Chemistry Reference
In-Depth Information
Fig. 1
8
.13
Decarboxylative ring-closure
reactions carried out in aqueous
solution and initiated by 30
8
-nm light
from an XeCl* excimer lamp.
Table 1
8
.1
Excimer light sources with peak output wave-
lengths in the range 100-400 nm
a
have described a falling-film reactor incorporating
a 308-nm excimer lamp (XeCl*). In this reactor
they have carried out some interesting decarboxyla-
tive ring-closures (see Fig. 18.13) using water as the
solvent. Because these reactions use light as the acti-
vating agent and an environmentally benign solvent,
they are good examples of what can be achieved in
cleaner synthesis utilising photochemistry.
Gas mixture
Emitting species
Peak wavelength (nm)
Ar
Ar
2
*
12
8
Kr
Kr
2
*
14
8
Xe
Xe
2
*
172
Ar
+
Cl
2
ArCl*
175
Ar
+
F
2
+
SF
6
(trace)
ArF*
193
Kr
+
Cl
2
KrCl*
222
4 Conclusions
Kr
+
F
2
+
SF
6
(trace)
KrF*
24
8
Xe
+
Cl
2
XeCl*
30
8
Several reasons for regarding photochemistry as a
viable clean technology for chemicals manufactur-
ing have been put forward and elaborated in this
chapter. Although thermal reactions will continue to
provide the largest proportion of synthetically useful
procedures, photochemical reactions can have sig-
nificant advantages and should not be neglected out
of hand. The current unpopularity of photochem-
istry may be traced to various influences, such as
the exclusion of preparative photochemistry from
many, if not most, chemistry courses, the need
for specialised reactors and the admittedly high
cost of photons. Technological problems, such as
window fouling, also contribute to the lack of enthu-
siasm for photochemistry, but such problems can be
overcome given sufficient incentive to develop novel
reactors.
Adaptations of the widely available stirred batch
reactors can be accomplished relatively easily and
inexpensively and may prove sufficient for some
photoreactions, but they will not yield all the poten-
tial benefits of photochemical processing. There
are, however, several designs of reactor that could be
developed for synthesis and that promise much
Xe
+
F
2
+
SF
6
(trace)
XeF*
352
a
Data from Ref. 24.
excimer sources that have been most studied gener-
ate either noble gas excimers (excited dimers), e.g.
Xe
2
*, which give out light in the vacuum-UV region
of the spectrum (wavelengths below 200 nm), or
exciplexes (excited complexes) of noble gases with
halogens, e.g. XeCl*, which tend to give out longer
wavelengths. Both types are referred to as excimer
sources, even though true excimers are involved
only with the unmixed gases. It seems probable that
other excimer systems, offering an even wider range
of output wavelengths, should be relatively easy to
develop.
As with many advances in photochemical tech-
nology, these new light sources so far have found
applications primarily in the photo-oxidative degra-
dation of organic materials [24-30], but synthetic
applications are beginning to appear.
In one of the few published papers on photo-
chemistry for clean synthesis, Griesbeck
et al.
[31]
