Factors affecting the selectivity of air oxidation of 2-ethyhexanal, an α-branched aliphatic aldehyde

Article abstract of DOI:10.1021/op000045k

Various solvents and metal catalysts were tested in air oxidation of 2-ethylhexanal to elucidate the effects of different variables on the reaction path of the substrate. The primary goal was to determine the optimal conditions for maximising the yield of carboxylic acid and minimising the formation of other products. Solvents and catalysts both had a significant impact on the product distribution. The best selectivity of 2-ethylhexanal to 2-ethylhexanoic acid, 84%, was obtained with octanoic acid as solvent and manganese(II) acetate as catalyst. Addition of an aromatic aldehyde, benzaldehyde, or p-methoxybenzaldehyde retarded the oxidation rate of both 2-ethylhexanal and aromatic aldehyde dramatically.

Full text of DOI:10.1021/op000045k

Organic Process Research & Development 2000, 4, 544−549
Factors Affecting the Selectivity of Air Oxidation Of 2-Ethyhexanal, an
r-Branched Aliphatic Aldehyde
Christel Lehtinen* and Go¨sta Brunow
Laboratory of Organic Chemistry, Department of Chemistry, UniVersity of Helsinki, P.O. Box 55,
FIN-00014 Helsinki, Finland
Abstract:
R-branched aliphatic aldehydes which are prone to side
reactions5 can be expected to yield more information on
possible reaction paths of aldehydes.
Various solvents and metal catalysts were tested in air oxidation
of 2-ethylhexanal to elucidate the effects of different variables
on the reaction path of the substrate. The primary goal was to
determine the optimal conditions for maximising the yield of
carboxylic acid and minimising the formation of other products.
Solvents and catalysts both had a significant impact on the
product distribution. The best selectivity of 2-ethylhexanal to
2-ethylhexanoic acid, 84%, was obtained with octanoic acid as
solvent and manganese(II) acetate as catalyst. Addition of an
aromatic aldehyde, benzaldehyde, or p-methoxybenzaldehyde
retarded the oxidation rate of both 2-ethylhexanal and aromatic
aldehyde dramatically.
Our special interest was in understanding how to diminish
the formation of by-productss3-heptanone, 3-heptanol,
3-heptyl formate, and low boiling hydrocarbonssand to
maximise the yield of 2-ethylhexanoic acid. Besides the
influence of solvent and catalyst, study was made of the
effects of temperature, oxygen concentration, amount of
water, and added NaOH on the oxidation of 2-ethylhexanal.
Also investigated was the effect on the oxidation of the
aliphatic aldehyde of adding an aromatic aldehyde, benzal-
dehyde, or p-methoxybenzaldehyde.
Results and Discussion
Introduction
Conversion of aldehyde to carboxylic acid takes place in
two stages: in a typical reaction the aldehyde is first
converted to peracid (Scheme 1, (II)) in a free radical chain
reaction, and then the peracid reacts with another aldehyde
molecule6 and forms an intermediate (III) as shown in
Scheme 1. In principle, the intermediate can decompose by
migration either an R-carbonyl group or hydrogen, leading
either to1 mol of acid and 1 mol of formate (path a) or to
two mol of acid (path b). Normally path b dominates.
2-Ethylhexanoic acid was the main product in all oxidation
reactions of 2-ethylhexanal.
Oxidation of 2-Ethylhexanal in the Absence of Cata-
lyst. Oxidation without Additional SolVent. The results of
noncatalysed oxidation of 2-ethylhexanal without solvent are
presented in Table 1. In noncatalysed reactions with no added
solvent the best yield of carboxylic acid was obtained at room
temperature with air bubbled through the reaction mixture.
