Chemistry Reference
In-Depth Information
tion equipment within a double envelope makes it possible to collect and destroy
any leakage of the phosgene in dedicated installations [50].
2.2
Phosgene ''Oligomers''
2.2.1
Diphosgene
Trichloromethyl chloroformate (diphosgene) 12 is a dense liquid, d
15
1.65, bp
128
C, and vapor pressure at 20
C of 1370 Pa or 10.3 mmHg (see Chapter 3 and
[51]). The compound was used as a warfare agent during World War I and has also
been called Supralite or Superpalite.
Cl
O
Cl
12
Cl
O
Cl
Kurita systematically investigated the reactivity of diphosgene and compared its
reactions with those of phosgene [52]. Ugi proposed diphosgene as the reagent of
choice for the synthesis of isocyanides [53].
Some of the safety hazards associated with phosgene could be circumvented by
the general availability of diphosgene [1, 54]. It is readily available from the regular
commercial suppliers; for sources, see Section 7.2.
The catalytic decomposition routes of diphosgene are extremely interesting. The
compound is stable at room temperature, but decomposes to phosgene when
heated above 300
C [55-57], or on contact with iron(III) oxide, iron(III) chloride,
or aluminum(III) chloride (less active) or activated charcoal (very active) [51].
The kinetics of the thermal decomposition of diphosgene has been studied over
a temperature range of 260-310
C and a pressure range of 4-17 mmHg. The re-
action has been found to be first order and homogeneous, with catalysis by the
glass walls of vessels having only a slight influence [56]. The rate constant is given
by the expression: k
1
10
13
e
14:500/RT
.
The possible equilibrium between phosgene, carbon monoxide, and chlorine was
not found to arise as a result of the reaction, the sole product of decomposition
being phosgene.
The formation of N-carboxy a-amino acid anhydrides with diphosgene is usually
unsuccessful without prior decomposition of the diphosgene to phosgene [58]. Di-
phosgene decomposes instantly to give phosgene under catalysis by active charcoal,
making the method as rapid as the phosgene stock solution method [58] (see Sec-
tion 4.3.5).
Decomposition to tetrachloromethane and carbon dioxide occurs on exposure to
alumina [51, 55, 57, 60].
ΒΌ
1
:
4
