Chemistry Reference
In-Depth Information
have dielectric properties suitable for efficient cou-
pling of microwave energy and rapid heating to tem-
peratures that, although high, are typically some
20-30°C below boiling [55]. With this technique,
reactions (on the milligram to several hundred gram
scale) have been performed within minutes. A
disadvantage is the limitation to high-boiling
polar solvents such as dimethylsulfoxide, ethylene
glycol, diglyme, triglyme,
N
-methylmorpholine,
N
,
N
-dimethylformamide and 1,2-dichlorobenzene,
which have relatively similar boiling points and can
present difficulties for recycling and for isolation of
products.
out at reflux include various organometallic and
Diels-Alder reactions, as well as arylsulfonation [9].
Reactions under pressure with solvents
Dedicated microwave reactors have been developed
that are capable of reliable and safe operation with
volatile, organic solvents at elevated temperatures
and pressures [9,64]. Independent investigations
into optimal parameters for microwave chemistry
support this approach [23,27].
The continuous microwave reactor (CMR)
The
continuous microwave reactor (CMR) was the first
microwave system designed for reactions in organic
solvents [9,65]. The microwave cavity (see Fig. 17.1)
is fitted with a vessel of microwave-transparent,
inert material. Plumbing in the microwave zone is
attached to a metering pump and pressure gauge at
the inlet end and a heat exchanger and pressure-
regulating valve at the effluent end. The heat
exchanger enables rapid cooling of the effluent,
under pressure, immediately after it exits the irradi-
ation zone. Temperature is monitored immediately
before and after cooling. Variables such as volume of
the vessel within the microwave zone, flow rate and
control of the applied microwave power allow flex-
ible operation. The plumbing was designed to with-
stand corrosive acids and bases and to minimise
contact between metal surfaces and reaction mix-
tures. Feedback microprocessor control allows the
setting of pump rates and temperatures for heating
and cooling of reactions. Fail-safe measures ensure
Superheated solvents and reactions at reflux
Enhancements in rates of reaction also have been
exploited through superheating of solvents under
microwave irradiation [35-37]. Baghurst & Mingos
obtained boiling points up to 26°C above equilibrium
values at atmospheric pressure and suggested that
nucleation-limited boiling (superheating) was
responsible [35]. Solvents with relaxation times
greater than 65 ps (corresponding to 2450 MHz) will
have loss tangents that increase with temperature
and will be prone to superheating [23].
For reactions at reflux [56-63], domestic micro-
wave ovens have been modified by making a
shielded opening to prevent microwave leakage and
through which the reaction vessel has been con-
nected to a condenser. Alternatively, microwave-
transparent coolants, including CO
2
, have been used
within the microwave cavity. Preparations carried
Fig. 17.1
Schematic diagram of the
CMR: 1, reactants for processing; 2,
metering pump; 3, pressure
transducer; 4, microwave cavity; 5,
reaction coil; 6, temperature sensor;
7, heat exchanger;
8
, pressure
regulator; 9, microprocessor controller;
10, product vessel. (Reproduced with
permission from Ref. 9.)
