Chemistry Reference
In-Depth Information
low adhesivity can help to minimise detergent and
organic solvent usage during cleaning operations
that otherwise would generate considerable effluent
[82-85].
transportation and storage is avoided by just-in-time,
point-of-use production.
The MBR and CMR are portable, multi-purpose
and self-contained advantages that could become
increasingly important for industrial chemical reac-
tors. Their capabilities for rapid throughput and the
materials of construction enable easy cleaning for
reuse and promote short turnaround times. Safety
advantages include control and method of energy
input, low volumes undergoing reaction at one
time and opportunities for remote, programmable
operation.
The microwave systems have facilitated thermal
processes for cleaner production, including applica-
tions of water as a solvent and reactions that require
elevated temperatures, and also have been useful
components of tandem processes, including coupling
with membrane techniques [64,91].
6.6 Reactions with a distillation step
With conductive heating, several factors are detri-
mental for distillation. Transferral of heat to the
sample usually occurs from the inner surfaces of
the vessel. Owing to vaporisation at the surface
of the liquid and to a thermal gradient established by
convection, not all of the sample will be at the same
temperature. Significantly higher temperatures are
required for the pot than for the distillate. With
microwaves, energy is absorbed more uniformly, a
larger volume of the sample in the pot is heated
simultaneously, convection is reduced and distilla-
tion is more rapid.
The MBR can facilitate distillation through an
outlet tube connected to a port on the top flange.
With that arrangement, a monodehydrobromination
of a dibromoalkane was performed in which the
product, monobromo-olefin, was removed from
the mixture before further HBr could be eliminated
[9,68].
7 High-temperature Water as a Medium
or Solvent for Microwave-assisted
Organic Synthesis
Priorities for safety, the environment and economy
of operation emphasise the need for decreased usage
of many organic solvents in chemical laboratories
and in industrial processes. The phasing out of some
halogenated solvents, particularly chlorofluorocar-
bons and CCl
4
, and the search for suitable replace-
ment media are indicative [64].
Since the pioneering work of Grieco & Breslow in
the late 1980s, water has been investigated more
intensively as a medium for non-enzymatic organic
reactions [92-98]. Temperatures below boiling have
been employed, mainly to exploit hydrophobic
effects brought about by the tendency of non-polar
species to aggregate and decrease the hydrocar-
bon/water interfacial area. Conditions at and near
supercritical, where hydrophobic effects are less
important, also have attracted increasing interest
[99-105].
At ambient temperature, water is a poor solvent
for most organic compounds. However, its ionic
product increases 1000-fold between 25°C and
240°C, so it becomes a stronger acid and base [101].
The dielectric constant decreases from 78 at 25°C to
20 at 300°C, indicating that the polarity is lowered
with temperature increase [101]. These apparently
anomalous properties suggest that as a medium for
organic reactions water could have a complex role
6.7 Flexible operation
Economic and safety considerations encourage
minimisation in the stockpiling of chemicals and
transportation of hazardous substances. Industrially,
reactor size is now important, with miniaturisation
becoming an attribute [86,87]. These factors suggest
that, in future, individual chemical reactors will be
required for diverse tasks and may need to be relo-
catable readily. Indicative of this trend is the DuPont
HCN process [88-90], involving catalysed condensa-
tion of methane and ammonia at 1200°C under
microwave heating (see Scheme 17.9). Advantages
include quick start-up and shut-down, high yield,
low holding capacity and almost complete elimina-
tion of waste. The potential for accidents during
Scheme 17.9
The DuPont HCN process [
88
-90].
