Preparation of monoalkyl terephthalates: An overview

Article abstract of DOI:10.1080/00397910601039226

Terephthalic acid can be readily converted to the corresponding monoalkyl terephthalate in high yield, via a two-step procedure. This method is advantageously compared to the more representative methods described in the literature. The purification of the expected monoester, a crucial problem for this synthetic pathway, is discussed, and an original procedure has been developed. Copyright Taylor & Francis Group, LLC.

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Preparation of Monoalkyl Terephthalates: An
Overview
Elodie‐Denise Chenot a , Dan Bernardi b , Alain Comel a & Gilbert Kirsch a
a Laboratoire d'Ingénierie Moléculaire et Biochimie Pharmacologique
(LIMBP), Université Paul Verlaine Metz, Metz, France
b Institut de Chimie Organique et Analytique, Université d'Orléans, Orléans,
France
Published online: 06 Feb 2007.
To cite this article: Elodie‐Denise Chenot , Dan Bernardi , Alain Comel & Gilbert Kirsch (2007): Preparation
of Monoalkyl Terephthalates: An Overview, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 37:3, 483-490
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Synthetic Communicationsw, 37: 483–490, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910601039226
Preparation of Monoalkyl Terephthalates:
An Overview
Elodie-Denise Chenot
´ ´
Laboratoire d’Ingenierie Moleculaire et Biochimie Pharmacologique
´
(LIMBP), Universite Paul Verlaine Metz, Metz, France
Dan Bernardi
Institut de Chimie Organique et Analytique, Universite d’Orleans,
´
Orleans, France
´
´
Alain Comel and Gilbert Kirsch
´ ´
Laboratoire d’Ingenierie Moleculaire et Biochimie Pharmacologique
´
(LIMBP), Universite Paul Verlaine Metz, Metz, France
Abstract: Terephthalic acid can be readily converted to the corresponding monoalkyl
terephthalate in high yield, via a two-step procedure. This method is advantageously
compared to the more representative methods described in the literature. The purifi-
cation of the expected monoester, a crucial problem for this synthetic pathway, is
discussed, and an original procedure has been developed.
Keywords: dialkyl terephthalates, monoalkyl terephthalates, monohydrolysis,
terephthalic acid
INTRODUCTION
Preparation of monoesters of symmetric dicarboxylic acids on a gram scale
remains a challenge for synthetic chemists when the corresponding cyclic
anhydrides are not readily available. Even recently, articles dealing with
Received in the U.K. April 24, 2006
Address correspondence to Alain Comel, Laboratoire d’Ingenierie Moleculaire
´
´
´
et Biochimie Pharmacologique (LIMBP), Universite Paul Verlaine Metz,
boulevard Arago, 57078, Metz Technopole, , France. E-mail: comel@univ-metz.fr
1
483

484
E.-D. Chenot et al.
this problem have been published, starting either from the diacid or the
dimethyl or diethyl esters.
When the diacid is the starting material, various supports acting as adsor-
bents[1] such as alumina or catalysts[2] (ion-exchange resins) are used in the
presence of alkylating agents, and high selectivity is claimed by each author
or research group. On the other hand, diesters require either selective
enzymatic[3] or basic monohydrolysis under anhydrous[4] or aqueous[5] con-
ditions. Although enzymatic hydrolysis is only used in specific cases, basic
hydrolysis seems to be more general, and authors often discussed the versati-
lity of their own method (e.g., Niwayama’s procedure,[6] which has been
applied successfully to several classes of aliphatic diesters).
Interesting, alternative ways are reported in the literature:
† esterification of a formyl-containing acid followed by oxidation of the
formyl group to obtain the monoester of the symmetric diacid[7] and
† carboxylation of arylcopper or arylpalladium compounds.[8]
In the latter case, the availability of the starting material is the crucial point,
and it can be more difficult to obtain than the monoester itself.
We were particularly interested in the preparation of monoesters of ter-
ephthalic acid 2 and describe here an easy-to-handle ester monohydrolysis
of various dialkyl terephthalates 1 using solutions of potassium hydroxide
in the corresponding alcohols. A comparison to the most representative
methods described in the literature is also provided.
RESULTS AND DISCUSSION
We decided first to investigate the use of alumina as an adsorbent to transform
specifically one of the acid groups to the corresponding ester. We therefore
tried to adsorb terephthalic acid on commercial alumina following the
procedure described in the literature.[1] Unfortunately, only traces of
terephthalic were retained on alumina, unlike what was reported by Chihara
et al.[1]
We also investigated the feasibility of a monohydrolysis of various
diesters of terephthalic acid 1 in a basic medium (Scheme 1).
The latter have been prepared following a procedure often used in the lab-
oratory (large excess of thionyl chloride added to an alcoholic solution of the
diacid). The yields are reported in Table 1 and are generally more than 90%
for primary alcohols, they obviously decrease for secondary alcohols (39%
for isopropanol) because of the steric hindrance. No reaction has been
observed when tertiobutanol was used.
Several procedures for monohydrolysis have been described in the litera-
ture,[5] generally dedicated to the formation of the monomethyl ester as
starting material for different purposes.

