An improved greener esterification of fatty alcohols using a renewable acid-ionic liquid couple as catalyst-solvent

Article abstract of DOI:10.1081/SCC-120037923

We have developed a simple and efficient method for the esterification of various organic acids with C4-C18 alcohols using the 1-octyl-3-methyl- imidazolium tetrafluoroborate-para-toluenesulphonic acid (OMIM/BF ₄-PTSA) couple, without organic solvent, either by classic heating or by microwave irradiation. The utilisation of the OMIM/BF₄-PTSA couple presents considerable advantages: high yield, low temperature, short reaction time, isolation of the ester by simple decantation, easy reuse of the catalyst couple.

Full text of DOI:10.1081/SCC-120037923

SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 11, pp. 2085–2093, 2004
An Improved Greener Esterification of
Fatty Alcohols Using a Renewable Acid–
Ionic Liquid Couple as Catalyst–Solvent
*
Hoang-Phuong Nguyen, Samira Znifeche,
`
and Michel Baboulene
´ ´ ´
Laboratoire des Interactions Moleculaires et Reactivite Chimique et
´
Photochimique, Universite Paul Sabatier, Toulouse, France
ABSTRACT
We have developed a simple and efficient method for the esterification of
various organic acids with C4–C18 alcohols using the 1-octyl-3-methyl-
imidazolium tetrafluoroborate–para-toluenesulphonic acid (OMIM/BF4–
PTSA) couple, without organic solvent, either by classic heating or by micro-
wave irradiation. The utilisation of the OMIM/BF4–PTSA couple presents
considerable advantages: high yield, low temperature, short reaction time,
isolation of the ester by simple decantation, easy reuse of the catalyst couple.
Key Words: Esterification; Ionic liquids; Green chemistry; Reaction time;
Fatty alcohols.
*
´
Correspondence: Hoang-Phuong Nguyen, Laboratoire des Interactions Moleculaires
´
´ ´
et Reactivite Chimique et Photochimique, UMR (CNRS) 5623, Universite Paul Saba-
´
tier, 118 route de Narbonne, 31062 Toulouse, Cedex 04, France; E-mail: nguyen@
chimie.ups-tlse.fr.
2085
DOI: 10.1081/SCC-120037923
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

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INTRODUCTION
The synthesis of organic acid esters is increasingly relevant owing to the
requirements of the chemical and pharmaceutical industries.[1] So, the trad-
itional method of esterification has given rise to numerous variants in which
the nature of the acid catalyst and/or the experimental conditions
are modified.[2,3] However, the necessity to eliminate the water formed, the
use of excess reagents in a large volume of organic solvent and the indispen-
sable neutralisation of the acid catalyst at the end of the reaction do represent
considerable obstacles. Thus, for economical and ecological reasons, esterifi-
cation using “green chemistry” approaches is currently in full expansion
owing to the particular properties of ionic liquids (IL).[4]
Recently, a few examples of esterification in the presence of IL have been
reported. Hence, IL with Brønsted acid characteristics such as 1-methylimidazo-
lium tetrafluoroborate,[5] 1-alkyl-3-methylimidazolium hydrogenosulphates,[6]
and 1-butylpyridinium chloride/aluminium chloride[7] have been proposed.
However, the preparation of these IL is not straightforward and requires the use
of volatile organic solvents. In addition, the rates of esterification drop rapidly
when fatty alcohols or acids are involved and the increase in temperature and
longer reaction times lead to degradation. It is for this reason that we considered
improving the process by a specific choice of IL in which the ester formed
would be insoluble (shifting the equilibrium to synthesis and facilitating the
recovery of the ester) in conjunction with the use of a strong acid catalyst,
easy to manipulate, cheap, and suited to the recycling that is indispensable
for industrial applications. We felt that these requirements could be satisfied
by using 1-octyl-3-methylimidazolium tetrafluoroborate (OMIM/BF4)
coupled with para-toluenesulphonic acid (PTSA). In this paper, we report a
simple and efficient esterification of various organic acids with fatty alcohols
based on the particular properties of the OMIM/BF4–PTSA couple.
RESULTS AND DISCUSSION
The IL, OMIM/BF4, is readily synthesised by simple exchange of the
bromide ion in OMIM/Br with NH4BF4, the OMIM/Br itself obtained by
quarternisation of 1-methylimidazole with octyl bromide. Purification and
recovery are made easier by its insolubility in numerous solvents such as
hexane, THF, or ethyl acetate. The AgNO3 test can then be used to check