Eighty percent of aldehyde was converted to carboxylic acid
and 12% to formate over 2 h. The amounts of 3-heptanone
and 3-heptanol were 2.4 and 2.1%, respectively, and only
1% of unidentified products were formed (entry 5). Gradual
addition of aldehyde to the reaction did not change the
product composition (entry 1). The main reaction path
leading to ketones and alcohols as by-products are presented
in Scheme 2.1,7
Many carboxylic acids are produced from the correspond-
ing aldehyde by dioxygen oxidation, both industrially and
on laboratory scale. The reaction mechanism and kinetics
of the oxidation of various aldehydes with dioxygen have
been widely studied, but few comparisons have been made
of the effects of different catalysts1,2 or solvents3 on the
oxidation path of aldehyde.
Larkin et al.1 have studied oxidation of 2-methylpropanal
presence of Cu(II), Co(II), and Mn(II)acetates and have got
some promising results with a mixture of these catalysts. In
addition, oxidation of 2-ethylhexanal has been researched
both in the presence2 and absence4 of metal soaps. The
highest selectivity to carboxylic acid in the absence of
catalyst was 71.4%, and the maximum selectivity, 80.8%,
was reached in the presence of Mn(II) ethylhexenoate. There
is a report by Yamada et al.3 on the effects of the presence
of solvents on nickel(II)complex in air oxidation of octanal,
but no comparative studies of different catalysts in various
solvents or solvent studies without catalyst present on the
oxidation of aliphatic aldehydes have been reported.
In the present study we have looked at the effects of
catalysts and solvents in different combinations on the
product formation in the oxidation of 2-ethylhexanal. We
chose 2-ethylhexanal as an example of an aliphatic aldehyde
as it is a starting material for 2-ethylhexanoic acid, which is
widely used in the dye and varnish industries. Oxidation of
(5) Lappe, P.; Schults, E. In Applied Homogeneous Catalysis with Organome-
tallic Compounds: a comprehensiVe handbook in two Volumes; Cornils,
B., Herrmann, W., Eds.; VCH: Weinheim, 1996; Vol. 1, p 424.
(6) (a) McNesby, J.; Heller, C. Chem. ReV. 1954, 54, 325. (b) Sajus, L.; Roch,
S. Compre. Chem. Kinet. 1980, 16, 89. (c) Maslow, S.; Bluymberg, E. Russ.
Chem. ReV. 1976, 45, 155.
* Author for correspondence. Telephone: 358-40-7032979. Fax: 358-0-
19130366. E-mail: christel.lehtinen@helsinki.fi.
(1) Larkin, D. J. Org. Chem. 1990, 55, 1563.
(2) Glin˜ski, M.; Kije´nski, J. React. Kinet. Catal. 1995, 55, 319.
(3) Yamada, T.; Rhode, O.; Takai, T.; Mukaiyama, T. Chem. Lett. 1991, 5.
(4) Glin˜ski, M.; Kije´nski, J. React. Kinet. Catal. 1995, 55, 311.
(7) (a) Clinton, N.; Kenley, R.; Traylor, T. J. Am. Chem. Soc. 1975, 97, 3746.
(b) Clinton, N.; Kenley, R.; Traylor, T. J. Am. Chem. Soc. 1975, 97, 3752.
544
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Vol. 4, No. 6, 2000 / Organic Process Research & Development
10.1021/op000045k CCC: $19.00 © 2000 American Chemical Society and The Royal Society of Chemistry
Published on Web 10/26/2000

Scheme 1. Two possible routes of decomposition of
aldehyde-peracid adduct in air oxidation of 2-ethylhexanal
reaction rate of aldehyde was not retarded at 60 °C in FC-
75, which is well-known for its ability to dissolve oxygen.
The effects of adding water or alkali to the reaction
mixture are presented in Table 1.
Adding 1.5 g of water (entry 7) or NaOH mixed with
2-ethylhexanoic acid (entry 9) did not have a marked effect
on the product distribution, even though in the patent
literature alkali metal salts are claimed to improve the yields
of carboxylic acid in air oxidation of aldehydes.8 When
NaOH was added to the system as an aqueous solution (entry
8), the selectivity of oxidation of aldehyde to acid was
slightly decreased. In addition to the usual by-products some
new unidentified products were formed.
