Phosgene dissociation

The following equation represents the equilibrium reaction for the dissociation of phosgene gas. 

Phosgene is a stable compound at ordinary temperatures and in absence of humidity. At somewhat elevated temperatures, however, it dissociates fairly easily into carbon monoxide and chlorine ... 

Chemically much more inert than chlorine, phosgene is a very stable compound and is not dissociated by cxplo. bursting charges. When dry, phosgene does not attack iron ami may, therefore, be kept indefinitely in iron and steel containers. It is, however, extremely sensitive to water, in contact with which it quickly breaks down into hydroehlorii acid and carbon dioxide, according to the equation... 

Very recently, phosgene-free methods for producing organic isocyanates have been developed. One method involves reductive carbonylation of a nitro compound in the presence of a monoalcohol to produce a urethane compound, followed by thermal dissociation of the resulting urethane compound, as shown below ... 

Surprisingly, under the conditions of flash photolysis, no detectable photochemical reaction is induced between CO and Clj, although almost complete dissociation of phosgene can be induced under identical conditions [1523,1647a] this has been ascribed to the very high radical concentrations present, and the high instantaneous temperatures which are generated [1523],... 

The extent of thermal dissociation of phosgene at 0.5, 1.0 and 10 bar pressure (0.05, 0.1 and 1 MPa, respectively) has been calculated [1764] based on the accepted ideal gas thermodynamic values [359aa], and is illustrated in Fig. 5.3, whilst the enthalpy of formation for this reaction has been based on actual measurements of the equilibrium constant in the temperature range of 645-725 K by heating together carbon monoxide and dichlorine [218], see Section 6.1. The equilibrium reaction depicted in Equation (5.1) has been measured experimentally both by dissociation of phosgene and by association of carbon monoxide and dichlorine [216]. At 603, 553 and 503 C, the dissociation was found to be 91, 80 and 67%, respectively, in reasonable agreement with the values based on the ideal gas calculations illustrated in Fig. 5.3. At temperatures above 800 C, the dissociation is essentially complete [216]. 

Even at temperatures as high as 503 C, however, the thermal reaction between carbon monoxide and dichlorine is far from instantaneous, as illustrated in Fig. 5.4, which indicates the time required for the close attainment of equilibrium for both the formation (upper curve) and dissociation (lower curve) of phosgene [216]. 

The mechanism for the formation of phosgene was represented originally by the following steps, in which the Cl, dissociation reaction is initiated thermally, and the Cl - and [Cl,] -radicals are the chain carriers for the process [218] ... 

A laboratory Liebig s condenser can be conveniently used as the reactor, since water cooling is necessary to remove the heat from the reaction which is highly exothermic (dH -108 kj mol" ). Since the thermal dissociation of phosgene (Fig. 5.3) becomes perceptible at above 200 C, it is desirable to regard this temperature as a ceiling for the temperature of the bulk catalyst. The principal function of the charcoal catalyst is believed to be the activation of the Cl [1053], and if dichlorine is adsorbed onto the activated carbon before... 

In view of the dissociation of phosgene at such high temperatures this reaction appears to be very unlikely. 

As seen in Chapter 6, phosgene is thermodynamicaliy stable under standard conditions with respect to its elements. Equation (8.1), and with respect to dissociation. Equation (8.2). 

This chapter describes the dissociation and dismutation processes for phosgene in some detail, as well as examining its photochemlcally induced decomposition. 

The thermal dissociation of phosgene into carbon monoxide and elemental chlorine. Equation (8.2), is the reverse of the reaction used widely, either thermally, catalytically or photochemlcally promoted, for the synthesis of phosgene (see Section 5.1). Thus, much of the chemistry relevant to this Section has already been covered in Chapters 5 and 6. Much of the early work, both kinetic and thermodynamic, was marred by a lack of appreciation of the importance of high purity, and the fact that the system takes a very long time to come... 

Although [COCl]- has been universally recognized as the key intermediate in the thermal dissociation of phosgene, and there is much evidence for its existence in... 

The thermal dissociation of phosgene is too slow at temperatures below 500 "C to act as a useful source of [COCl]- radicals [2027]. However, at 14(X)-2(XX) K, under shock tube conditions, [COCl]- is the primary decompostion product [1253a]. The bond dissociation... 

Fig. 8.3 The calculated [1825a] effect of temperature upon log, (j(/fj/bar) for the phosgene dissociation reaction, based on data from [3S9aaj.

The dismutation reaction for phosgene is given in Equation (8.3), and is in competition with the dissociation reaction expressed by Equation (8.2). 

The photochemical dissociation of phosgene was first reported by Weigert in 1907... 

The photochemical dissociation of phosgene has been used as the basis of a chemical laser [1386]. When a mixture of COCl, and H,/D, (which is unreactive in the ground state see Section 9.7.1) is irradiated with the P line of a CO, laser in a laser cavity, HCl or DCl are produced in excited vibrational states, thus producing a chemical HCl or DCl laser. The (1012 cm ) line of CO, is resonant with a phosgene combination band, rj+rj... 

Huston s work [997] also indicated that the reaction between aluminium(III) chloride and COCIj resulted in the formation of the 1 1 adduct, AlCIg.COClj, which readily loses phosgene in vacuo at room temperature to yield pure aluminium(III) chloride. This was later confirmed independently by tensimetric titration (see Fig. 9.5) [376], and the i.r. spectrum (see Table 9.1) of the white product (m.pt. 25 C dissociation pressures of 4.8 kPa at 0 C and 58.66 kPa at 25 C) indicated that the phosgene was O-bonded, viz. [Cl2C=0—>A1C13], and that no [A1C1 ] was present [376]. From the data in this paper, the enthalpy of dissociation of the adduct can be calculated ... 

The dissociation of phosgene into carbon monoxide and dichiorine, and CO exchange reactions, are discussed in Section 8.2. However, there is a report that under the right conditions, phosgene will react with carbon monoxide [2184] ... 

As is common for many of the reactions with metal oxides and phosgene, the reaction rate goes through a maximum at ca. 650 C, drops to a minimum at ca. 850 C, and then starts to rise again (see Fig. 9.10). This is reasonably explained in terms of phosgene dissociation into carbon monoxide and dichlorine (see Chapter 8) becoming appreciable at above 600 C,... 

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