Chemistry Reference
In-Depth Information
conditions were optimised readily through the capa-
bilities of the MBR for rapid heating and cooling. The
substantially shorter reaction times probably resulted
from the convenience in the operation of the reactor
in comparison with a standard autoclave, indicating
broader opportunities for microwave heating to
improve the conditions for established reactions that
require high temperatures.
increased at 1.6°C min
-1
to 250°C and then lowered
[9]. The temperature response was rapid when
required and the low thermal inertia of the system
facilitated subsequent cooling.
6.4 Exothermic reactions, differential
heating and viscous reaction mixtures
In the MBR, heating and cooling can be performed
concurrently, thereby helping to control reactions
that require heat for induction but then develop
exotherms that could result in unwanted by-
products or decomposition [9,68].
The individual phases in two-phase systems can be
heated at different rates owing to differences in the
dielectric properties. In some cases a sizeable tem-
perature difference can be maintained for several
minutes. This technique also was useful for prepar-
ing aryl vinyl ketones batchwise by Hofmann elimi-
nation in a two-phase system comprising water
and chloroform [9,68]. Reactions took place in the
aqueous phase and the thermally unstable products
were extracted simultaneously and diluted into the
cooler organic phase, which could be recycled. Yields
were nearly quantitative and twice those obtained by
the traditional method of pyrolysis under vacuum
with distillation (see Section 6.2 for discussion of an
alternative method employing the CMR).
Typically, viscous materials transfer energy poorly.
With conductively heated vessels, pyrolytic degrada-
tion on the walls can co-occur with incomplete
reaction towards the centre of the container. Large
thermal gradients can result in suboptimal conver-
sions, loss of product and laborious clean-up proce-
dures. Also, when high temperatures are required,
heat losses increase and conductive heating becomes
inefficient. Under microwave conditions these prob-
lems are diminished.
6.3 Control of heating
With conventionally heated reactions, a constant
temperature can be attained under reflux conditions.
For temperatures below boiling though, a feedback
system normally is employed but adjustments can
be slow to take effect owing to the thermal inertia
of associated vessels, oil baths and heating mantles.
With microwave irradiation, the response to changes
in energy input is immediate. This makes heating at
high or low rates possible in the MBR, with periods
at temperature plateaux if so desired. Because the
system can operate under pressure, this control can
be applied at temperatures above the normal boiling
point of the solvent.
To illustrate, temperature profiles were obtained
for two comparable aqueous reactions (100-ml scale;
Fig. 17.3). In the experiment depicted by the unbro-
ken line, the reaction was heated rapidly from
ambient to 100°C and held for 2 h. The temperature
then was increased at ca. 60°C min
-1
to 250°C,
held for 10 min, and rapidly decreased. In the other
experiment, after 2 h at 100°C the temperature was
6.5 Reaction vessels
Because the reaction vessels are microwave-
transparent, they will be no hotter than their con-
tents. They usually are made from insulating
polymeric materials such as polytetrafluoroethene
(PTFE), which have inherent advantages for cleaner
processing. In contrast with glass, PTFE is resistant to
attack by strong bases or HF. In contrast with stain-
less steel, it is not corroded by halide ions [81]. Also,
conductive heat loss by PTFE is minimal and the
Fig. 17.3
Heating profiles for two aqueous reactions.
(Reproduced with permission from Ref. 9.)
