Barbosa et al.
Synthesis of Phenyl Esters Using SiO2–SO3H Catalyst in Conventional Heating
Phenyl Acetate.22ꢀaꢁbꢂ MS m/z 136 (18.00) [M]+
C8H8O+2 , 94 (100.00) [M-C2H2O]+ C6H6O+. IR (KBr)
vmax 3068, 3044, 3034, 2943, 1765, 1694, 1493, 1484,
1456, 1433, 1371, 1291, 1194, 1163, 1070, 1046,
group directly bound to an aromatic ring can be straight-
forwardly related to the greater basicity of that hydroxyl,
since the resulting negative charge of the anion is sta-
bilized within the aromatic ring. The consequence is the
lowering of the nucleophilic strength of that –OH group,
which will have difficulty in attacking the partner of the
reaction, which is the activated form of the acid. These
effects should lead to very poor yields, if one assumes
that the esterification reaction under those conditions fol-
lows the Fischer esterification mechanism, and this is the
main reason why industrial processes rely in acid chlo-
rides or activated anhydrides for the preparation of phenyl
esters. In the present synthesis of the esters, the phenyl
benzoate was formed in the highest yield using a reac-
tion time of nine minutes, under microwave irradiation
and without solvents, followed by the rather good yields
of phenyl salicylate and phenyl nicotinate, prepared under
the same circumstances. It is possible that the presence
of extra functional groups in the molecules of the lat-
ter reagents increase their affinity for the Bronsted cata-
lyst surface, hence lowering their overall reactivity towards
esterification by phenol molecules. In fact, this behavior of
the catalyst could be an indication that under the present
circumstances, the Fischer mechanism may still be in oper-
ation, the microwaves helping phenol to attack the organic
acids which have been highly activated by the combina-
tion of MW irradiation and strong Bronsted SiO2–SO3H
1027 and 1014 cm−1 1H NMR (400 MHz, CDCl3,
.
ppm), 7.43 (m-phenyl), 7.28 (p-phenyl), 7.17 (o-phenyl),
2.31 (methyl). 13C NMR (100 MHz) 169.70 (car-
bonyl), 150.80 (quaternary), 129.50 (m-phenyl), 125.92
(p-phenyl), 121.70 (o-phenyl), 21.06 (methyl).
Diphenyl Oxalate.23 White solid; m.p. 136–138 ꢀC
(lit. 136 C). MS m/z 242 (15.91) [M]+ C14H10O4+, 77
ꢀ
(100.0) [M-C7H5O4]+ C6H5+. IR (KBr) vmax 3043, 2954,
2924, 2866, 1776, 1699, 1586, 1487, 1456, 1378, 1366,
1317, 1304, 1290, 1276, 1255, 1201, 1188, 1181, 1165,
1145, 1070, 1019, 1007, 936, 926, 846, 751, 745, 723,
1
695 and 689 cm−1. H NMR (400 MHz, CDCl3, ppm),
7.43 (m-phenyl), 7.32 (o-phenyl), 7.28 (p-phenyl); 13C
NMR (100 MHz), 154.96 (carbonyl), 150.32 (quaternary),
129.63 (m-phenyl), 126.61 (o-phenyl), 121.59 (p-phenyl).
3. DISCUSSION SECTION
One of the interests of our research group is in the develop-
ment of new substrates for heterogeneous catalysis,4ꢁ5ꢀaꢁbꢂ
and our attention was attracted to a silica support that
could be made directly from inexpensive construction
sand. To access the potential of that support, the silica
(surface area 507.00 m2g−1ꢂ was impregnated with con-
IP: 91.243.90.254 On: Tue, 19 Mar 2019 13:26:29
catalyst surface.
centrated sulfuric acid and after that the mixture was kept
under stirring for 12 h at room temperature. WashDienlgivaenrded by Ingenta
Copyright: American Scientific Publishers
The time under ordinary reflux necessary to obtain high
yields of the phenyl formate and phenyl acetate was 5 h,
drying resulted in a material, SiO2–SO3H, with a sur-
face area 115.00 m2g−1, that could catalyze the esteri-
fication of carboxylic acids with phenol, with excellent
yields.
which is still a reasonably short time if considering the
participation of the acids as reagents. These reactions were
not tested in our unmodified MW oven. The formation of
by-products was not detected in any of the esterification
reactions with phenol.
3.1. Esterification Reactions
The main challenge involved in the direct esterification
reaction between a carboxylic acid and phenol is the pro-
duction of water, which has to be removed by adsorption
on the silanol groups to impede the reversibility of the
reaction. The yields of esters obtained using microwave
radiation were higher than those obtained by heating
under reflux with a heating mantle, probably because of
the increased rotational modes of the polar reagents car-
boxylic acids and phenol caused by the microwaves irra-
diation. This increase in vibration must enhance the ease
of flow of the reactants through the pores of the catalyst,
reducing the reaction times and, consequently, improv-
ing the performance of all reaction processes (Table I).
In the microwave-irradiated processes, the temperatures
4. CONCLUSION
A solid Bronsted acid catalyst, obtained by treating silica
gel produced from sand with concentrated sulfuric acid,
was employed for the esterification of phenol with aro-
matic and short chain aliphatic acids. Yields of over 96%
phenyl benzoate were obtained with this catalyst using
microwave irradiation, and the catalyst could be reused
three times before deactivation. The presence of extra
functional groups in the acids i.e., pyridine in nicotinic
acid, or hydroxyl in salicylic acid, decreases the activity
without affecting the catalytic formation of the respec-
tive phenyl esters. The high polarity of the reactants and
the strong Bronsted character of the catalyst lead to a
high reagent-catalyst affinity, and this affinity appears to
increase when the reaction medium suffers the effect of
microwave radiation, resulting in good to excellent yields
of the esterification processes of the otherwise unreac-
tive phenol alcohol. The diphenyl ester of oxalic acid,
the important intermediate for the production of diphenyl
ꢀ
of the reaction media did not exceed 75 C, unlike pro-
cesses with heating in a thermal blanket, where tempera-
ꢀ
tures reached 120 C.
Under conventional reaction conditions, i.e., dispersion
of the reagents by a suitable solvent and conventional
heating, the longer times required to esterify a hydroxyl
J. Nanosci. Nanotechnol. 19, 3663–3668, 2019
3667