Effect of Metal Complexes in the Oxidation of 2-Eth-
ylhexanal. Reactions without Additional SolVent. Metal(II)
acetates of copper, manganese, cobalt, and nickel were tested
as catalysts in the oxidation of 2-ethylhexanal. Manganese
and cobalt in particular5 are often used as catalysts in
aldehyde oxidation. In most tests oxygen provided the oxygen
source. The results of the metal-catalysed oxidations of
2-ethylhexanal are presented in Table 3.
Two reaction temperatures, room temperature and 60 °C,
were tested in the oxidation of 2-ethylhexanal with Cu(II)
acetate as catalyst. The higher reaction temperature (60 °C,
entry 23) decreased the formation of carboxylic acid and also
retarded the overall reaction rate. The effect is the result of
oxygen starvation in liquid phase and was observed also in
oxidations of 2-ethylhexanal without metal complex present.
The reaction of 2-ethylhexanal was often slower in the
presence than in the absence of metal complex. Ni(II)- and
Mn(II)acetate, in particular, retarded the oxidation 34.5%,
and 36% of aldehyde was still present after 4 h (entries 29
and 26). The retarding effect of some nickel catalysts in the
oxidation reaction of aldehyde has been reported by Nam et
al.9
Another difference besides the slower rate of reaction in
the presence of metal acetate was the lower conversion of
aldehyde to formate. The suggested reason for this is the
ability of metal catalyst to decompose the peracid formed
in the oxidation reaction faster than the Baeyer-Villiger
reaction takes place.10 The effect was most notable with Mn-
(II) acetate where only traces of formate were detected. The
yield of formate was also low in the cobalt- and nickel-
catalysed reactions. The conversion of aldehyde to formate
was least affected by Cu(II)acetate.
The reaction without added solvent was influenced by
oxygen concentration in the reaction. Only 9.5% of 2-eth-
ylhexanal reacted when the liquid was stirred under air (entry
2), whereas with continuous air bubbling only 3.5% of
aldehyde remained after 2 h (entry 5). The use of pure oxygen
as oxidant increased the amount of by-products (entry 3).
Likewise, raising the temperature to 60 °C increased the
amount of by-products and also slightly retarded the reaction
rate (entry 4). Presumably, the reason for the slower reaction
rate and increased amount of by-products at 60 °C is that
oxygen solubility in the reaction mixture at higher temper-
ature is lower, causing oxygen starvation in the reaction.
Oxidation Using Additional SolVent. The influence of
various solvents on the oxidation path of 2-ethylhexanal was
tested both in pure oxygen atmosphere and under continuous
air bubbling with and without catalyst. The results of
noncatalysed oxidation in solvent are presented in Table 2.
In chloroform 59% of reacted aldehyde converted to acid
and 23.6% to formate (entry 12) and, correspondingly, in
dichloroethane, 63% to acid and 26.7% to formate (entry
14). The best conversions of 2-ethylhexanal to 2-ethylhex-
anoic acid were afforded in octanoic acid (79%, entry 11),
acetonitrile (82%, entry 17) ,and especially methanol (100%,
entry 16), even though in the last case reaction was very
slow. These solvents are known for their ability to form
hydrogen bonds.
Even though less 3-heptyl formate was formed in the
presence of catalyst, the quantity of other by-products was
greater. Usually, a few percent of 3-heptanol and a few
percent of 3-heptanone were formed. When Co(II) acetate
was used as catalyst, 3-heptanol represented almost 8% of
the products (entry 28). Heptanol is formed either in the
termination reaction of peroxyradical, formed in the reaction
of 2-ethylhexyl acyl radical decarbonylation product and
dioxygen,1 or in a decomposition reaction of 3-heptyl
2-Ethylhexanal did not dissolve in the perfluorous solvents
perfluoro-2-butyltetrahydrofuran (FC-75) and perfluorodeca-
lin either at room temperature or at 60 °C (FC-75), and the
reaction mixture was a two-phase system. The trends of
aldehyde oxidation in perfluorous solvents were similar to
those in the more polar solvents, acetonitrile and octanoic
acid. Contrary to the results in nonsolvent conditions, the
(8) For example: Fisher, K.; Fritz, K. U.S. Patent 5,504,229, 1996.
