Excellent synthesis of adipic acid

Article abstract of DOI:10.1080/00397911003629408

A simple, straightforward, and environmentally benign protocol for the synthesis of adipic acid from oxidation of cyclohexanone with Oxone in the presence of 0.5mol% RuCl₃·nH₂O is reported. The reaction completes within a very short time even at room temperature. The generality of the method is shown successfully for synthesis of other C-5 to C-8 dicarboxylic acids.

Full text of DOI:10.1080/00397911003629408

Synthetic Communications1, 41: 548–552, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911003629408
EXCELLENT SYNTHESIS OF ADIPIC ACID
Lalthazuala Rokhum and Ghanashyam Bez
Department of Chemistry, North Eastern Hill University, Shillong, India
Abstract A simple, straightforward, and environmentally benign protocol for the synthesis
of adipic acid from oxidation of cyclohexanone with Oxone1 in the presence of 0.5 mol%
RuCl3 ꢀ nH2O is reported. The reaction completes within a very short time even at room
temperature. The generality of the method is shown successfully for synthesis of other
C-5 to C-8 dicarboxylic acids.
Keywords Adipic acid; oxidation; Oxone; ruthenium chloride; water
The industrial application of adipic acid in the production of Nylon-6 along with
increased environmental concerns associated with the traditional method of
using nitric acid[1] as oxidant for oxidation of cyclohexanone or a cyclohexanone=
cyclohexanol mixture have led to the development of a number of methods for its syn-
thesis. There are only a few methods that carry all the traits to justify themselves as
environmentally and industrially benign. Use of oxidizing agents such as KMnO4,[2,3]
[5]
CrO3,[4] or KO2 lack the desired ingredients to attract industry because of tedious
purification process from their deoxygenated counterparts, while use of oxygen as
the oxidizing agent in the presence of the catalyst uses hexamethyldisilazane (HMPA)
or acetic acid as solvent. Recently, Sato et al.[6] reported the use of 30% hydrogen per-
oxide as oxidizing agent in the presence of a catalytic amount of tungstic acid for the
synthesis of adipic acid from cyclohexanol=cyclohexanone. Although easy separation
technique, easily available or by-product-free reagent system, and good conversion
are the keys to judge its efficacy, use of high temperature (at 90 ꢁC) for about 20 h left
further scope for better energy- and time-efficient protocols.
Use of Oxone1, a potassium triple salt with potassium monopersulfate as an
oxidizing agent, is finding a lot of applications in oxidation of boron, nitrogen,
Received October 14, 2009.
Address correspondence to Ghanashyam Bez, Department of Chemistry, North Eastern Hill
University, Shillong 793 022, India. E-mail: ghanashyambez@yahoo.com
548

SYNTHESIS OF ADIPIC ACID
549
sulfur, and phosphorus compounds.[7] It has already been demonstrated that Oxone is
very useful for epoxidation of olefins, oxidation of aldehydes to carboxylic acids
and esters, oxidative cyclization, and cleavage of olefins. Being a water-soluble and
nonhazardous solid reagent, its application is further bolstered by a recent surge of con-
sciousness for design and development of reaction protocols using environmentally
benign reagents and solvent. In continuation of our recent interest in Oxone-mediated
epoxidation of olefins[8] coupled with the versatility of Oxone as powerful oxidizing
source, we wanted to explore the applicability of Oxone for the synthesis of adipic acid.
Initially, we tried to oxidize cyclohexanone dissolved in acetonitrile by heating with
an aqueous solution of Oxone at 60 ꢁC. Unfortunately, formation of the desired adipic
acid was not observed even after 36 h. This finding led us to believe that Oxone, by itself,
may not be robust enough to accomplish this oxidation, but we decided to look for a cata-
lyst that can change the reactivity profile of Oxone. Because Ru catalysts, such as RuO2
and RuO4, are known oxidizing agents,[9] we decided to use RuCl3 ꢀ nH2O as catalyst for
the conversion. A literature survey revealed that a similar oxidizing system has already
been developed by Bressan et al., consisting of water-soluble metal sulfophthalocyanines
and Oxone to oxidize cyclohexanone to adipic acid.[10] We reasoned that instead of
RuCl3 ꢀ nH2O-derived complexes, if RuCl3 ꢀ nH2O can be used directly, it may lead to
the development of a more user-friendly protocol. To that effect, when the aforesaid reac-
tion was carried out in the presence of 1 mol% RuCl3 ꢀ nH2O, we could observe the com-
plete conversion of cyclohexanone to adipic acid within 4 h. Here, we encountered a
problem in purification of the product, because the sulfate derivatives of Oxone also gave
solid precipitates upon cooling. Finally, it was averted by diluting the reaction mixture
with ethanol and cooling the mixture at 5–10 ꢁC overnight. The solid sulfates were filtered
off, and the filtrate was cooled to 0 ꢁC to get the solid adipic acid in absolutely pure form.
Following our successful attempt to purify adipic acid, we tried the same reaction in the
absence of acetonitrile only to find that the reaction works well here, too. It is interesting to
note that reaction in water takes less time (2 h) than in an acetonitrile–water mixture (4 h).
In our effort to optimize the amount of Oxone required for the reaction, we
observed that use of 2 equiv. of Oxone generated adipic acid in 62% yield only,
whereas use of 3 equiv. of Oxone gave 83% yield. Optimum conversion was found
by using 4 equiv. of Oxone to obtain the purified product in 98% yield. The optimum
amount of the catalyst, RuCl3 ꢀ H2O, was determined by keeping the ratio of cyclo-
hexanone and Oxone (1:4) constant and varying the amount of the catalyst. It was
observed that 0.5 mol% of the catalyst is optimum and an increase of catalyst ratio
up to 1 mol% affected neither the yield nor the reaction time. Use of 0.1 mol% of
the catalyst led to increase in the reaction time (4 h) but gave a similar yield. Increase
in reaction temperature up to 70 ꢁC had only a little effect on the rate of the reaction:
the reaction underwent completion within 1.5 h at 70 ꢁC as compared 2 h at 25 ꢁC.[11]
Having optimized the aforementioned parameters, we extended the applicability of
our method to synthesize C-5 to C-8 dicarboxylic acids from their corresponding cyc-
lic ketones. When the reaction was carried out with cyclic alcohols, it was observed
that reaction is very slow at RT and the percentage of yield also gets adversely affec-
ted. Increasing the reaction temperature to 70 ꢁC considerably reduces the reaction
time as well as enhances the yields. Our results are summarized in Table 1.
It has been observed that reactions of cyclohexanone and cyclopentanone with
Oxone–RuCl3 ꢀ H2O reagent system undergo completion within a very short reaction