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2087
for the absence of bromide. The yield is almost quantitative (ꢀ97%). The IL
is then used to dissolve the organic acid and the alcohol in equimolar
proportions. Esterification does not occur at room temperature, even over
long periods of time. However, upon heating to 808C for 1 hr good yields of
isolated esters are obtained (Table 1).
The process proposed here has a very broad range of applications since it
can be applied not only to acids from C2 to C12 and to aromatic acids but also
to amino acids (in progress). Moreover, the insolubility of the fatty acid ester
in the reaction mixture offers excellent recovery of the ester, at the same time
favouring recycling of the OMIM/BF4–PTSA couple after elimination of the
water formed under moderate vacuum.
However, as certain authors[8] reported instability of IL with BF4 or PF6
anions on strong or prolonged heating, leading to the release of HF, we experi-
mented with a process involving microwave heating[9] to reduce the risks
of degradation. We subjected the acid/alcohol/PTSA/IL reaction mixture
to irradiation (see Experimental) by 5-sec bursts of low-power microwaves
separated by periods of 10-sec manual stirring. Conversion yields of 87–95%
were achieved in this way after 1 min of irradiation as opposed to the 1 hr
or more required when heating to 808C (Table 2).
Table 1. Examples of esterification of carboxylic acids by the couple
OMIM/BF4–PTSA.
Entry
Carboxylic acid
Alcohol
Yield (%)a
1
2
3
4
a-Bromoacetic acid
Octanol
Dodecanol
90
98
93
97
Tetradecanol
Octadecanol
5
6
7
8
Propionic acid
Lauric acid
Octanol
82
81
98
90
Dodecanol
Tetradecanol
Octadecanol
9
10
11
12
Butanol
95
90
96
94
Hexanol
Octanol
Dodecanol
13
14
15
Benzoic acid
Hexanol
Octanol
Dodecanol
90
96
85
aReaction conditions: 808C for 1 hr; isolated yields.

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Table 2. Selected examples of the esterification under microwave irradiation.a
Carboxylic
acid
Effective
time (min)
Yield
(%)
Entry
Alcohol
IL/cat.
1
2
3
4
5
6
7
Lauric acid
Lauric acid
Lauric acid
Lauric acid
Lauric acid
Benzoic acid
Benzoic acid
Octanol
Octanol
OMIM/BF4–PTSA
HMIM/BF4
PTSA only
OMIM/BF4–PTSAc
OMIM/BF4–PTSA
OMIM/BF4–PTSA
OMIM/BF4–PTSA
1
3
3
1
1
1
1
92
10
50
90
90
95
87
b
Octanol
Octanol
Dodecanol
Octanol
Dodecanol
aReaction conditions: see Experimental part; isolated yields.
b1-Methylimidazolium tetrafluoroborate.
cReused.
Compared to the results obtained using 1-methylimidazolium tetrafluoro-
boric acid (HMIM/BF4) described recently,[5] the present results clearly show
the advantages of the couple OMIM/BF4–PTSA that we propose here
(Table 2, entry 2).
It should be noted that the presence of IL is indispensable for the pro-
gression of the esterification reaction as shown by the 50% yields obtained
without IL (Table 2, entry 3). To simplify the process, it could be possible
to skip the initial synthesis of OMIM/BF4 and use OMIM/Br directly. This
requires, taking into account, potential competition between halogenation as
recently described[10] and esterification. Below 808C, the reaction is exclu-
sively one of esterification but the conversion yields remain average to
mediocre (10–50%) even after heating for several hours. Microwave
irradiation, however, is totally unsuitable with this system as the acceleration
of the reaction eliminates selectivity. In addition, it should be mentioned that
recently Villa et al.[11] proposed the synthesis of fatty cosmetic esters working
under microwave irradiation at 1608C for 10 min, without organic solvent.
In general, the yields were good but our process seems easier to use
owing to the mild experimental conditions by comparison with Villa’s drastic
conditions.
In conclusion, the couple OMIM/BF4–PTSA proves to be an excellent
“catalyst–solvent” for the high-yield esterification of various carboxylic
acids by fatty alcohols. The advantages it presents are due to its efficiency
and low cost, it is readily recycled and simple to manipulate. Moreover, it
is an excellent example of “green chemistry,” contributing to the increasing
use of IL.

