phase-transfer catalyst. The phase-transfer catalysis results in nearly complete oxidation of the 2-ethylhexanol in 40 min at room temperature, with high selectivity to the 2-ethylhexanal, compared to 48 h in its absence. Kinetic studies show the reaction occurs via transfer of Cr₂O ₇^2- into the organic phase. The emphasis will be on simplicity of the condition as a preparative organic method, selectivity with regard to over-oxidation, efficiency, and mildness of conditions.

Full text of DOI:10.1021/op034120d

Organic Process Research & Development 2003, 7, 936−938
A Kinetic Study of 2-Ethyl-1-hexanol Oxidation by Dichromate Using
Clay-Supported 1-Butyl 4-aza-1-azonia Bicyclo[2.2.2]octane Chloride as the
Phase-Transfer Catalyst
M. Ghiaci,* R. J. Kalbasi, and M. E. Sedaghat
Department of Chemistry, Isfahan UniVersity of Technology, Isfahan, Iran, 84156
Abstract:
in catalyzing aqueous-organic biphase reactions. Interest in
such phase-transfer and related two-phase catalytic tech-
niques continues to grow in synthetic and mechanistic
organic chemistry.14,15 It seems that the development of a
technique which uses insoluble catalysts to accelerate aque-
ous-organic phase reactions would not only be an interesting
possibility but would also reinforce claims16-18 that the future
of synthetic organic chemistry rests with heterogeneous
media rather than the currently predominant homogeneous
systems. Modified-clay constitutes one such medium.
Our main purpose in this study was to examine the
usefulness of surfactant-modified clay for oxidation of an
industrially important lipophilic alcohol, i.e., 2-ethyl-1-
hexanol. The other goal was to introduce easily made
surfactant-modified clay to be compared with a series of
cross-linked polystyrene resins bearing quaternary am-
monium groups.19-22
Selective oxidation of primary alcohols to aldehydes is a long-
standing problem of organic chemistry (Bueler, C. A.; Pearson,
D. E. SurWey of Organic Synthesis; Wiley-Interscience: New
York, 1977; Vol. 2, p 480; House, H. O. Modern Synthetic
Reactions, 2nd. ed.; W. A. Benjamin: Menlo Park, California,
1972; p 257; Epstein, W. W.; Sweet, F. W. Chem. ReW. 1967,
67, 247; Landini, D.; Montanari, F.; Rolla, F. Synthesis 1979,
134). The use of potassium dichromate as a synthetically useful
oxidizing agent is reported for the oxidation of an industrially
important lipophilic alcohol, employing modified clay as the
phase-transfer catalyst. The phase-transfer catalysis results in
nearly complete oxidation of the 2-ethylhexanol in 40 min at
room temperature, with high selectivity to the 2-ethylhexanal,
compared to 48 h in its absence. Kinetic studies show the
reaction occurs via transfer of Cr2O72- into the organic phase.
The emphasis will be on simplicity of the condition as a
preparative organic method, selectivity with regard to over-
oxidation, efficiency, and mildness of conditions.
Results and Discussion
1-Butyl-4-aza-1-azonia bicyclo[2.2.2]octane chloride was
selected as the surfactant for modifying the clay. On the basis
of preliminary phase distribution studies of dichromate
between the aqueous phase and various organic solvents
(petroleum ether, cyclohexane, toluene, carbon tetrachloride,
chloroform, and methylene chloride) petroleum ether was
selected because of the phase-transferred Cr2O72- concentra-
tion in this solvent remains relatively constant during several
hours of stirring with surfactant present.
According to the mechanism proposed by Wiberg23 for
oxidation of alcohols by KMnO4 in the biphasic system, we
proposed eqs 1, 2, and 3 as the overall stoichiometry of the
oxidation reaction.
Introduction
A significant and recurring problem facing the synthetic
organic chemist5,6 is how to effectively carry out reaction
between a water-soluble reagent and a water-insoluble
substrate. When reaction is attempted as an aqueous-organic
biphase process, very low rates are usually observed due to
the low concentration of at least one of the compounds in
each phase. Rapid stirring has been shown, in certain cases,
to have an accelerating effect by increasing the surface
contact between the layers and thereby increasing the
interfacial component of the reaction.7,8 Soluble ammonium
and phosphonium salts,9 as well as crown ethers,10,11
cryptands,12 and surfactants13 have proven extremely useful
Results When No PTC was Used. To establish a blank
for the phase-transfer catalyzed system, the reaction rate for
the oxidation of 2-ethylhexanol by dichromate in the biphasic
* Corresponding author. E-mail: mghiaci@cc.iut.ac.ir. Fax: +98-311-391-
2350.
(1) Buehler, C. A.; Pearson, D. E. SurVey of Organic Synthesis; Wiley-
(14) Yadav, G. D.; Haldavanekar, B. V. J. Phys. Chem. A 1997, 101, 36.
(15) Mukhopadhyay, J. K.; Mukhopadhyaya, D. E.; Ponde, D. E.; Cohen, S.;
Kurkalli, B. G. S. Org. Process Res. DeVel. 2000, 4, 509.
