Chemistry Reference
In-Depth Information
that the reaction rate depended upon the extent of
hydration of the enzyme [148]. Although further
work will be required to determine more fully the
advantages and disadvantages of microwave heating
for enzymatic reactions, completed studies have
demonstrated the feasibility of combining both
techniques.
11 Tandem Technologies
Reactors have been constructed that combine
microwave heating with other technologies includ-
ing sonication [153], ultraviolet radiation [154] and
electrochemistry [155]. With the microwave-
ultrasound reactor, the esterification of acetic acid
with
n
-propanol was studied along with the pyroly-
sis of urea to afford a mixture of cyanuric acid,
ameline and amelide [153]. Improved results were
claimed compared with those from conventional and
microwave heating. Ultrasound is propagated
through a series of compression and rarefaction
waves induced within the medium. At sufficiently
high power, cavitation bubbles form and grow over
a few cycles, accruing vapour or gas from the
medium. The acoustic field associated with the
bubbles is unstable, leading to sudden expansion and
violent collapse that generates energy for chemical
and mechanical effects [156]. The reported outcome
from the microwave-ultrasound reactor is somewhat
surprising because ultrasonic reactions usually are
favoured at low temperatures and microwave
heating could be expected to inhibit cavitation.
The efficacy of the microwave-UV reactor was
demonstrated through the rearrangement of 2-
benzoyloxyacetophenone to 1-(2'-hydroxyphenyl)-
3-phenylpropan-1,3-dione.
In 1997, electrochemical processes and devices
that can contribute to a cleaner environment were
reviewed [157]. The first report of microwave-
activated voltammetry has since appeared [155]. The
technique involves the focusing of microwave
energy at the electrode/solution (electrolyte) inter-
face of an electrode immersed in a solution and
placed in a microwave cavity. Either superheating
or a stable high temperature of the solution near
the electrode can be accommodated [158]. So far,
microwave-activated voltammetry has been applied
to metallic complexes, including the ferrocyanide/
ferricyanide redox couple [156] and to the reduction
of a ruthenium complex [158] and it will be inter-
esting to discover whether or not it could benefit
organic reactions involving oxidation or reduction.
10 Deuteration and Tritiation
Deuterium (
2
H)- and tritium (
3
H)-labelled com-
pounds have important roles in the physical and life
sciences [18]. Tritiation usually is performed on a
much smaller (milligram) scale than deuteration
(gram) and attention must be paid to the radioactive
waste produced. Hence, there is a need for environ-
mentally friendly procedures. Jones & Lu have
employed microwave dielectric heating to facilitate
catalysis for radiosynthesis [149]. Their work (see
Scheme 17.14) has embraced a wide range of
reactions, including hydrogenations, borohydride
reductions, aromatic dehalogenations, decarboxy-
lations and hydrogen isotope exchange processes.
In addition to accelerated rates of reaction, new
environmentally friendly routes have been devel-
oped, particularly solventless reactions that mini-
mise waste production and facilitate containment
[150-152].
12 Conclusion
From tentative beginnings in 1986, microwave-
assisted organic chemistry has emerged as a field
Scheme 17.14
Examples of microwave-assisted deuterium
exchange [149].
