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XU ET AL.
tedious operation are difficult to avoid.2,5 In recent years, esterification using
aqueous sulfate as catalyst has been reported,5,6 and shows some advantages
such as short reaction time, high yields and easy work-up for a variety of
aromatic acids.
In our research, we need various of ethyl and methyl arylcarboxylates
on a large scale. We find that the reported methods do not give satisfactory
results and the yields are relatively low with methanol and ethanol.
However, using a mixed catalyst system, Fe2(SO4)3ÁxH2O/concentrated
H2SO4, instead of Fe2(SO4)3ÁxH2O we observe effective esterification of
aromatic acids with methanol or ethanol, and excellent yields in short reac-
tion times.
First, we used Fe2(SO4)3ÁxH2O to catalyze the esterification of m-nitro-
benzoic acid with ethanol using the procedure reported in the literature.5
Only about 40% of the acid had been transformed (tested by TLC) after
refluxing for 5 hours. When a drop of concentrated sulfuric acid was added
into it, the esterification rapidly continued and, surprisingly, it had under-
gone nearly quantitative transition after successively refluxing for 3 hours.
Next we examined the esterification of 3,5-dinitrobenzoic acid in presence of
concentrated sulfuric acid as catalyst under the same conditions, but only
about 35% of the acid was transformed after 8 hours, while nearly quanti-
tative esterification was observed upon adding some Fe2(SO4)3ÁxH2O after a
further 4 hours.
We thought that this mixed catalyst perhaps was more active, so we
carried out parallel esterification of many aromatic acids with ethanol.7 We
obtained some excellent results as compared with using Fe2(SO4)3ÁxH2O or
H2SO4 alone, as shown in Table 1. It is shown that the esterification reaction
carried out by the mixed catalyst requires shorter times and gives higher yields.
Next we applied this mixed catalyst to the esterification of various
aromatic acids with methanol and ethanol. These esterification processes
all give satisfactory results as shown in Table 2.
Choshi, T. carried out the esterification of o-methoxybenzoic acid
by treating 40mmol o-methoxybenzoic acid with 40–50 ml absolute ethanol
in the presence of 2 ml concentrated sulfuric acid, but only 87% yield was
obtained.9 Hosangadi, B. reported the esterification of acid with alcohol in
the presence of equivalent thionyl chloride. The operation is complicated and
the thionyl chloride is toxic and acrid, although esterification yields obtained
by their method are similar to our result.10
In conclusion, using the available mixed catalyst system, the esterifica-
tion process requires even shorter reaction time and gets much higher yield
than using Fe2(SO4)3ÁxH2O or concentrated H2SO4 alone as catalyst.
The system is especially effective for acids with steric hindrance and for
a,b-unsaturated acids (Entry 5–14, 16–17). In addition, it has other
Xu
