Chemistry Reference
In-Depth Information
Fig. 22.1
Biphasic system consisting
of a reagent/catalyst phase, containing
a preferentially soluble reagent or
catalyst, and a product phase that has
limited solubility in the
reagent/catalyst phase.
Fig. 22.2
Typical solvents and their
Hildebrand solubility parameters
d
.
(From Ref. 3.)
biphasic product separation by running the reaction
at higher temperatures and separating the products
at lower temperatures [5,8,9,14-19].
compatible perfluorooctyl bromide, like other
fluorous solvents, is non-toxic and manufactured at
an industrial scale. However, some perfluorinated
olefins are toxic, e.g. CF
2
=C(CF
3
)
2
[24].
The unique solvent behaviour of fluorous solvents
was clearly recognised and described by Hildebrand
and co-workers in the 1940s: '
The large molal volumes
of the fluorocarbons compared with the corresponding
hydrocarbons lead to unusually low internal pressures or
cohesive energy densities, while their high weights aid in
maintaining them in the liquid state, the net result of which
is to make many of their solutions with ordinary liquid
hydrocarbons deviate from Raoult's law to such an extent
as to form two liquid phases, a state of affairs compara-
tively rare among non-polar liquids
' [25]. Both the
experimental data and the explanations for the
anomalous solution behaviour of two-component
systems involving fluorocarbons have been critically
reviewed [26].
In general, fluorophilic, organophilic or hydro-
philic solvents can be identified by their Hildebrand
solubility parameters: d
(fluorous)
<d
(organic)
<< d
(water)
.
These values are known for several compounds
or can be estimated by a group contribution method
[3]. Solvents having higher Hildebrand parameter
values than 18 MPa
1/2
3 Fluorous Solvents
Perfluorocarbon liquids have unique properties
making them suitable for a wide range of applica-
tions in medicine, biotechnology, electronics and the
oil and gas industries. It should be emphasised that
only perfluoroalkanes, perfluorodialkyl ethers and
perfluorotrialkyl amines are considered here as flu-
orous solvents. Many of their applications rely on
properties such as hydrophobicity, non-toxicity, high
oxygen solubility, inertness, high thermal stability,
non-flammability and efficient heat-transfer capac-
ity. These characteristics are due to the remarkably
high stability of the C-F bonds [20,21]. Fluorous sol-
vents are used in degreasing technologies (replacing
toxic, chlorinated and ozone-depleting solvents),
non-toxic fire-extinguishers, gas-tracers, plasticisers
of poly(tetrafluoroethylene) and medical applica-
tions [22]. A recently developed fluorocarbon emul-
sion called
Oxygent
TM
is now in the final stages of
clinical evaluation for use as a therapeutic oxygen
delivery system [23]. Its main ingredient, the bio-
(or located right from