(9) Nam, W.; Baek, S.; Lee, K.; Ahn, B.; Muller, J.; Burrows, C.; Valentine,
J. Inorg. Chem. 1996, 35, 6632.
(10) Sheldon, R.; Kochi, J. Metal Catalyzed Oxidations of Organic Compounds;
Academic Press: New York, 1981; p 44.
Vol. 4, No. 6, 2000 / Organic Process Research & Development
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545

Table 1. Effects of different reaction parameters on the product distribution in O2 oxidation of 2-ethylhexanal (%) in the
absence of solventa
2-ethyl
aldehyde
3-heptyl
sum of
2-ethyl
hexanoic converted to
entry
conditions
3-heptanone 3-heptanol formate othere products by-products hexanal
acid
acid(%)
1
2
3
4
5
6
7
8
b
c
d
trace
-
3.0
4.7
2.4
2.6
-
trace
-
3.2
7.2
2.1
2.0
-
12.9
trace
12.7
5.7
12.0
12.5
11.2
12.4
2.6
2.0
8.0
31.0
-
5.9
6.4
16.0
15.5
2.0
9.1
90.5
1.6
14.3
3.5
2.5
11.3
9.3
75.4
7.5
83
79
73
44
83
76
80
69
26.9
47.7
16.5
23.0
17.6
28.4
71.5
38.0
80.0
74.5
71.1
62.3
d, 60 °C
25.7 mg H2O
1.52 g H2O
90 mg NaOH/
1.52 g H2O
-
-
9
90 mg NaOH/
0.32 g acid b,d
1.3
0.6
12.7
3.4
18.0
4.0
78.0
81
a Reaction time 2 h and reactions performed in room temperature under continuous air bubbling 34 mL/min if not otherwise mentioned. b Continuous addition of
aldehyde. c Air atmosphere, no bubbling. d O2 atmosphere. e NaOH was mixed into 2-ethylhexanoic acid and added to reaction.
Scheme 2. Formation of by-product: ketone (IIX), alcohol(IX), and CO2 in air oxidation of 2-ethylhexanal
formate.2 No significant differences between the metal
complexes were detected in the conversion of aldehyde to
3-heptanone. In general after 6 or 7 h reaction the amount
of unidentified products was 30-35%.
The amount of catalyst had little effect on the product
distribution, but it affected the reaction rate. Reducing the
amount of catalyst to 10% of normal (entries 22 and 27)
lowered the rate of aldehyde oxidation by 30% with the
manganese catalyst and by about 10% with the copper
catalyst.
Metal-Catalysed Reactions in Dichloroethane and Oc-
tanoic Acid. Octanoic acid and dichloroethane were tested
as solvents in reactions catalysed with manganese and copper.
The amount of aldehyde oxidised was ∼25% of that in the
nonsolvent system. The oxidation was slow in air atmosphere
(C2H4Cl2, Cu(OAc)2), and some aldehyde was still present
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Table 2. Product distribution in O2 oxidation of 2-ethylhexanal (%) in various solventsa
dielectric
aldehyde constant
3-heptyl
otherb
sum of
2-ethyl
2-ethyl
converted
ꢀ
20,25 of
entry
solvent
octane
octanoic acid
chloroform
ethyl acetate
C2H4Cl2
acetone
methanol
acetonitrile
3-heptanone 3-heptanol formate products by-products hexanal hexanoic acid to acid %
solvent
10
11
12
13
14
15
16
17
1.0
0.8
13.4
16.0
15.3
8.7
13.0
3.5
6.6
3.5
11.1
9.7
4.5
14.0
21.8
19.5
26.4
21.3
18.5
20.5
-
2.3
8.5
35.8
5.9
51.0
2.0
92.0
6.5
75.9
72.0
37.8
72.8
30.5
77.5
8.0
78
79
59
78
63
79
100
82
1.9
2.5
4.8
6.0
10.4
21
2.9
1.0
2.0
1.0
1.7
32.6
38.8
1.6
12.7
16.0
77.5
a By method b, reaction 2 h, room temperature, oxidant source O2 balloon. b Amount of aldehyde reacted to other products than mentioned in the table.
