Russian Journal of General Chemistry, Vol. 75, No. 9, 2005, pp. 1498 1499. Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 9, 2005,
pp. 1573 1574.
Original Russian Text Copyright
2005 by Suerbaev, Akhmetova, Shalmagambetov.
LETTERS
TO THE EDITOR
Carboxylation of Phenol with Potassium Ethyl Carbonate.
A New Method of Synthesis of p-Hydroxybenzoic Acid
Kh. A. Suerbaev, G. B. Akhmetova, and K. M. Shalmagambetov
Al’-Farabi Kazakh National University, Almaty, Kazakhstan
Received January 17, 2005
p-Hydroxybenzoic acid and its derivatives have
found wide application as food preservatives and
stabilizers (antioxidants), as well as for synthesis of
liquid crystalline polyethers [1 3]. Up to now no
approaches to direct selective carboxylation of phenol
into the para position have been found. p-Hydroxy-
benzoic acid is most commonly prepared by Kolbe
Schmidt via carboxylation of potassium phenolate
with carbon dioxide under pressure. However, this
reaction always provides a mixture of o- and p-hydr-
oxybenzoic acids, and the yield of the latter is no
higher than 40% [4]. One mode drawback of this
synthesis is that it makes use of anhydrous potassium
phenolate whose synthesis is associated with con-
siderable experimental difficulties, such as vacuum
distillation of water and extreme sensitivity of an-
hydrous porassium phenolate to moisture.
wever, by performing the reaction in the conditions in
[5] we failed to obtain the target products (o- and
p-hydroxybenzoic acids) because of the oxidative con-
densation and compaction that resulted in tarring and
charring of the reaction mixture.
Previously we studied in detail carboxylation of
arenes with metal alkyl carbonates [6]. We found that
alkaline metal alkyl carbonates can be used to success
in carboxylation of phenols and naphthols. The reac-
tions were shown to be strongly affected by the nature
of the metal in the salt, nature of the gas medium (air,
argon, carbon dioxide), temperature, and pressure.
The carboxylation of phenol (I) with potassium
ethyl carbonate at temperatures below 200 C provides
two isomers: o- and p-hydroxybenzoic acids (II and
III), the former isomer prevailing. At 170 C, a mix-
ture of 60.8% of isomer II and 5.1% of isomer III is
formed. The reaction above 200 C results in exclusive
formation of isomer III. The optimal conditions of the
synthesis of acid III are as follows: gas medium car-
bon dioxide, 215 C, pressure 25 atm, and reaction
time 7 h. The yield of acid III is this case reaches
93%.
We found a method for para carboxylation of
phenol with potassium ethyl carbonate. Previously
Jones [5] reported the synthesis in 50% yield of sa-
licylic acid by heating to 175 C of a mixture of
sodium ethyl carbonate with phenol; with potasiium
ethyl carbonate, a mixture of salicylic acid and p-
hydroxybenzoic acid was obtained (yield 30%). Ho-
OH
OH
OH
KOC(O)OEt
KOC(O)OEt
<200 C
COOH
III
+
>
200 C
COOH
I
II
III
Thus, we have developed a facile and convenient
synthesis of acid III via carboxylation of phenol with
potassium ethyl carbonate, that gives the target pro-
duct not contaminated with acid II.
p-Hydroxybenzoic acid (III). A glass reactor
placed in a steel autoclave was charged with
0.025 mol of phenol I and 0.027 mol of potassium
ethyl carbonate. The autoclave was closed, and carbon
1
070-3632/05/7509-1498 2005 Pleiades Publishing, Inc.