550
L. ROKHUM AND G. BEZ
Table 1. Oxidation of cyclic ketones and alcohols via Scheme 1a,b
Entry
Substrate
Time (h)
Yield
Product
1
2
3
Cyclohexanone
Cyclopentanone
Cycloheptanone
2c
5c
7c
98
90
75
4
5
6
Cyclooctanone
Cyclooctanone
Cyclohexanol
16c
6d
49
81
73
20c
7
8
9
Cyclohexanol
Cyclopentanol
Cycloheptanol
6d
8d
8d
92
95
78
10
11
Cyclooctanol
Octan-3-one
8d
10d
89
—
No reaction
aRatio of ketone Oxone RuCl3 ꢀ nH2O ¼ 1:4:0.005.
bRatio of alcohol Oxone RuCl3 ꢀ nH2O ¼ 1:5:0.005.
cReaction was carried out at RT.
dAt 70 ꢁC.
time at room temperature to give their corresponding dicarboxylic acids. In contrast,
cyclooctanone reacts relatively slowly and does not give complete conversion of the
starting material at room temperature even after 16 h. At elevated temperature
(70 ꢁC), cyclooctanone gave 81% yield of the corresponding dicarboxylic acid after
6 h. Comparatively lower yield of the reaction with no starting material left may
be due to the formation of smaller dicarboxylic acids as the side products. When
cyclohexanol (entry 6) was treated with 5 equiv. of Oxone in the presence of
0.5 mol% RuCl3 ꢀ H2O at room temperature, it took 20 h for complete consumption
of cyclohexanone and gave adipic acid in 73% yields. The reaction time was drasti-
cally reduced to 6 h to give the product in 92% yield (entry 7) when the reactions were
carried out at 70 ꢁC. It has also been observed that irrespective of ring sizes, all the
cyclic alcohols (entries 7–10) gave the optimum yield at 70 ꢁC within 6–8 h of reac-
tion time. When the reaction was carried out for octan-3-one, the starting material
remained intact even after heating for 10 h at 70 ꢁC.
In conclusion, we have developed an easy and straightforward method for
synthesis of adipic acid from cyclohexanone at ambient temperature within 2 h,
which is far superior to existing methods in terms of reaction time and temperature.
Nevertheless, the method demonstrates synthesis of adipic acid without using any
harmful solvent in the entire process. The method was successfully applied for
synthesis of other C-5 to C-8 dicarboxylic acids from their cycloalkanone and
cycloalkanol counterparts just by varying the temperature and reaction time.
TYPICAL PROCEDURE
An aqueous solution of Oxone (24.6 g, 40 mmol) in water (15 mL) was added to
cyclohexanone (0.98 g, 10 mmol) and RuCl3 ꢀ nH2O (0.011 g, 0.05 mmol) and allowed

SYNTHESIS OF ADIPIC ACID
551
Scheme 1. Synthesis of C5–C8 dicarboxylic acids.
to stir at room temperature, while monitoring the progress of the reaction by
thin-layer chromatography (TLC). After 2 h, the reaction mixture was diluted with
ethanol (15 mL) and kept at 0–5 ꢁC overnight. The mixure was then filtered, and
the residue was washed with ethanol (3 ꢂ 30 mL). The combined ethanol–water
solution was then kept at 0–5 ꢁC overnight to get colorless precipitates, which upon
drying gave solid adipic acid in 98% yield (1.431 g, 9.8 mmol). Mp 151–153 ꢁC (lit.[7]
1
151–154 ꢁC). IR (KBr): 1685, 2500–3100 cmꢃ1; H NMR (400 MHz, CD3OD): 1.74
(m, 4H), 2.42 (m, 4H), 5.34 (s, 2H); 13CNMR (100 MHz, CD3OD): 176.90, 34.21,
25.21; MS (m=z): 128, 100, 87, 73, 69, 60, 43, 41; Elemental analysis for C6H10O4
(%) calcd: C, 49.31; H, 6.90. Found: C, 49.38; H, 6.73.
ACKNOWLEDGMENTS
Financial grants from the Council of Scientific and Industrial Research
[No. 01(1992)=05=EMR-II], the Department of Science and Technology [SR=S1=
OC-25=2007], New Delhi, and University Grant Commission, New Delhi, are grate-
fully acknowledged.
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