Monoalkyl Terephthalates
485
Scheme 1.
Surprisingly, the ratios of potassium hydroxide/diester of terephthalic
acid reported in the literature[5] are not exactly equal to one. Indeed, a
slight excess of potassium hydroxide is used by some authors even
though Hotten[5a] reported high yields of crude monomethyl terephthalic
acid using a ratio of potassium hydroxide/dimethyl terephthalate equal to
0.97.
We therefore decided to evaluate the value of this ratio, and we observed
that the best results were obtained when the value is logically equal to 1.
The solvent used is generally the corresponding alcohol, in order to avoid
transesterification, but authors generally have not specified if anhydrous con-
ditions are required. Nevertheless, we clearly observed an increase of ter-
ephthalic acid formation closely related to the presence of water.
The purification of the monoester obtained following this procedure is
another crucial step, but still poorly described in the literature.
All the previous reports indicated that the expected monoester is always
contaminated by small amounts of terephthalic acid as a by-product. The
Table 1. Preparation of dialkyl terephthalates
Mp (8C)
Entry
R
Product
Yield (%)
Found
Literaturea
1
2
3
4
5
Me
Et
1a
1b
1c
1d
1e
99
89
99
39
95
140–141
46–47
29–31
57–59
Oilb
141–142
44
31
Pr
i-Pr
Bu
55–56
6
aSee Ref. [9].
bAt room temperature.

486
E.-D. Chenot et al.
authors claimed that this monoester can be purified by recrystallization from
water or acetone. Conversely, we observed that although purity really
increases, traces of terephthalic acid are still observable by thin-layer chrom-
atography (TLC).
However, terephthalic acid can be efficiently eliminated from the crude
product when the latter is dissolved in a potassium hydroxide solution and
quickly filtered to eliminate what remains in suspension.
Although the yields of monoesters decrease because of the presence of
appreciable amounts of the latter in the filtration residues, this was the only
practical way to obtain pure (diacid free especially) monoesters of terephthalic
acid. This easy-to-handle procedure has been succesfully applied for all the
monoesters listed in Table 2.
The low yield observed for monobutyl ester of terephthalic acid is only
attributable to the poor solubility of potassium hydroxide in n-butanol.
In summary, we have clearly demonstrated that the easiest and less
expensive way to prepare monoalkylesters of terephthalic acid consists of
the monohydrolysis of the corresponding diester in the alcohol used for the
preparation of the diester. The latter can be easily obtained in high yields
from thionyl chloride in the corresponding alcohol.
High purity can be reached following the original procedure we describe
here.
EXPERIMENTAL
Nuclear magnetic resonance spectra (1H NMR and 13C NMR) were expressed
in parts per million and were recorded on Bruker-AC 250-MHz spectrometer.
The chemicals shifts were obtained with CDCl3 and DMSO-d6 as solvent and
TMS as internal standard. Melting points were determined on a Stuart SMP3
binocular and are uncorrected. Mass spectra for diester were measured with
Hewlett Packard 5971A electron impact (70 eV) mass spectrometer,
equipped with a Hewlett Packard 5890 series II gas chromatograph.
Table 2. Monohydrolysis of dialkyl terephthalates
Mp (8C)
Entry
R
Product
Yield (%)
Found
Literaturea
1
2
3
4
5
Me
Et
2a
2b
2c
2d
2e
77
72
72
62
13
219–220
168–169
130
230
171
Pr
127–129
166
122–124
i-Pr
Bu
164
123–125
aSee Ref. [9].

