Dehydrochlorination thermal

Pyrolysis. Vinyl chloride is more stable than saturated chloroalkanes to thermal pyrolysis, which is why nearly all vinyl chloride made commercially comes from thermal dehydrochlorination of EDC. When vinyl chloride is heated to 450°C, only small amounts of acetylene form. Litde conversion of vinyl chloride occurs, even at 525—575°C, and the main products are chloroprene [126-99-8] and acetylene. The presence of HCl lowers the amount of chloroprene formed. 

Dichloroethane, an important intermediate for vinyl chloride production, is produced by catalytic chlorination of ethylene in either vapor or Hquid phase or by oxychlorination of ethylene. Thermal dehydrochlorination of 1,2-dichloroethane produces vinyl chloride and coproduct hydrogen chloride. Hydrogen chloride is commonly recycled to an oxychlorination unit to produce 1,2-dichloroethane or is processed into sales-grade anhydrous or aqueous hydrogen chloride. 

Dehydrochlorination of chlorinated derivatives such as 1,1,2-trichloroethane may be carried out with a variety of catalytic materials, including Lewis acids such as aluminum chloride. Refluxing 1,1,2-trichlorethane with aqueous calcium hydroxide or sodium hydroxide produces 1,1-dichloroethylene in good yields (22), although other bases such as magnesium hydroxide have been reported (23). Dehydrochlorination of the 1,1,1-trichloroethane isomer with catalytic amounts of a Lewis acid also yields 1,1-dichloroethylene. Other methods to dehydrochlorinate 1,1,1-trichloroethane include thermal dehydrochlorination (24) and by gas-phase reaction over an alumina catalyst or siUca catalyst (25). 

Oxychlorination. This is an important process for the production of 1,2-dichloroethane which is mainly produced as an intermediate for the production of vinyl chloride. The reaction consists of combining hydrogen chloride, ethylene, and oxygen (air) in the presence of a cupric chloride catalyst to produce 1,2-dichloroethane (eq. 24). The hydrogen chloride produced from thermal dehydrochlorination of 1,2-dichloroethane to produce vinyl chloride (eq. 25) is usually recycled back to the oxychlorination reactor. The oxychlorination process has been reviewed (31). 

A second method for synthesis of aHyl chloride is thermal dehydrochlorination, ie, cracking, of 1,2-dichloropropane, but this method is generally less satisfactory because of low aHyl chloride selectivity (50—60%) and operating temperatures of 500—600°C (4,7—10). The by-products of cracking are 1-chloropropene and 2-chloropropene, which have no significant commercial use. 

Naqvi [134] has proposed an alternative model to the Frye and Horst mechanism for the degradation and stabilization of PVC. At room temperature, PVC is well below its glass transition temperature (about 81°C). The low thermal stability of the polymer may be due to the presence of undesirable concentrations of like-poles in the more or less frozen matrix with strong dipoles. Such concentrations, randomly distributed in the polymer matrix, may be considered to constitute weak or high energy spots in the polymer, the possible sites of initiation of thermal dehydrochlorination. 

Thermal stabilities of modified PVC samples acet-oxylated to varying degrees (reaction temperature 46°C) were determined [45]. Rate of thermal dehydrochlorination at 1% degradation was taken as a measure of thermal stability. The log of the degradation rate is plotted against the acetate content of the polymer in Fig. 2. 

In keeping with earlier observations (19,98), the nonoxidative thermal dehydrochlorination of PVC has been shown recently to be facilitated by preliminary photodegradation of the polymer (10,99). The thermal sensitivity enhancement increases with decreasing wavelength of irradiation (10) and undoubtedly results from the photolytic formation of thermally labile defect sites (10). 

The parent hydrocarbon 4 has been obtained by several routes91-93, with the thermal dehydrochlorination of the readily available 2,4,6-tris(chloromethyl)mesitylene (148, equation 12) being particularly valuable92. The yields of this process (close to 50%) are reproducible, making 4 a readily available, albeit difficult-to-handle, highly reactive starting material for further transformations (see below). 

Secondly, the carbon framework holding the exocyclic double bonds could be extended. This is demonstrated by naphtharadialene 5, a highly reactive intermediate which has been generated by thermal dehydrochlorination from either the tetrachloride 178 or its isomer 179106. Radialene 5 has not been detected as such in these eliminations rather, its temporary formation was inferred from the isolation of the thermolysis product 180 which was isolated in 15% yield (equation 25). Formally, 5 may also be regarded as an [8]radialene into whose center an ethylene unit has been inserted. In principle, other center units—cyclobutadiene, suitable aromatic systems—may be introduced in this manner, thus generating a plethora of novel radialene structures. 

Thermal dehydrochlorination of hydroximoyl chlorides affords nitrile oxides (50-52). O-Ethoxycarbonylbenzohydroximoyl chloride, generating benzonitrile oxide, was used as a stable nitrile oxide precursor, which was efficiently used in... 

Arylsydnone-4-carbonitrile oxides, which are generated in situ by thermal dehydrochlorination of the corresponding hydroximic acid chlorides, undergo 1,3-dipolar cycloadditions with sydnone-4-carbonitriles to give 3-aryl-4-[5-(3-arylsydnonyl)-l,2,4-oxadiazol-3- yl]sydnones 228 (392). 

Tetrafluoroethylene is produced from the thermal dehydrochlorination of chlorodifluoro-methane (equation 17.35) which in turn is produced from chloroform and HF (equation 17.34)... 