Table 3. Product distribution in O2 oxidation of 2-ethylhexanal (%) with various catalysts
metal acetate
reaction
2-ethyl
aldehyde
reaction
3-heptyl
otherf
sum of
2-ethyl hexanoic converted
entry
18
conditions
time (h) 3-heptanone 3-heptanol formate products by-products hexanal
acid
to acid (%)
Cub,e
C2H4Cl2,
air atmosphere
Cub
6
23
1.5
3.2
1.5
2.3
3.9
5.7
20.7
33.8
27.6
45.0
54.0
12.2
18.4
42.8
40
50
19
20
1
2
1.1
0.5
21.0
18.0
7.3
2.3
29.9
20.3
11.5
3.0
58.6
76.7
66
79
C2H4Cl2,
Cub
trace
trace
octanoic acid
Cuc
21
22
2
2
1.8
1.8
1.0
1.8
7.8
6.1
13.5
17.4
24.1
27.1
28.6
32.4
47.3
40.5
66
60
Cu 1/10d
no solvent
Cuc
23
24
25
2
2
2
2.7
11.0
6.9
1.8
5.0
2.7
6.0
0.7
-
16.6
19.3
6.0
27.1
36.0
15.6
34.9
5.0
38.0
59.0
82.9
58
62
84
60 °C
Mnb
C2H4Cl2,
Mnb
1.5
octanoic acid
Mnc
26
27
28
29
2
2
6
4
2.9
2.1
3.9
2.2
3.5
4.0
7.7
5.2
0.4
1.0
1.1
3.3
19.2
15.9
30.6
15.8
26.0
23.0
43.3
26.5
36.0
53.8
9.5
38.0
23.2
47.2
39.0
59
50
52
60
Mn 1/10d
Coc
Nic
34.5
a Reactions performed under O2 atmosphere without additional solvent present if not otherwise mentioned. b Amount of aldehyde 0.012 mol and catalyst 0.012
mmol. c Amount of aldehyde 0.035 mol and catalyst 0.035 mmol. d Amount of aldehyde 0.035 and catalyst 0.0035 mmol. e Air atmosphere, no bubbling. f Amount of
aldehyde reacted to products other than those mentioned in the table.
after 23 h (entry 18). The reaction under oxygen was slightly
more selective and markedly faster (entry 19). The higher
oxygen concentration in the system also considerably ac-
celerated the Baeyer-Villiger reaction of 2-ethylhexanal. The
most significant difference between Cu(II)- and Mn(II)
acetate-catalysed reactions in solvents was the absence of
3-heptyl formate as the product when manganese was the
metal.
The best selectivity of aldehyde to carboxylic acid (84%)
was achieved in octanoic acid in the presence of Mn(II)
acetate (entry 25). An explanation for the small amount of
by-products and high conversion to acid could be the ability
of octanoic acid to reduce the acyl radical formation from
2-ethylhexanal, while at the same time Mn(II)acetate im-
proves the efficiency of conversion of 2-ethyl perhexanoic
acid to carboxylic acid.10
products were formed. Benzaldehyde is a frequently used
coreactant in Baeyer-Villiger reactions carried out in aerial
conditions.11
Addition of benzaldehyde did not significantly affect the
reaction of 2-ethylhexanal, but the 2-ethylhexanal unexpect-
edly retarded the oxidation of benzaldehyde. Only 15% (total
9 mL) of benzaldehyde had converted to acid after 4 h,
whereas benzaldehyde alone (6 mL) was fully oxidised in 2
h.
When benzaldehyde was replaced with p-methoxyben-
zaldehyde, which is known to react through Baeyer-Villiger
reaction in the presence of i-butyryl aldehyde,12 the oxidation
of 2-ethylhexanal was hindered almost completely; only
about 7% acid and 0.5% of 3-heptyl formate were formed
(11) For example: (a) Kaneda, K.; Ueno, S.; Imanaka, T.; Shimotsuma, E.;
Nishiyama, Y.; Ishii, Y. J. Org. Chem. 1994, 59, 2915. (b) Yan, Y.-Y.;
Dong, L.-M.; Huang, M.-Y.; Jiang, Y.-Y. Pure Appl. Chem. 1997, A34,
1097.
