Chemistry Reference
In-Depth Information
reductions in reaction times in comparison with con-
ventional reflux conditions, but that rate enhance-
ments of 100-1000-fold at atmospheric pressure
would be required before specific microwave effects
could be invoked [37].
For kinetics studies, the temperature must be
known and the reaction solution must be either
thermally homogeneous or possess thermal gradi-
ents that are known or can be modelled [38,39]. In
1992, a microwave unit was reported that could
conduct reactions under these circumstances [40].
Reaction kinetics were determined with that system
and a conventionally heated oil bath. Rates were the
same within experimental error, regardless of the
heating method [38,39].
Some of the discussion about non-thermal 'micro-
wave effects' appears to have stemmed from a
misconception that microwave radiation at 2.45 GHz
can excite rotational transitions. The frequencies at
which molecules undergo rotational transitions are
higher, however. Internal bond rotations also require
higher frequencies for excitation. These 'rotations'
should be referred to as torsional vibrations and are
excited by infrared radiation at ca. 100-400 cm
-1
ceeds first (i.e. that with the lower activation energy)
is undesired [26]. In such circumstances rapid
microwave heating can be advantageous, as demon-
strated by Stuerga
et al
. [45] for the sulfonation of
naphthalene to give 1- and 2-naphthalenesulfonic
acids. At 130°C, the 2-substituted isomer was pro-
duced almost exclusively, whereas at lower heating
rates a mixture of both regioisomers was obtained.
Similarly, Bose
et al
. [46] controlled the steric course
of b-lactam formation by varying the microwave
power input. In the example illustrated in Scheme
17.1, after heating for 1 min the
trans
to
cis
isomeric
ratio was 16 : 84, but after 4 min the
cis
isomer pre-
dominated, with the ratio being 55 : 45.
To summarise, when reaction kinetics have been
determined in homogeneous systems, the rates of
microwave- and conventionally heated reactions
were found to be the same, within experimental
error. In cases where the reaction kinetics were not
determined, rate enhancements probably resulted
from superheating, non-uniform heating, differen-
tial heating and/or transport processes, rather than
from 'specific microwave effects.'
or
3000-12 000 GHz [39].
Heterogeneous systems are difficult to study. The
influence of microwave radiation on the catalytic
transfer hydrogenation of soybean oil from an
aqueous sodium formate solution in the presence of
Pd/C has been reported [41]. Reaction rates were up
to eight times greater with microwave irradiation
compared with conventional heating at the same
temperature. This was attributed to microwave assis-
tance in transport processes at the catalyst and
oil/water interfaces. In support of this rationale,
microwave extraction of plants, foods and soils also
can be rapid and efficient [20,42-44]. In some cases,
the extracted material absorbs the energy preferen-
tially and the cellular structure becomes disrupted
and readily releases components into the solvent.
Heating rate will be important in reactions that can
proceed to more than one product by separate reac-
tion pathways, especially if the reaction that pro-
5 Approaches to Microwave-assisted
Organic Chemistry
Because, for the majority of applications, the rates of
microwave-heated reactions will be comparable with
those from conventional heating, what advantages
do microwaves offer for clean processing? Briefly,
there are general advantages, others that are more
specific and some that are relevant to particular
methods.
The main general advantages include:
(1)
Microwave energy can be introduced remotely,
without contact between the source and the
chemicals.
(2)
Energy input to the sample starts and stops
immediately when the power is turned on or off.
(3)
Heating rates are higher than can be achieved
conventionally if at least one of the components
can couple strongly with microwaves.
Scheme 17.1
b
-Lactam formation [46].