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2089
EXPERIMENTAL
General
Commercial quality reagents were used without further purification. IR
spectra were recorded on a Perkin–Elmer 683 spectrophotometer. 1H spectra
were obtained on Bruker AC 200 instrument. Microwave irradiation was con-
ducted using a Moulinex Micro-FM 1515 (750 W) domestic oven equipped
with operator-selectable power output from 0–750 W and with an operator-
selectable time. When the power selector was positioned on 75 W, the oven
magnetron delivers 5-sec impulse for 30-sec cycle. The esters were analysed
by using a Varian CP-3800 chromatograph equipped with non-apolar capillary
column BPX 5 (5% phenyl(equiv)polysilylphenylene-siloxane) and by TLC
on precoated plates of silica gel 60 F254 using hexane–ethyl acetate as eluent.
MS data were measured with a MALDI-TOF instrument using electrospray
technique.
Synthesis of IL
IL, OMIM/Br, and OMIM/BF4 were synthesised by following described
methods[12] HMIM/BF4 was obtained according to Ref.[5]
Typical Procedure for Synthesis of the Ester
By Heating
To a solution of organic acid (3 mmol) and alcohol (3 mmol) in OMIM/
BF4 (3 mmol) was added para-toluene sulphonic acid (0.5 mmol). The mixture
was stirred at 808C for 1 hr. Two phases appeared. The ester phase was sepa-
rated. The reaction mixture was extracted by hexane (3 ꢀ 2 mL). The com-
bined hexane extracts was treated with a pinch of silica gel 60 to remove
traces of IL. The filtrate was evaporated in vacuo to give almost pure product.
Further purification was achieved through a short column of silica gel 60 with
hexane–ethyl acetate: 3 : 1 (v/v) to afford the pure ester or by distillation.
By Microwave Irradiation
The previous solution was submitted to irradiation during several cycles:
5-sec, effective time magnetron (75 W), separated by periods of 10-sec manual
stirring. After the usual treatment, the ester was isolated as before.

