Tetrachloromethane, from chlorination

Figure 7.13 presents a simplified flowsheet, which concentrates the essential features the balanced VCM technology, as conceptually developed in the previous sections, but this time with the three plants and recycles in place chlorination of ethylene (Rl), thermal cracking of EDC (R2) and oxyclorinahon of ethylene (R3). As mentioned in Section 7.3, from plantwide control three impurities are of particular interest (I]) chloroprene (nbp 332.5 K), (12) trichloroethylene (nbp 359.9K), and (13) tetrachloromethane (nbp 349.8). I, and 12 are bad , since the first can polymerize and plug the equipment, while the second favors the coke formation by EDC pyrolysis. On the contrary, I3 has a catalytic effect on the VCM formation, in some patents being introduced deliberately. 

The classical Hunsdiecker reaction (equation 18), involving the reaction of silver carboxylates with halogens, and the various associated side reactions, has been reviewed several times. Optimum yields are obtained with bromine, followed by chlorine. Iodine gives acceptable yields provid that the correct stoichiometry of 1 1 is used. The reaction is most frequently carried out in tetrachloromethane at reflux. From a practical point of view, one drawback is the difficulty encountered in the preparation of dry silver carboxylates the reaction of silver oxide on the acyl chloride in tetrachloromeAane at reflux has been employed to circumvent this problem. Evidently the use of molecular bromine limits the range of functional groups compatible with the reaction the different reaction pathways followed by the silver salts of electron poor (equation 19) and electron rich (equation 20) aryl carboxylates illustrate this point well. 

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 ... 

Introduction of tetrachloromethane (TCM) to the feedstream in the CO oxidation process inhibits the formation of CO2 on stoichiometric CaHAp but has relatively little effect on a nonstoichiometric sample. The effect of TCM results from the interaction of chlorine with the surface of the catalyst to form chlorapatite. 

The tetrathiadiazepine (60) was obtained by chlorination of lenthionine in tetrachloromethane at 0°C, followed by cyclization with bis(trimethylsilyl)sulfurdiimide to give two products. The first was trithiadiazine (12%) and the second was an orange solid which, on the basis of its mass spectrum, was tentatively assigned structure (60) (20%) <90JCS(Pi)509> this presumably arose from incomplete sulfur-sulfur bond cleavage by chlorine (Scheme 35). 

The source of chloroprene (Ii) is the cracking section, trichloroethylene (I2) appears mainly in the oxychlorination, while tetrachloromethane (I3) is produced both in chlorination and oxy-chlorination reactors. These Impurities must be removed selectively from the high purity DCE (nbp 356.8 K) sent to cracking. Both ft and I2 must be kept low, while I3 be kept close to an optimal value. This is the incentive of the plantwide control problem. 

Any combination of atomic and hybrid orbitals may overlap to form bonds. For example, the four sp orbitals of carbon can combine with four chlorine 3p orbitals, resulting in tetrachloromethane, CCI4. Carbon-carbon bonds are generated by overlap of hybrid orbitals. In ethane, CH3-CH3 (Figure 1-19), this bond consists of two sp hybrids, one from each of two CH3 units. Any hydrogen atom in methane and ethane may be replaced by CH3 or other groups to give new combinations. 

When methane and chlorine gas are mixed in the dark at room temperature, no reaction occurs. The mixture must be heated to a temperature above 300°C (denoted by A) or irradiated with ultraviolet light (denoted by hv) before a reaction takes place. One of the two initial products is chloromethane, derived from methane in which a hydrogen atom is removed and replaced by chlorine. The other product of this transformation is hydrogen chloride. Further substitution leads to dichloromethane (methylene chloride), CH2CI2 tri-chloromethane (chloroform), CHCI3 and tetrachloromethane (carbon tetrachloride), CCI4. 

This reaction is not really suitable for preparing specific halogenoalkanes, because we get a mixture of substitution products. In the reaction between methane and chlorine, the products can include dichloromethane, trichloromethane and tetrachloromethane as well as chloromethane. These other products result from propagation steps in which a chlorine free radical attacks a halogenoalkane already formed. For example ... 

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