Chemistry Reference
In-Depth Information
4.1 Oxidations at carbon
(peracetic or perpropionic) was prepared and con-
tained in a recycling loop such that only H
2
O
2
itself
was consumed stoichiometrically (Fig. 11.18).
These processes were not adopted at the time (late
1970s) for a combination of reasons including cost
and complexity of plant and cost of H
2
O
2
. In current
industrial use, olefins that do not have active allylic
hydrogen can be epoxidised with oxygen, commonly
with a silver-containing catalyst. Ethylene oxide is
the prime example. Quite recently, a process also has
been introduced for mono-epoxidation of butadiene
using oxygen. Like most oxidations with air/oxygen,
it is carried out at elevated temperature and pressure
to achieve acceptable acitivity and at low conversion
per pass to achieve acceptable selectivity.
Propylene oxide is currently manufactured by one
of two types of process. In the chlorhydrin route,
olefin is reacted with hypochlorous acid (from chlo-
rine) followed by ring closure of the chlorhydrin
with lime:
Epoxidation
Epoxidation of olefins is a single oxygen transfer
reaction to which peroxygen systems ideally—
perhaps uniquely—are suited. The reactivity of
olefins varies widely: for example, the trisubstituted
2-methylbut-2-ene is 6000 times more reactive than
ethylene. It is very important to bear this in mind
when assessing published work. Many results are
reported for cyclooctene, which not only is pro-
bably the easiest common olefin to epoxidise but also
forms epoxides readily with dioxygen and other oxi-
dants that do not usually transfer single oxygen
atoms. Industrial demand, on the other hand, is most
often for epoxidation of substrates at the very far end
of the reactivity series—terminal olefins and allylic
compounds—and no conclusions on this can be
drawn from cyclooctene results. An indicative order
of reactivity is cyclooctene > methylcyclohexene >
cyclohexene > 2-octene > 1-octene > allyl chloride
(~other allylics, propylene). Cyclohexene is a useful
selectivity probe because it readily undergoes allylic
oxidation to give cyclohex-2-en-1-ol/one (Fig.
11.17).
Historically, organic peracids have been used to
prepare epoxides on small to medium scale, and
processes have even been worked out for such
preparations on the large commodity scale for propy-
lene oxide and epichlorhydrin [27,28]. The peracid
CH
3
CH=CH
2
+ HOCl Æ CH
3
CH(OH)CH
2
Cl
+ by-products
CH
3
CH(OH)CH
2
Cl + OH
-
Æ epoxide + H
2
O + Cl
-
This route produces both organic and inorganic
effluent and measures up poorly against the 'atom
utilisation' principle. The other major route is the
(Halcon) co-oxidation of propylene with either
isobutane (giving
t
-butanol co-product) or ethylben-
zene (leading to styrene co-product). The co-
O
OH
allylic
oxidation
+
OH
O
OH
epoxidation
+
Fig. 11.17
Oxidation products of
cyclohexene.
