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RSC Advances
DOI: 10.1039/C6RA01029F
ARTICLE
Journal Name
o
0.05 mol), and DES (0.25 mmol) at room temperature (25 C) Materials. All reagents and solvents for syntheses were
(
for 24h
purchased from commercial sources and used without further
purification.
b Yield are based on GC-MS
c isolated yield
Table 5 Recycling of [ChCl][CrCl
a
3
·6H
2
O]
2
DES in the synthesis of Measurements. The analysis of the reaction mixture was
n-butyl acetate
carried out on a gas chromatograph (SP-6890) equipped with a
b
flame ionization detector (FID) and a capillary column (HP-5,
0 m 0.25 mm 0.25 mm). The column temperature was 80
. The temperatures of the injector and detector were
Recycle times
Yield of n-butyl acetate
3
×
×
0
1
2
3
4
93.0%
91.8%
90.7%
90.0%
89.5%
℃
o
o
maintained at 320 C and 320 C, respectively. The products
were further identified by GC (Varian 3900)
–
MS (Varian Saturn
2
100 T) quipped with a flame ionization detector (FID) and a
1
×0.25 mm).The H
capillary column (DB-5, 30 m
×
0.25 mm
a Reaction conditions: n-butanol/acetic acid/ DES =1:5:0.025
o
at room temperature (25 C) for 24h
13
and C NMR spectra were recorded on a Bruker 400MHz NMR
spectrometer at 298K. The chemical shifts (δ) were given in
part per million relative to internal tetramethylsilane (TMS, 0
b Yield are based on GC. 1,4-dimethyl-benzene as intemal
standard
3
ppm for 1H), CDCl (77.3 ppm for 13C).
Benzyl alcohol's hydroxide radical formed the conjugated
system with the benzene ring decreasing the strict hindrance,
and its methylene was mobilizable, so the yield of methyl
phenylacetate was higher than cyclohexyl acetate(Table 4,
Entry 13-14). Also, the alcohols with electron-withdrawing
groups had poorer reactivity (Table 4, Entry 15-16). Moreover,
esterification of alcohols and formic acid (Table 4, Entry 17-20),
catalyzed by DES also had high efficiency. These results
General preparation of deep eutectic solvent (see ESI for
more details). A mixture of the metal halides and choline
chloride in a molar ratio of 2:1 was heated to 70-100
gentle stirring until a liquid formed.
with
General synthetic process of organic esters (see ESI for more
details). A typical reaction mixture consisted of 0.01 mol
alcohols and 0.05 mol carboxylic acids without solvent. The
deep eutectic solvent was added to initiate the reaction. The
molar ratio of deep eutectic solvent/alcohol was 1/40. The
3 2 2
indicated that [ChCl][CrCl ·6H O] DES could be a very
potential catalyst for the synthesis of formate and acetate.
Once the reactions were complete, more attention was
paid to separate esters from the reaction system and recycle
the DES. In fact, the product can be very easily isolated, DES
and water formed the lighter phase while the ester product
and reactants remained in the upper liquid phase and it was
simply and conveniently separated by decanting. After that,
the reusability test was conducted by using the former model
reaction. As shown in Table 5, although the yield of n-butyl
o
reaction mixture was stirred at 25 C for 24h. After the reaction
was completed, liquid samples were analyzed on a gas
chromatograph after addition of the internal standard.
Recycle process of DES. After the reaction was completed, two
clearly separated phases are formed, the upper liquid layer
was spilled off with a Pasteur pipette, the lower layer formed
by DES, water and excess starting material was extracted with
acetate slightly decreased, the reused DES [ChCl][CrCl
3
·6H
2
O]
2
o
diethyl ether three times. A drying in vacuo at 60 C overnight
still had higher activity for the yield of n-butyl acetate after
reusing 4 times. The slight decrease of activity might be
ascribed to the slight deactivation and loss of DES during the
process of reuse.
was carried out on DES for further cycles.
Conclusions
Deep eutectic solvents are formed by mixing metal halides
and choline chloride. So, it shows both Lewis acidity and polar
solubility in an esterification reaction. During the reaction, the
reactants alcohols and carboxylic acids are dissolved in the DES
phase, and the product ester are separated to a non-polar
In summary, a new method for the esterification of carboxylic
3 2 2
acids with alcohols catalyzed by [ChCl][CrCl ·6H O] deep
eutectic solvent under mild conditions have been achieved.
This method makes it possible to synthesize organic esters
easily and efficiently at room temperature. The effect of
different Lewis acids DESs and the amount of DES on the
phase. As
a result, the chemical equilibrium of the
esterification was pulled forward, and the reaction occurred
simultaneously without removing the by-product water.
3
esterification reaction are investigated. The CrCl based DES
stands out among other catalysts. The effect of molar ratio of
carboxylic acids to alcohols and reaction time on the
esterification reaction are also studied. Furthermore, different
organic esters are obtained through this method, and the
chain length of reactants can obviously influence the reaction
rate and yield. Compared with traditional methods, using the
3 2 2
As a conclusion, the DES [ChCl][CrCl ·6H O] showed good
catalytic ability in the reaction between organic acids and
alcohols, and the unique solubility of the DES was beneficial to
generate more products. Further experiments, aiming at the
specific reaction mechanism, are now ongoing in our labs.
DES [ChCl][CrCl
3
·6H
2
O]
2
as catalyst has following advantages:
·6H O] has excellent activity, high
Experimental
(a) the DES [ChCl][CrCl
3
2
2
4
| J. Name., 2012, 00, 1-3
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