Vinylidene fluoride is produced by the thermal dehydrochlorination of 1-monochloro-1,1 -difluoroethane. 

Suppression of the Thermal Dehydrochlorination of Poiy(vinyl Chloride) by Addition of Stabilizers... 

Taking this data into account, we subjected the chemically dehydrochlorinated polymer to thermal treatment firstly at 200°C for 2 h to enrich the product with carbon via thermal dehydrochlorination and then at 350°C for 30 min to allow the formation of carbon-like structures. 

Of the numerous reactions the most thoroughly studied is dehydrochlorination [12] (Scheme 2.3). Common dehydrochlorinating agents include alcoholic alkali [4], liquid ammonia [13], methylamine [13], LiCl in dimethylformamide (DMF) [14], MOH or M2CO3 (M = K or Na) in DMF [15] and tertiary and heterocyclic amines [16]. Moreover, some diaryltrichloroethanes, such as dichlordiphenyl-trichloroethane (DDT), may undergo thermal dehydrochlorination near 170-200 °C. 

Thermal dehydrochlorination of 1,2-dichloroethane188-190 272 273 takes place at temperatures above 450°C and at pressures about 25-30 atm. A gas-phase free-radical chain reaction with chlorine radical as the chain-transfer agent is operative. Careful purification of 1,2-dichloroethane is required to get high-purity vinyl chloride. Numerous byproducts and coke are produced in the process. The amount of these increases with increasing conversion and temperature. Conversion levels, therefore, are kept at about 50-60%. Vinyl chloride selectivities in the range of 93-96% are usually achieved. 

Hi) By formation of seven- from six-membered rings The expansion of a di- or a tetra-hydropyran ring fused to a three-membered ring has been used as a synthetic approach to oxepins. Thus the synthesis of oxepin (193) was attempted by thermal dehydrochlorination of a tetrahydropyran (equation 57) obtained from a dichlorocarbene addition reaction (65CI(L)184). Unfortunately the equilibrium appeared to favor the keto tautomer to the apparent exclusion of the enolic oxepin form (193). 

This process is shown schematically in Figure 7. The ethylene part of the feed reacts with chlorine in the liquid phase to produce 1,2-di-chloroethane (EDC) by a simple addition reaction, in the presence of a ferric chloride catalyst (9). Thermal dehydrochlorination, or cracking, of the intermediate EDC then produces the vinyl chloride monomer and by-product HC1 (1). Acetylene is still needed as the other part of the over-all feed, to react with this by-product HC1 and produce VCM as in the all-acetylene route. 

POLVINYLIDENE FLUORIDE. This product is made by the free-radical chain polymerization of vinylidene fluoride (H2C=CF2). This odorless gas which has a boiling point of —82°C is produced by the thermal dehydrochlorination of 1,1,1-chlorodifluoroethane or by the dechlorination of 1,2-dichloro-l.l-difluoro-ethane. As shown by the following equations, 1,1,1-chlorodifluoroethane may be obtained by the bydroflnorination and... 

Equations (2) and (3) could permit a simple determination of fc 2/A-B, and the authors started their experimental work with this objective in mind. The values of Ez and A3 for the C2H2CI3 radicals were known from the work of Ayscough et al. (7) in which they applied the rotating sector technique to the photochlorination of cis-l,2-dichloroethylene and found logio A3 (1. mole"1 sec.-1) = 8.7 0.3 and i 3 = 2.7 0.6 kcal. mole-1. In view of these values, and Howlett s conclusion, based on thermal dehydrochlorination studies, that for C2H3CI2 radical A 2 = 1013 sec.-1 and f 2 = 22 kcal. mole-1, no detectable isomerization was expected below 200°C. However, in contrast to this, a concurrent isomerization was easily detectable at 30°C even at a chlorine pressure of 200 mm. A detailed investigation was therefore necessary to explain the reasons for the discrepancy. 

The curve obtained with PVC prepared by suspension polymerization generally indicated autocatalytic thermal dehydrochlorination, and the time required for 0.1 mole % decomposition was usually less than 35 minutes (Figure 1). When PVC was prepared by bulk polymerization, the dehydrochlorination plot was generally slightly more linear, showed little autocatalytic character, and the time for 0.1 mole % decomposition was approximately 40 minutes (Figure 2). 

The process is combined with the process in which hydrogen chloride is produced by thermal dehydrochlorination of ethylene dichloride. 

Thus, vinyl chloride is manufactured by the thermal dehydrochlorination of ethylene dichloride at 95 percent yield at temperatures of 480 to 510°C under a pressure of 50 psi with a charcoal catalyst. 

The influence of dicarboxylic acid ester plasticisers on the thermal degradation of PVC significantly depends on the physical state of the PVC-plasticiser system. If PVC retains the structure formed in the stage of suspension polymerisation, the additive produces inhibition of the process of thermal dehydrochlorination. In the case of true diluted PVC solutions in ester plasticisers, the polymer exhibits accelerated degradation, in accordance with a high value of the solvent basicity. 7 refs. 

Production and use of PVC occur in the presence of air, i. e. in the presence of oxygen. Therefore, it is surprising that the mechanistic details of thermooxidative degradation of PVC are still not fiilly revealed. The major reactions of this process are shown in Scheme 1. As indicated in this Scheme, thermal dehydrochlorination yields HCl and simultaneously sequences of conjugated double bonds (polyenes) in the chain. The reactive polyenes lead to peroxides in a reaction with oxygen followed by the formation of radicals. Subsequent chain reactions result in additional initiation of HCl loss and further oxidative processes (/, 8). 

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