(12) Anoune, N.; Lante´ri, P.; Longeray, R.; Arnaud, C. Tetrahedron Lett. 1996,
36, 6679.
Simultaneous Oxidation of Two Aldehydes. On the
oxidation of 2-ethylhexanal the effect of adding another
aldehyde was studied to find out if Baeyer-Villiger reaction
Vol. 4, No. 6, 2000 / Organic Process Research & Development
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547

Table 4. Product distribution in simultaneous oxidation of p-methoxybenzaldehyde and 2-ethylhexanal by method b
products of p-methoxybenzaldehyde products of 2-ethylhexanal
code oxidant source reaction time metal acetate aldehyde acid formate othera aldehyde acid formate othera
31
32
33
34
35
36
air
air
O2
O2
O2
O2
19
24
24
20
19
20
Co
Cu
Cu
-
Cu
-
4
67
46
100
-
100
90
13
16
-
60
-
4
20
34
-
38
-
1
-
4
-
2
-
-
24
9
73
56
60
5
6
20
28
-
21
-
3
-
95
b
b
a Aldehyde reacted to products other than those identified in the table. b No 2-ethylhexanal present.
in 6 h. p-Methoxybenzaldehyde did not react at all. Results
of the simultaneous oxidation of 2-ethylhexanal and p-
methoxybenzaldehyde in dichloroethane are presented in
Table 4.
Oxidation of aliphatic and aromatic aldehydes simulta-
neously retarded the reaction rate of both aliphatic and
especially that of the aromatic aldehydes. The catalysts tested
and higher oxygen concentration both had a positive effect
on the reaction rate of the aldehydes, mostly due to the
accleration of the formate production.
The oxidation of p-methoxybenzaldehyde did not proceed
in dichloroethane alone (entry 34) (the amount of aldehydes
was only 5% of that reported above). However, the addition
of Co(II) acetate as catalyst significantly accelerated the
oxidation of both aldehydes (entry 31). All 2-ethylhexanal
had reacted in 4 h, 73% of it converted to acid, and besides
this also some by-products were formed. Oxidation of
p-methoxybenzaldehyde was slower: about one-third had
converted in 4 h, mostly to corresponding acid (90%), but
also some formate (4%) was detected.
Experimental Section
General 2-Ethylhexanal, p-methoxybenzaldehyde, ben-
zaldehyde (Aldrich), and all solvents were dried, distilled
(except fluorous solvents), and preserved under inert atmo-
sphere until use. The metal complexes Co(NO3)2‚6H2O
(Merck), Co(OAc)2‚4H2O (J.T. Baker), Ni(OAc)2‚4H2O
(Fluka), Fe(OAc)2 (Aldrich), and Mn(OAc)2‚2H2O (Fluka)
were used as obtained from the supplier.
Relative to the noncatalysed reaction, also Cu(II) acetate
slightly promoted the oxidation of both aldehydes (entry 32).
The impact of higher oxygen concentration in the liquid was
tested in the presence of copper catalyst (entry 33). The faster
conversion rate was mostly due to the acceleration of the
formate production of both aldehydes. The presence of
2-ethylhexanal accelerated the formate route of p-methoxy-
benzaldehyde, but the aromatic aldehyde had no particular
influence on the oxidation path of the aliphatic aldehyde.
As in the noncatalysed reaction, the single aldehyde was
oxidised much faster than when aliphatic and aromatic
aldehydes were oxidised simultaneously. To our knowledge
this phenomenon has not been observed earlier. No mecha-
nistic explanation for this effect can be offered at this time.