(16) Laszlo, P., Ed. PreparatiVe Chemistry using Supported Reagents; Academic
Press: San Diego, 1987.
(17) Balogh, M.; Laszlo, P. Organic Chemistry Using Clays; Springer: Berlin,
1993.
(18) Varma, R. S. Tetrahedron 2002, 58, 1235.
(19) Regen, S. L. J. Am. Chem. Soc. 1975, 97, 5956.
(20) Desikan, S.; Doraiswamy, L. K. Chem. Eng. Sci. 2000, 55, 6119.
(21) Annunziata, R.; Benaglia, M.; Cinquini, M.; Cozzi, F.; Tocco, G. Org.
Lett. 2000, 2, 1737.
(22) Wu, H. S.; Lee, C. S. J. Catal. 2001, 199, 217.
(23) Wiberg, K. B. Oxidation in Organic Chemistry, Part A; Academic Press:
New York and London, 1965.
Interscience: New York, 1977; Vol. 2, p 480.
(2) House, H. O. Modern Synthetic Reactions, 2nd. ed.; W. A. Benjamin:
Menlo Park, California, 1972; p 257.
(3) Epstein, W. W.; Sweet, F. W. Chem. ReV. 1967, 67, 247.
(4) Landini, D.; Montanari, F.; Rolla, F. Synthesis 1979, 134.
(5) Dehmlow, E. V. Angew. Chem. Int. Ed. Engl. 1974, 13, 170.
(6) Regen, S. L. Angew. Chem. Int. Ed. Engl. 1979, 18, 421.
(7) Menger, F. M. J. Am. Chem. Soc. 1970, 92, 5965.
(8) Tomita, A.; Ebina, N.; Tamai, Y. J. Am. Chem. Soc. 1977, 99, 5725.
(9) Starks, C. M. J. Am. Chem. Soc. 1971, 93, 195.
(10) Pedersen, C. J. J. Am. Chem. Soc. 1967, 89, 2495.
(11) Sam, D. J.; Simmons, H. E. J. Am. Chem. Soc. 1972, 94, 4024.
(12) Lehn, J. M. Struct. Bonding 1973, 16, 1.
(13) Menger, F. M.; Rhee, J. U.; Rhee, H. K. J. Org. Chem. 1975, 40, 3803.
936
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Vol. 7, No. 6, 2003 / Organic Process Research & Development
10.1021/op034120d CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/23/2003

Table 1. Initial ratesa of 2-ethylhexanal formation with no
PTC
amounts of reactants, mmol
initial rate ×
expt
2-ethylhexanol
K2Cr2O7
103 M s-1
1
2
3
50
100
50
25
25
100
0.051
0.073
0.148
a The initial rate r0 can be found by plotting [alcohol] versus t and drawing
the tangent line at t ) 0.
system was studied without phase-transfer catalyst present.
Table 1 shows the amounts of reactants and initial rates that
were determined from the graph of 2-ethylhexanal product
formed versus time (Figure 1). Even though very little
dichromate ion is extracted into the petroleum ether phase
and very little 2-ethylhexanol is soluble in the aqueous phase,
the 2-ethylhexanol is oxidized mainly to corresponding acid
and ester after 48 h. Since the two phases are immiscible, a
classical rate law analysis is not appropriate here; however,
the comparison of initial rates does provide valuable data.
Comparing experiments 1 and 3, when the amount of
2-ethylhexanol was kept constant, a 4-fold increase in the
amount of Cr2O72- resulted in a 3-fold increase in the initial
rate. Comparing experiments 1 and 2, when the amount of
Figure 1. Production of 2-ethylhexanal versus time for experi-
ments 1-3.
Table 2. Initial rates of 2-ethylhexanal formation with PTC
amounts of reactants, mmol
initial rate ×
expt
2-ethylhexanol
K2Cr2O7
PTC
103 M s-1
2-
Cr2O7 was kept constant, a 2-fold increase in the amount
of 2-ethylhexanol resulted in an initial rate only 1.4 times
faster.
4
5
6
7
8
50
100
50
50
100
25
25
50
25
25
1.75
1.75
1.75
8.75
8.75
0.54
1.088
1.08
5.9
11.54
There are several possible paths for the reaction of
dichromate ion with 2-ethylhexanol when no phase-transfer
agent is used. These types of reactions tend to be limited
not only by the concentration of the reactants at the interfaces
but also by the interfacial area available and hence the stirring
rate.23,24 When the stirring rate was decreased, there was a
decrease in the initial rate of formation of the 2-ethylhexanal.
Influence of Reactant and PTC Concentration on the
Rate. The amount of 2-ethylhexanol, Cr2O72-, and PTC (1-
butyl-4-aza-1-azonia-bicyclo[2.2.2]octane chloride) were sys-
tematically varied in order to determine the rate dependence
on each species. Table 2 contains a summary of the results
of these experiments. The initial rates were determined from
the graphs of 2-ethylhexanal product formed versus time
(Figures 2 and 3).
