Received: January 22, 2016 | Accepted: February 12, 2016 | Web Released: February 18, 2016
CL-160067
Microwave Effect on Fischer Esterification
Hideko Koshima,*1,2 Kiminori Miyazaki,2 Saori Ishii,3 and Toru Asahi1,4
1Research Organization for Nano and Life Innovation, Waseda University, 513 Wasedatsurumaki-cho, Shinjuku-ku, Tokyo 162-0041
2Department of Applied Chemistry, Faculty of Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama, Ehime 790-8577
3School of Advanced Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555
4Graduate School of Advanced Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555
(E-mail: hkoshima@aoni.waseda.jp)
Upon microwave irradiation, esterification of octanoic acid
with 1-octanol in the presence of hydrochloric acid as an acid
catalyst proceeded more efficiently in glass vessels than in silicon
carbide (SiC) vessels, affording 1-octyl octanoate in higher yields
and revealing microwave effects during esterification.
Keywords: Microwave effect
| Fischer esterification |
Acid catalyst
Since the acceleration of organic reactions by microwave
irradiation was first reported in 1986,1 a number of studies using
microwave-assisted organic syntheses have been published, and
several reviews2 and books3 are also available. Microwave heating
is becoming popular in organic synthesis laboratories due to its
convenience. Reaction acceleration is caused mainly by the
thermal effect, which is the rapid temperature increase because
of microwave heating. In addition, acceleration by the direct
microwave effect (non-thermal effect) has been discussed.4
Recently, Yamada et al. reported significant enhancement of
catalytic enantioselective reactions by microwave irradiation,
ring-opening,5 reduction6 of biaryl lactones, and Claisen rearrange-
ment,7 and demonstrated non-thermal effects in organic reactions.
Dudley et al. reported the acceleration of Friedel-Crafts benzyla-
tion8 and an aryl Claisen rearrangement9 by microwave irradiation
in homogeneous solution, and they provided a detailed account of
their theoretical rationalization for these observations.10 However,
the non-thermal effects remain unclear.
Borosilicate glass allows microwaves to pass through due to
the low dissipation factor tan ¤ (0.0025, 2.45 GHz, 25 °C). In
contrast, silicone carbide (SiC) strongly absorbs microwaves due
to the large tan ¤ (0.15, 2.45 GHz, 50 °C) to virtually change the
microwave energy to thermal energy.11 Furthermore, SiC has
excellent chemical stabilization and high thermal conductivity.
Kappe et al. utilized sintered SiC as the heating element in
microwave-assisted organic synthesis to attain rapid heating of
nonpolar solvents.12,13 They also performed several organic
reactions using SiC vessels, but could not obtain different
reactivity compared to that in glass vessels.14 However, microwave
heating in reaction vessels consisting of SiC is almost equivalent
to conventional heating in oil bath or electric heaters. Hence,
accurate comparison of the reactivity in glass and SiC vessels may
be used to separate the thermal effects from non-thermal effects.
Fischer esterification of carboxylic acids with alcohols in the
presence of acid catalysts is one of the most well-known organic
reactions.15 We applied microwave-assisted esterification in glass
and SiC reaction vessels of the same shape and size by employing
a microwave reactor to provide higher yields of the product ester
in the glass vessels compared to that in the SiC vessels, showing
microwave effects (Scheme 1).
Scheme 1. Microwave-assisted esterification of octanoic acid
with 1-octanol in the presence of acid catalyst in glass and SiC
reaction vessels.
A single-mode microwave reactor (Anton Paar, monomode
300, 2.45 GHz, 850 W magnetron, maximum temperature up to
300 °C, maximum pressure-resistant up to 30 bar) was employed.
Two types of reaction vessels, a 10 mL borosilicate glass vessel
(G10) and 10 mL SiC vessel (S10), with completely similar
dimensions were used (Scheme 1). The magnetic stirrer speed was
600 rpm, which was sufficient to mix the sample solution.
Two types of thermometers, a ruby fiber optic thermometer
and an IR sensor, are equipped in this reactor. The IR sensor is
adjusted towards the ruby thermometer. The accuracy for the ruby
thermometer is specified as «2 °C and that for the IR sensor is
«5 °C in the instruction manual.
The temperature of the reactant solution was accurately
measured using the ruby thermometer immersed into the reactant
solution in a vessel. The heating temperature was controlled by
monitoring the surface temperature of the vessel using the IR
sensor, because the temperature stability at preset 80 °C was 79.0 «
0.4 °C (standard deviation) and 81.0 « 0.1 °C in the glass and SiC
vessel, respectively, which gave higher stability than 80.3 « 1.0
and 82.6 « 3.1 °C by ruby thermometer control (Figure S1).
Esterification in the presence of hydrochloric acid as an acid
catalyst was performed without the solvent. Reactants octanoic
acid (1) (10 mmol, 1.442 g) and 1-octanol (2) (10 mmol, 1.302 g),
as well as hydrochloric acid (1.0 mmol, 0.083 mL of 12 M HCl),
were put into a glass or SiC reaction vessel, and mixed
homogeneously with a magnetic stirrer. The vessel was covered
with a silicone septum and snap cap, and was set into the
microwave reactor. A ruby sensor was immersed into the solution
in the vessel. Microwave was irradiated for 3 min to reach the
preset temperature, after which the irradiation was continued for
10 min at the same temperature.
© 2016 The Chemical Society of Japan | 505