Monoalkyl Terephthalates
487
General Procedure for the Preparation of Dialkyl Terephthalates
A solution of terephthalic acid (1.0 g, 6 mmol) and corresponding alcohol
(50 mL) was refluxed for 30 min. Thionyl chloride (18 mL, 20 eq) was
added dropwise, and the mixture was maintained 4 to 10 h under reflux.
After being cooled to room temperature, the solvent was removed under
reduced pressure. The mixture was extracted twice with ethyl ether (50 mL)
and washed with a basic (potassium hydroxide) solution. The combined
organic layers were dried over MgSO4. Removal of the solvent allowed as
to obtain the diester as a pure product. Yields: 99% for 1a, 89% for 1b,
99% for 1c, 39% for 1d, 95% for 1e.
Data
1
Terephthalic acid dimethyl ester (1a): H NMR (CDCl3): 3.95 (s, 6H, -O-
CH3), 8.10 (m, 4H, HAr); 13C NMR (CDCl3): 52.4 (-O-CH3), 129.5 (CHAr),
133.9 (CAr), 166.2 (C55O); GC-MS/EI: m/z (%): 194 (Mþ, 25), 179 (4),
163 (100), 135 (24), 120 (9), 103 (15), 92 (4), 76 (17).
1
Terephthalic acid diethyl ester (1b): H NMR (CDCl3): 1.41 (t, J ¼ 7.1 Hz,
6H, -O-CH2-CH3), 4.41 (q, J ¼ 7.1 Hz, 4H, -O-CH2-CH3), 8.10 (m, 4H,
HAr); 13C NMR (CDCl3): 14.1 (-O-CH2-CH3), 61.2 (-O-CH2-CH3), 129.2
(CHAr), 133.9 (CAr), 165.6 (C55O); GC-MS/EI: m/z (%): 222 (Mþ, 13),
194 (21), 177 (100), 149 (62), 121 (13), 104 (18), 76 (20).
Terephthalic acid dipropyl ester (1c): 1H NMR (CDCl3): 1.04 (t, J ¼ 7.3 Hz,
6H, -O-CH2-CH2-CH3), 1.81 (m, 4H, -O-CH2-CH2-CH3), 4.31 (t, J ¼ 6.7 Hz,
4H, -O-CH2-CH2-CH3), 8.10 (m, 4H, HAr); 13C NMR (CDCl3): 10.4 (-O-CH2-
CH2-CH3), 22.0 (-O-CH2-CH2-CH3), 66.9 (-O-CH2-CH2-CH3), 129.4 (CHAr),
134.1 (CAr), 165.8 (C55O); GC-MS/EI: m/z (%): 250 (Mþ, 4), 191 (100), 167
(94), 149 (85), 104 (34), 41 (24).
Terephthalic acid diisopropyl ester (1d): 1H NMR (CDCl3): 1.39 [d,
J ¼ 6.7 Hz, 12H, -O-CH-(CH3)2], 5.27 [sep, J ¼ 6.7 Hz, 2H, -O-CH-
(CH3)2], 8.08 (m, 4H, HAr); 13C NMR (CDCl3): 21.9 (-O-CH-(CH3)2), 69.0
[-O-CH-(CH3)2], 129.4 (CHAr), 134.5 (CAr), 165.4 (C55O); GC-MS/EI: m/z
(%): 250 (Mþ, 6), 209 (23), 197 (56), 167 (34), 149 (100), 121 (17), 104
(24), 76 (28), 59 (45), 43 (42).
1
Terephthalic acid dibutyl ester (1e): H NMR (CDCl3): 0.99 (t, J ¼ 7.3 Hz,
6H, -O-CH2-CH2-CH2-CH3), 1.48 (m, 4H, -O-CH2-CH2-CH2-CH3), 1.77
(m, 4H, -O-CH2-CH2-CH2-CH3), 4.35 (t, J ¼ 6.4 Hz, 4H, -O-CH2-CH2-
CH2-CH3), 8.10 (m, 4H, HAr); 13C NMR (CDCl3): 13.6 (-O-CH2-CH2-CH2
-CH3), 19.1 (-O-CH2-CH2-CH2-CH3), 30.6 (-O-CH2-CH2-CH2-CH3), 65.1

