Phosgene-tetrachloromethane

In the production of polycarbonates from dihydroxylic compounds and phosgene, tetrachloromethane also causes a yellowing of the material, which is disadvantageous for optical applications of the polymers a colorless product can only be achieved when the phosgene has a CCI4 content <150 ppm [44). 

Carbon tetrachloride [56-23-5] (tetrachloromethane), CCl, at ordinary temperature and pressure is a heavy, colorless Hquid with a characteristic nonirritant odor it is nonflammable. Carbon tetrachloride contains 92 wt % chlorine. When in contact with a flame or very hot surface, the vapor decomposes to give toxic products, such as phosgene. It is the most toxic of the chloromethanes and the most unstable upon thermal oxidation. The commercial product frequendy contains added stabilizers. Carbon tetrachloride is miscible with many common organic Hquids and is a powerhil solvent for asphalt, benzyl resin (polymerized benzyl chloride), bitumens, chlorinated mbber, ethylceUulose, fats, gums, rosin, and waxes. 

In this lecture some new routes to phosphorus-carbon compounds with P-C multiple bonds, found in connection with our investigations on reactions of tertiary phosphanes with chlorinated carbon compounds, such as tetrachloromethane, hexachloroethane, phosgene, and isocyanide dichlorides are reported. Furthermore some stereochemical problems concerning this type of compound will be discussed. 

It has long been speculated, since the pioneering work of Muller in 1911 [1459], that phosgene may be an important intermediate in the toxicity of trichloromethane and tetrachloromethane. Experiments on rat liver microsomes have demonstrated that phosgene is generated when CHClj is metabolized both in vitro [1316,1631,1635,1636,1846] and in vivo... 

Colorimetric methods have been developed for the detection of phosgene in trichloromethane or tetrachloromethane [33,52,425,1360b,1504,1815]. A sensitive visual test involves the addition of an excess of phenylhydrazine (usually as its tra/w-3-phenylpropenoate salt), which reacts with phosgene according to equation (3.4) [52,425]. The diphenylcarbazide... 

Although the employment of tetrachloromethane (carbon tetrachloride) in fire extinguishers has now largely been superseded by more efficient and inherently safer materials, this type of extinguisher is undoubtedly still in use, despite the fact that phosgene is a major product of its oxidative thermal decomposition. 

In real fire situations, phosgene may also result from the thermal hydrolysis of tetrachloromethane [649b] ... 

The two most probable loss processes of phosgene in the stratosphere are photolysis, and transport into the troposphere. Phosgene can be formed in situ in the stratosphere [36a], and indeed it is one of the main photo-oxidation products in the upper troposphere and lower stratosphere from the breakdown of chlorinated hydrocarbons (of both natural and anthropogenic origin). For example, the relatively inert tetrachloromethane accumulates in the air, but can photodissociate in the stratosphere according to ... 

The earliest methods for the manufacture of phosgene were based upon John Davy s original procedure of exposing a mixture of carbon monoxide and dichlorine to sunlight [577]. Later methods (used to a limited extent in Italy and France during World War I) involved the oxidation of tetrachloromethane or hexachloroethane with suIfur(VI) oxide or fuming sulfuric acid (see Chapter 5) [577,1778]. Alternative methods proposed for the manufacture of phosgene, but which have not been commercialized, are described in Chapter 5. 

Tetrachloromethane and hexachloroethane in phosgene are determined following their extraction into tetrachloroethene. The phosgene is destroyed using aqueous sodium hydroxide, and the tetrachloroethene layer separated prior to analysis by gas-liquid chromatography. 

A possible exception to the foregoing statement is the so-called "disproportionation" of phosgene (see Chapter 8). This process. Equation (4.15), has been proposed as an efficient method for the manufacture of tetrachloromethane, although in the present feedstock situation this process would not be economical. Indeed, at the current prices commanded for phosgene, and with the perceived availability of carbon tetrachloride, it would be more beneficial to be able to derive phosgene from CC1 , for example by oxidation or hydrolysis. 

The presence of tetrachloromethane inhibits the formation of phosgene, and this has been ascribed to its action as an inert third body for the recombination of chlorine atoms to form dichlorine, thus inhibiting both the formation of [COClj- and its reaction with Cl-[1328] ... 

C) and pressures. The phosgene is recovered by heating the solution or by reducing the pressure. Alternatively, depending upon the requirements of subsequent processes, the solution of phosgene, in tetrachloromethane or chlorobenzene say, may be used directly. 

Tetrachloromethane is relatively cheap and abundantly available, and it can be converted to phosgene by a variety of methods. The oxidation or hydrolysis of CCl is... 

A convenient laboratory preparation of phosgene involves the oxidation of tetrachloromethane with oleum (or strong sulfuric acid with an SO 3 content less than 45%) according to Equation (5.11) [829,1231] ... 

If an excess of tetrachloromethane is used in the preparation, phosgene is also generated according to Equation (5.12) [829] ... 

Diatomaceous earth is also reported to catalyse the reaction between oleum and CC1 [1464]. The reagents are slowly mixed at 90 C, and the phosgene escapes from the reaction vessel leaving behind a mixture of sulfuric and chlorosulfonic acids. However, ordinary concentrated sulfuric acid may be used to react with tetrachloromethane in the presence of diatomaceous earth at 150-160 C, according to Equation (5.15) [829] ... 

When CCl is heated with a large amount of water in a sealed tube at 250 C, only CO 3 and HCl are formed. In order to produce phosgene by this procedure, the use of only a small amount of water is essential [649b]. Similarly, the hydrolysis of tetrachloromethane in the gas phase (300-450 C) proceeds according to the overall reaction [708] ... 

Tetrachloromethane reacts with an excess of dioxygen, in the gas phase above 300 C, to form phosgene according to Equation (5.17) [1288b] ... 

Fig. 5.7 The oxidation of tetrachloromethane to phosgene by Oj as a function of time [1088a].

The reactions of tetrachloromethane with sulfur(VI) oxide have been discussed in Section 5.2.1.1. Phosgene has been detected in the synthesis of TeCl from TeOj and CCl, [1103a]. 

The 7-radiolysis of tributylphosphate, (Bu0)3P=0, in tetrachloromethane produces dibutylphosphate, (Bu0)2P(0)0H, as the primary product, along with phosgene and CjClg, inter alia [303]. 

Passage of the off-gases through a mass of bone-black, coke, or pumice results in the dismutation of the phosgene into tetrachloromethane and carbon dioxide (see Section 8.2). 

During the production of CFC-11 and CFC-12 from the antimony-catalyzed reaction of CCl with HF, the small amount of moisture in the commercial HF is converted into COClj by reaction with the tetrachloromethane (see Section 5.2.1.2). The accumulation of phosgene... 

A solution of phosgene in tetrachloromethane exhibits a single C resonance (natural abundance) at 142.1 p.p.m. to low field of TMS [2141]. This is 62 p.p.m. to higher field than the carbonyl group in propanone and 30.8 p.p.m. to higher field than the carbonyl group... 

Although the reverse of the dismutation reaction (see Section 5.4) was first observed experimentally in 1868 [1824a], when it was observed that phosgene was produced when carbon dioxide and tetrachloromethane vapour were passed over pumice heated to 350-400 "C, the forward reaction has proved more elusive (despite the overwhelmingly favourable thermodynamics). The potential value of the phosgene dismutation was recognised as early as... 

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