Photochemical decomposition phosgene

Owing to their low flammability and excellent degreasing properties, the chlorinated hydrocarbons are commonly used as solvents in the laboratory, office and industrial workplaces. These materials, however, are particularly susceptible to thermal or photochemical decomposition to phosgene in normal environments. 

Phosgene was detected, by g.c.-m.s. and n.m.r. spectroscopy, in a commercial solvent mixture of trichloroethene and tetrachloroethene (80 20) [1254]. Such contamination is likely to arise as a result of photochemical decomposition. The primary product of the oxidation of trichloroethene under ultraviolet radiation is trichloroepoxyethane cf. the thermal decomposition product, structure (3.4), which rearranges to dichloroethanoyl chloride and chloral. Secondary decomposition of one of these compounds occurs to give CO, COj, HCl and COCI2 [ICI55]. 

The experimental conditions in this procedure resemble those described for the formation of phosgene from CCl. The photochemical decomposition is discussed in Section 5.2.5. 

In 1930, Henri and Howell [942] incorrectly assumed that the initial step of this reaction involved the formation of CO and 2C1 . A few years later, Montgomery and Rollefson [1428] studied the kinetics of the photochemical decomposition of phosgene, and derived the rate law ... 

Emissions of phosgene most commonly arise as a result of its release during manufacture and use, its formation from the decomposition of chlorinated hydrocarbons, and its formation from the photochemical oxidation of air-borne chlorinated organic materials, particularly the C, and C chloroalkanes, and chloroethenes. The location and estimation of air emissions from sources of phosgene have been described by the US Environmental Protection Agency [2088b], Catastrophic emissions and accidental spills and leaks are discussed in Section 3.6. 

The equilibrium constants calculated from rate measurements are in agreement with those obtained directly from equilibrium measurements [218]. For the decomposition of phosgene, a similar rate law is given with the signs reversed [387]. The full mechanism is essentially similar to that developed for the photochemical reaction (apart from initiation), and is discussed in more detail in Section 5.1.1. 

PROBABLE FATE photolysis-, photochemical reactions in aqueous media are probably unimportant, slow decomposition in the troposphere in the presence of nitrogen oxides is possible, appreciable photodissociation may occur in stratosphere, photooxidation half-life in air 19.1-191 days oxidation-, probably unimportant, in troposphere, oxidation by hydroxyl radicals to CO2, CO, and phosgene is important fate mechanism hydrolysis not an important fate process, first-order hydrolytic half-life 704 yrs volatilization due to high vapor pressure, volatilization to the atmosphere is rapid and is a major transport process sorption sorption to inorganic and organic materials is not expected to be an important fate mechanism biological processes bioaccumulation is not expected, biodegradation may be possible but very slow compared with evaporation... 

The thermal decomposition of phosgene occurs by a complex free radical route, just as does the reverse reaction (15). It can easily be seen that (6l)->-(ai) excitation is necessary to make the forbidden direct decomposition into CO and CI2 occur. This does not correspond to the lowest excited state of phosgene, which is a B2 state due to m — i excitation. Instead reaction would require an n to a excitation. Experimentally COCI2 and C0Br2 do decompose photochemically and apparently by a molecular process. 22) However the excited state involved is not known. 

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