488
E.-D. Chenot et al.
(-O-CH2-CH2-CH2-CH3), 129.3 (CHAr), 134.0 (CAr), 165.7 (C55O); GC-MS/
EI (m/z): 278 (Mþ, 2), 235 (2), 223 (77), 205 (100), 177 (2), 167 (79), 149
(72), 132 (2), 121 (24), 104 (19).
General Procedure for the Synthesis of Monoesters
of Terephthalic Acid
To the starting diester (10 mmol), 30 mL of corresponding alcohol was added,
and the mixture was heated for 15 min. Potassium hydroxide (1 eq) was
poured into the solution, and then the mixture was heated for 3.5 h under
reflux. The mixture was cooled, and the alcohol was evaporated. The
resulting crude product was dissolved in water and extracted with dichloro-
methane. The aqueous layer was acidified with concentrated hydrochloric
acid. The precipitate was then filtered or extracted with ethyl ether, dried
over magnesium sulphate, and concentrated under reduced pressure.
Data
Monomethyl ester (2a): 1H NMR (CDCl3): 3.76 (s, 3H, -O-CH3), 7.91 (m, 4H,
HAr); 13C NMR (CDCl3): 51.2 (-O-CH3), 128.2 (CHAr), 128.4 (CHAr), 132.4
(CAr), 132.9 (CAr), 165.0 (C55O), 166.1 (C55O).
1
Monoethyl ester (2b): H NMR (CDCl3): 1.43 (t, J ¼ 7.3 Hz, 3H, -O-CH2-
CH3), 4.42 (q, J ¼ 7.3 Hz, 2H, -O-CH2-CH3), 8.17 (m, 4H, HAr); 13C NMR
(CDCl3): 13.6 (-O-CH2-CH3), 60.6 (-O-CH2-CH3), 128.6 (CHAr), 128.9
(CHAr), 133.2 (CAr), 134.2 (CAr), 165.1 (C55O), 166.9 (C55O).
1
Monopropyl ester (2c): H NMR (CDCl3): 0.67 (t, J ¼ 7.3 Hz, 3H, -O-CH2-
CH2-CH3), 1.44 (m, 2H, -O-CH2-CH2-CH3), 3.92 (t, J ¼ 6.8 Hz, 2H, -O-CH2
-CH2-CH3), 7.71 (m, 4H, HAr); 13C NMR (CDCl3): 10.0 (-O-CH2-CH2-CH3),
20.7 (-O-CH2-CH2-CH3), 65.4 (-O-CH2-CH2-CH3), 127.6 (CHAr), 128.3
(CHAr), 132.5 (CAr), 133.6 (CAr), 164.3 (C55O), 165.9 (C55O).
1
Monoisopropyl ester (2d): H NMR (CDCl3): 1.00 [d, J ¼ 6.1 Hz, 6H, -O-
CH-(CH3)2], 4.85 [sep, J ¼ 6.1 Hz, 1H, -O-CH-(CH3)2], 7.69 (m, 4H, HAr);
13C NMR (CDCl3): 21.7 [-O-CH-(CH3)2], 68.6 [-O-CH-(CH3)2], 129.2
(CHAr), 129.4 (CHAr), 134.0 (CAr), 134.7 (CAr), 165.2 (C55O), 167.1
(C55O).
1
Monobutyl ester (2e): H NMR (CDCl3): 0.59 (t, J ¼ 7.3 Hz, 3H, -O-CH2-
CH2-CH2-CH3), 1.08 (m, 2H, -O-CH2-CH2-CH2-CH3), 1.38 (m, 2H, -O-
CH2-CH2-CH2-CH3), 3.93 (t, J ¼ 6.7 Hz, 2H, -O-CH2-CH2-CH2-CH3), 7.68

Monoalkyl Terephthalates
489
(m, 4H, HAr); 13C NMR (CDCl3): 12.8 (-O-CH2-CH2-CH2-CH3), 18.2 (-O-
CH2-CH2-CH2-CH3), 29.7 (-O-CH2-CH2-CH2-CH3), 64.2 (-O-CH2-CH2-
CH2-CH3), 128.4 (CHAr), 128.6 (CHAr), 132.9 (CAr), 134.0 (CAr), 164.8
(C55O), 166.5 (C55O).
ACKNOWLEDGMENT
The authors thank G. Sommen for helpful bibliographic assistance.
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