Gas chromatographic analyses were performed on a HP
6890 instrument: Polar Innowax column 30 m; initial column
temperature 40 °C; final column temperature 250 °C;
progress rate 10 °C/min; constant flow 6.3 mL/min of carrier
gas; initial pressure 0.93 bar. The main oxidation products
of 2-ethylhexanal and p-methoxybenzaldehyde were identi-
fied and quantified by comparison with authentic samples.
(for preparation, see below). Decane or tetradecane was used
as internal standard to calculate the exact amount of
substance present in the reaction mixture. The oxidation
products were also identified by GC-MS and compared with
library spectra.
Oxidation of Aldehyde. Method a. A flat-bottomed glass
vessel equipped with a condenser and an oxygen balloon
was charged with aldehyde (usually 0.035 mol) and decane
and with catalyst (usually 0.35 mmol) where this was used.
Magnetic stirring (1000-1250 rpm) was commenced, and
the reaction mixture was evacuated and oxygenated three
times at the chosen temperature. When reactions were done
in air atmosphere, no evacuation was performed, and the
oxygen balloon was replaced with a CaCl2 tube. The same
applies to air oxidations where air was bubbled to the reaction
mixture. The airflow in synthesis was 34 mL/min if not
otherwise mentioned.
Conclusions
2-Ethylhexanoic acid was the main product in all oxida-
tion reactions of 2-ethylhexanal. The best yield in nonca-
talysed reactions was 80%, while the yield of 3-heptyl
formate was 12%. Higher temperature and oxygen concen-
tration increased the amount of other by-products.
Solvents had considerable effect on the product distribu-
tion of 2-ethylhexanal. Chloroform and dichloroethane
enhanced the reaction route leading to formate. Certain
solvents, octanoic acid, acetonitrile, and especially methanol,
possibly because of the hydrogen bonding between solvent
and substrate, preferred the conversion of 2-ethyhexanal to
2-ethylhexanoic acid. The best selectivity of 2-ethylhexanal
to carboxylic acid, 84%, was obtained in octanoic acid in
the presence of Mn(II)acetate. The reason for this might be
that simultaneously when manganese catalyst acclerates the
conversion of peracid to carboxylic acid the solvent reduces
the acyl radical formation.
Method b. Oxidation of aldehyde in solvent was done
under air, air bubbling, and oxygen. Aldehyde (∼0.012 mol),
decane, catalyst (0.012 mmol), and 10-15 mL of solvent
were added to a flat-bottomed reaction vessel. Start-up of
the reaction was as in method a.
Oxidation of Two Aldehydes Simultaneously. Oxida-
tions of 2-ethylhexanal in the presence of benzaldehyde or
p-methoxyaldehyde were performed at normal pressure with
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Vol. 4, No. 6, 2000 / Organic Process Research & Development

air bubbling at 70 mL/min at 40 °C. At the beginning of the
reaction 1 mL of each of the two aldehydes was added to
the vessel, and magnetic stirring of 1250 rpm was com-
menced. Continuous substrate addition of 2 mL/h was then
begun. When the reaction was performed in solvent without
continuous addition of substrates, the reaction procedure was
similar to oxidation of a single aldehyde as in method b.
Preparation of Formates used as Model Compounds.
3-Heptyl formate was prepared from the corresponding
alcohol (20 mmol) with dimethylformamide (20 mmol) and
benzoyl chloride (20 mmol) in 12 mL of dichloroethane
according to the method of Barluenga et al.13 Besides formate
a small amount of unreacted 3-heptanol was present in the
crude product. The formate was purified with vacuum
distillation, and the yield was 39%.
Methoxyphenyl formate was prepared from 4-methoxy-
phenol (0.1 mol) with formic acid/acetic anhydride (1:1)
solution (0.1 mol) and pyridine (0.0015 mol) according to
Van Es et al.14 The yield of formate was 80%. For GC
analysis the formate was twice recrystallised from ethanol/
hexane solution.
(13) Barluenga, J.; Campos, P.; Gonzales-Nune´z, E.; Asensio, G. Synthesis 1985,
426.
(14) Van Es, A.; Stevens, W. Recl. TraV. Chim. Pays-Bas 1965, 84, 1247.
Received for review April 27, 2000.
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