Thermal dissociation phosgene

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 ... 

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]. 

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... 

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... 

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] ... 

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