Chemistry Reference
In-Depth Information
3.3 Some inherently atom
uneconomic reactions
same starting material as used in the Wittig reaction)
with trichloromethyl anion generated
in situ
, acety-
lation and removal of acetate and chloride groups.
The process still produces significant amounts of
waste but it is much more environmentally benign
waste.
Similarly there are reactions that will usually
produce some waste material; these are typified by
substitution and elimination reactions. These reac-
tions should be viewed with caution when design-
ing green syntheses and, if a viable alternative is not
possible, attempts made either to recycle or to find a
use for the eliminated or substituted product.
The Wittig reaction is highly useful for forming
carbon-carbon double bonds and is widely used
industrially in the manufacture of vitamins and
pharmaceuticals. Although normally proceeding in
high yield under mild conditions, it is an inherently
wasteful reaction producing a mole equivalent of
phosphine oxide per mole of product (Fig. 2.9).
The phosphine oxide normally is converted to
calcium phosphate for disposal. It is this 0% phos-
phorus atom efficiency that makes the Wittig
reaction expensive, as well as environmentally prob-
lematic, and limits its usefulness to the production of
high-value-added products. The greenness of the
reaction can be improved, however, by converting
the oxide back to triphenyl phosphine [16,17]. This
recycling process, developed by the multinational
chemical company BASF, involves chlorination of
the phosphine oxide with phosgene, reduction with
aluminium powder and hydrolysis. Although not a
particularly green process (because it involves the
use of hazardous reagents and produces aluminium
hydroxide waste), overall, comparing the whole
processes including triphenyl phosphine manufac-
ture (Equation 2.2), the BASF route is more envi-
ronmentally benign and cost effective.
4 Reduction of Materials Use
Frequently, many chemical reactions involve the use
of reagents such as protecting groups and so-called
catalysts that do not end up in the useful product.
Organic solvents, often thought to be essential but
sometimes not actually required at all, fall into this
category. Some of these materials end up as waste
and some are recovered, but in all cases valuable
resources and energy are consumed that do not form
part of the required product.
Materials and money often are wasted in the
design of chemical reactors, and new thinking about
plant and ancillary equipment design (the process
intensification concept) is part of the chemical engi-
neering solution to greener chemical processes.
There is a significant amount of synergy between
the chemistry and engineering approaches to mate-
rials reduction. Frequently, low reactor utilisation,
because of large solvent volumes for example, may
necessitate the building of additional plant. By using
the concepts of green chemistry to integrate the
(2.2)
PCl
+
3
C H Cl + 6Na
Æ
P C H
(
)
+
6
NaCl
3
6
5
6
5
3
Specific, more environmentally benign alternatives
to the Wittig reaction now are being sought. The key
(intermediate
1
) to the anti-HIV drug Efavirenza has
been produced in a one-pot process (see Scheme 2.1)
with an overall yield of 92% [18]. The process
involves reaction of cyclopropylcarboxaldehyde (the
Scheme 2.1
Fig. 2.9
Typical Wittig reaction.
