Chemistry Reference
In-Depth Information
O
SO
2
Cl
S
120-130ºC 6-12h
/
+
O
EPZE
Scheme 13.9
organic and mineral acids in high-temperature
esterifications.
Transformer fluids need to exhibit very low electri-
cal conductance and possess a particular viscosity. Sil-
icone oils can be manufactured to meet these criteria.
However, with time they degrade, lose their low con-
ductance and become coloured. Unfortunately, sili-
cones are not biodegradable and therefore the
disposal has an impact on the environment. A com-
mercial alternative to silicone oils has been found: the
fatty acid ester of pentaerythritol. The esterification is
performed with a mixture of C8-C12 fatty acids,
where the proportion of acids used is critical to
obtaining the required viscosity. As in the previous
example, organic acids can be used to catalyse this
process. However, a successful trial using Envirocat
EPIC at 200-250°C for 6 h gave a >90% yield of the
tetra-ester, showing that EPIC can be used as a com-
mercial replacement for the homogeneous catalyst,
eliminating the need for aqueous washes and
decolourising procedures (see Scheme 13.12).
O
O
O
O
Scheme 13.10
7.7 Esterifications
Esters are used in a wide variety of applications,
either as an intermediate or as the final product.
Commercially, esters are used as flavours and fra-
grances, sunscreens, plasticisers and transformer
fluids. Plasticisers and transformer fluids are of
particular interest because they also have an envi-
ronmental impact.
One of the most widely used plasticisers is dioctyl
phthalate (see Scheme 13.10). However, over the
last 2 years there have been a considerable number
of reports in the literature [34] on the use of these
compounds in children's toys. It appears that dioctyl
phthalates leach from the plastic when sucked by
children and these esters are toxic. In view of such
adverse publicity, the industry has been looking for
a non-toxic alternative and has come up with tri-
butyl citrate. This class of esters is prepared at high
temperature using catalysts such as methanesulfonic
acid. However, these organic acids are toxic and their
use results in a highly coloured product. However,
EPIC can be used successfully to replace these
acid catalysts. The reaction conditions shown in
Scheme 13.11 are not developed in any way, but
over 10 h at 150°C 100% conversion is achieved
with an isolated yield of 95%. This demonstrates
the effectiveness of Envirocat EPIC to replace
7.8 Aerobic oxidations
Of all the transformations discussed so far, oxidations
possess the potential to have the greatest impact on
the environment. Traditionally, reagents such as per-
manganate, dichromate, MnO
2
, OsO
4
, etc. have been
used to oxidise selectively a particular functional
group within a molecule. These reagents are highly
toxic and corrosive and the resulting wastestreams
are composed of large volumes of toxic, aqueous,
low-oxidation-state transition metal salts [28,30].
Even when used catalytically, the secondary oxidants
(e.g.
t
-butylhydroperoxide, 4-methylmorpholine-
N
-
oxide) result in the production of a large volume of
toxic waste.
The commercial process used to oxidise an aro-
matic methyl, for example, is a classic case of an
environmentally hostile process. The reaction condi-
tions used to achieve this oxidation involve the use
of cobalt acetate in acetic acid, usually at >250°C