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The structure of the esters obtained was confirmed by IR (
:
1725 cm21, C(O)–O–C: 1190 cm21, C–O–C: 1050–1000 cm21) and 1H
NMR spectra and by comparison of the data with those of authentic samples.
The following compounds were obtained:
Octyl bromoacetate. Oil: b.p. 1288C, 3 Torr (Lit[13]); 1H NMR (CDCl3)
d: 0.91 (t, 3H, J ¼ 7.1 Hz), 1.23–2.27 (m, 12H), 3.80 (s, 2H), 4.18 (t, 2H,
J ¼ 6.2 Hz); MS 251(Mþ).
Dodecyl bromoacetate. Oil: b.p. 1308C, 0.5 Torr (Lit[13]); 1H NMR
(CDCl3) d: 0.87 (t, 3H, J ¼ 7.1 Hz), 1.23–2.30 (m, 20H), 3.70 (s, 2H), 4.18
(t, 2H, J ¼ 6.5 Hz).
1
Tetradecyl bromoacetate. Oil: b.p. 1558C, 1 Torr (Lit[14]); H NMR
(CDCl3) d: 0.87 (t, 3H, J ¼ 7.2 Hz), 1.22–2.35 (m, 24H), 3.7 (s, 2H), 4.16
(t, 2H, J ¼ 6.3 Hz); MS 335 (Mþ).
1
Octadecyl bromoacetate. Yellow oil: b.p. 1858C, 5 Torr (Lit[14]); H
NMR (CDCl3) d: 0.91 (t, 3H, J ¼ 7.1 Hz), 1.23–2.27 (m, 32H), 3.80 (s, 2H),
4.18 (t, 2H, J ¼ 6.2 Hz); MS 391 (Mþ).
Butyl dodecanoate. Oil: b.p. 1508C, 10 Torr (Lit[15]); 1H NMR
(CDCl3) d: 0.83 (m, 6H), 1.20–1.6 (m, 22H), 2.20 (t, 2H), 4.02 (t, 2H,
J ¼ 6.3 Hz).
Hexyl dodecanoate. Oil: b.p. 1348C, 5 Torr (Lit[16]); 1H NMR (CDCl3)
d: 0.87 (m, 6H), 1.20–1.55 (m, 26H), 2.23 (t, 2H), 4.02 (t, 2H, J ¼ 6.3 Hz);
MS 284 (Mþ).
1
Octyl dodecanoate. Yellow oil: b.p. 1858C, 5 Torr (Lit[17]); H NMR
(CDCl3) d: 0.87 (m, 6H), 1.25–1.75 (m, 30H), 2.20 (t, 2H), 4.04 (t, 2H,
J ¼ 6.4 Hz); MS 312 (Mþ).
Dodecyl dodecanoate. Yellow oil: b.p. 2108C, 1 Torr (Lit[18]); 1H NMR
(CDCl3) d: 0.90 (m, 6H), 1.25–1.65 (m, 38H), 2.29 (t, 2H), 4.07 (t, 2H,
J ¼ 6.2 Hz).
1
Hexyl benzoate. Oil: b.p. 1538C, 12 Torr (Lit[19]); H NMR (CDCl3)
d: 0.91 (t, 3H, J ¼ 7.2 Hz), 1.20–1.74 (m, 8H), 4.37 (t, 2H, J ¼ 6.2 Hz),
7.5–8 (m, 5H).
Octyl benzoate. Yellow oil: b.p. 1508C, 5 Torr (Lit[20]); 1H NMR
(CDCl3) d: 0.90 (t, 3H, J ¼ 7.2 Hz), 1.35–1.74 (m, 12H), 4.32 (t, 2H,
J ¼ 6.3 Hz), 7.5–8 (m, 5 H).
1
Dodecyl benzoate. Yellow oil: b.p. 1858C, 5 Torr (Lit[21]); H NMR
(CDCl3) d: 0.92 (t, 3H, J ¼ 7.3 Hz), 1.35–1.75 (m, 20H), 4.32 (t, 2H,
J ¼ 6.4 Hz), 7.5–8 (m, 5H).
1
Octyl propionate. Oil: b.p. 1208C, 30 Torr, (Lit[22]); H NMR (CDCl3)
d: 0.91 (m, 6H), 1.23–1.65 (m, 12H), 2.27 (t, 2H), 4.18 (t, 2H, J ¼ 6.2 Hz).
1
Dodecyl propionate. Yellow oil: b.p. 1458C, 5 Torr (Lit[23]); H NMR
(CDCl3) d: 0.87 (m, 6H), 1.23–1.64 (m, 20H), 2.30 (t, 2H), 4.18 (t, 2H,
J ¼ 6.1 Hz).

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Tetradecyl propionate. Oil: b.p. 1508C, 3 Torr (Lit[24]); 1H NMR
(CDCl3) d: 0.90 (t, 6H); 1.22–1.63 (m, 26H); 2.25 (t, 2H), 4.16 (t, 2H,
J ¼ 6.2 Hz).
Octadecyl propionate. Yellow oil: b.p. 1858C, 5 Torr (Lit[25]); 1H NMR
(CDCl3) d: 0.91 (m, 6H), 1.23–1.65 (m, 32 H), 2.27 (t, 2H), 4.18 (t, 2H,
J ¼ 6.2 Hz); MS 326 (Mþ).
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