A comparison of the initial rates of 2-ethylhexanal
formation for experiments 4 and 5 reveals that doubling the
amount of 2-ethylhexanol increases the rate by a factor of
2. This is higher than the 1.4 factor observed when no PTC
was used (see experiments 1 and 2). In experiments 4 and
Figure 2. Production of 2-ethylhexanal versus time for experi-
ments 4-6.
6, doubling the amount of Cr2O72- does, in fact, double the
rate. This is higher than the 3-fold increase in the initial rate,
when the amount of Cr2O72- 4-fold increased in the absence
of PTC (see experiment 3).
(24) Starks, C. M., Ed. Phase-Transfer Catalysis: New Chemistry, Catalysts
and Applications; American Chemical Society: Washington, DC, 1987.
(25) Starks, C. M.; Liotta, C. Phase-Transfer Catalysis: Principles and
Techniques; Academic Press: New York, 1978.
Vol. 7, No. 6, 2003 / Organic Process Research & Development
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937

requirements for processing. Furthermore, the data presented
in this work amply demonstrate the efficiency of the
surfactant-modified clay towards the selective oxidation of
a lipophilic alcohol to the corresponding aldehyde.
Conclusions
The practical potential of this technique for laboratory as
well as industrial applications is great. Preliminary studies
in our laboratory reveal that a high degree of selectivity
toward reactants is possible by using surfactant-modified clay
as triphase catalysis. It appears highly probable that the use
of surfactant-modified clay will expand significantly in
future.
Experimental Section
Figure 3. Production of 2-ethylhexanal versus time for experi-
ments 7-9.
Natural bentonite from Salafchegan, Iran, gray in color
was used; it was sieved to a size range of 0.1-0.2 mm. It
was homoionized by sodium exchange before use. The CEC
measured was 70 meq/100 g of ignited materials. The BET
surface area of the unmodified Na bentonite was 63 m2/g.
The petroleum ether (boiling range ) 40-60 °C) was
obtained from Isfahan refinery company, and the n-octane
was from Merck Products.
The kinetic studies were carried out at ambient temper-
ature (25 °C) in Morton flasks equipped with a mechanical
stirrer (Heidolf type RZR50, stirring rate ) 800 rpm) by
first adding petroleum ether (250 mL), water (250 mL),
2-ethylhexanol (8.2 mL, 25 mmol), surfactant-modified clay,
and an internal standard. Potassium dichromate (7.35 g, 50
mmol) dissolved in 5.5 mL of H2SO4 and 1.9 mL of HOAc
was added last. Small samples of the organic phase were
then analyzed on a Shimadzu GC-14A gas chromatograph
(FID), using a 6 ft × 0.125 in. column packed with 10%
SE-30 on Chromosorb PAW (80-100 mesh). An internal
standard (n-octane) was used in all reactions, and calibration
curves for the oxidation products of 2-ethylhexanal, 2-eth-
ylhexanoic acid, and (2-ethylhexyl)-2-ethylhexanoate were
prepared so that the number of moles of each product could
be determined at various reaction times.
Table 3. Initial rates of 2-ethylhexanal formation with PTC
supported on clay
amounts of reactants, mmol
PTC supported initial rate ×
expt 2-ethylhexanol K2Cr2O7
50 25
on clay
103 M s-1
6.43
9
0.358 g PTC
on 1 g clay
Comparing experiments 4 and 7, we see that a 5-fold
increase in the amount of PTC increases the rate more than
10-fold. A comparison of experiments 7 and 8 show that
with substantially larger amount of PTC, doubling of the
amount of 2-ethylhexanol does double the rate of oxidation.
Results When PTC Supported on Clay. A comparison
of the initial rates of 2-ethylhexanal formation for experi-
ments 4 and 9 reveals that by supporting 0.358 g of PTC
(equivalent to 1.75 mmol used in experiment 4) on 1 g of
clay increases the rate more than 11-fold (Table 3). Under
this condition 2-ethylhexanol was oxidized to aldehyde in
about 40 min in more than 90% yield. The amounts of
2-ethylhexanoic acid and corresponding ester were negligible.
To determine the benefits of a PTC supported on clay, a
separate run with unmodified clay (without phase-transfer
catalyst) was performed. The result of this experiment was
similar to that of experiment 4. In experiment 4, the
2-ethylhexanal was contaminated with 2-ethylhexanoic acid
(9.5%) and corresponding ester (10.5%). Two other immedi-
ate advantages of this condition would be (1) simplified
product workup and (2) easy and quantitative catalyst
recovery. From the stand-point of industrial applications, this
concept is a very attractive one due to low energy and capital
Acknowledgment
We thank the research council of Isfahan University of
Technology and Center of Excellency in the Chemistry
Department of Isfahan University of Technology for the
support of this work.
Received for review August 25, 2003.
OP034120D
938
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Vol. 7, No. 6, 2003 / Organic Process Research & Development