Chemistry Reference
In-Depth Information
chemical reaction between propionic acid and SO
2
Cl
2
requires no peroxide initiator and can be carried
out at temperatures down to 0°C. At low tempera-
tures, products from 3-chlorosulfonation predom-
inate (the products actually isolated vary with
the work-up procedure) and no 3-chloropropionic
acid is obtained. When the photoreaction is carried
out at 80-100°C, however, 2- and 3-chloropropionic
acids but no chlorosulfonation products are
obtained.
This type of temperature dependence of the
product distribution may prove to be fairly common
in photoinitiated processes, and could become one of
the main driving forces for a greater utilisation of
photochemistry in chemicals manufacturing.
1.4 Photochemical reactions for industry
The general advantages of photochemistry as a clean
technology, which have been outlined above, will
not of course provide the solutions to every problem
in chemicals manufacturing. Nevertheless, there is
enough evidence to suggest that a photochemical
approach could prove optimum in a significant
minority of cases. In short, photochemistry should
not be neglected by process development chemists
and engineers.
It would be easy to conclude from the vast litera-
ture on laboratory photochemical reactions that the
main obstacle to exploiting photochemistry industri-
ally is the lack of well-developed technologies rather
than a lack of known photoreactions. To some extent
this is true. Nevertheless, photochemical research
has, naturally enough, been concentrated largely
on those photoreactions that have no thermal
counterparts. There is still a need for studies of
photoreactions that could serve as substitutes for
well-established thermal processes, especially those
with recognised problems of waste generation, e.g.
Friedel-Crafts acylation. There have been very few
reports of investigations of the effect of temperature
on product distributions in photoreactions, so little
is known about the temperature sensitivity of the
majority of photoprocesses. It is likely, therefore, that
future progress in the exploitation of photochemistry
in chemicals manufacturing will depend on a com-
bination of new or more fully researched photo-
chemistry as well as new technology. The remainder
of this chapter will, however, deal mainly with the
latter, because it is in the technological area that
most of the perceived drawbacks of photochemistry
lie.
Fig. 1
8
.7
Energy diagram for a reaction that, in the electronic
ground state, has a high-energy first transition state leading to
an intermediate from which two pathways diverge, both with
lower energy transition states. The intermediate can be
reached by thermal activation (a) or by photochemical
activation (b).
Fig. 1
8
.
8
Thermal (peroxide initiated)
and photochemical reactions of
sulfuryl chloride with propionic acid.
