Chloroalkanes, dehydrochlorination

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. 

Typically, the saponification is run with 10% sodium hydroxide solution in a reactor cascade at 95-98°C under stringent pH control. The saponification mixture is separated in a settler. The upper phase consists of alkanes with a small proportion of chloroalkanes, which is removed by oleum refining or dehydrochlorination and high-pressure hydrogenation. The refined alkanes can be recycled to the reactor. In the aqueous lower phase are alkanesulfonates, sodium chloride, and between 4 and 8 wt % hydrotropically dissolved alkanes. An optimal separation can be approached at 95 °C, and residence times of less than 60 min if Fe(III) ions are added and pH values of 3-5 are maintained. 

Previously, it was shown that, when chloroalkanes or chloroolefins are subjected to temperatures of 350—450 C, dehydrochlorination occurs. In the presence of oxygen and an oxidizing catalyst, however, most of the hydrogen chloride is converted to chlorine, which is found in the resultant chloroolefin. 

Bacaloglu, R., Fisch, M. Degradation and stabilization of poly(vinyl chloride). IV. Molecular orbital calculations of activation enthalpies for dehydrochlorination of chloroalkanes and chloroalkenes. Polym. Degrad. Stabil. 47(1), 9-32 (1995)... 

Chlorinated organic compounds may be converted and dechlorinated using oxide heterogeneous catalysts. Acidic solids such as silica-alumina are active for dehydrochlorination of chloroalkanes in mild conditions to the corresponding olefins. IR spectroscopy shows that this occurs through a previous nucleo-phylic substitution step giving rise to adsorbed alkoxide species. 

The formation of 1,2-dichloroethane from ethane and ethylene is described in patents issued to National Distillers (11) and I.C.I. (12), respectively. Englin et at. (13) report the formation of vinyl chloride from chloroalkanes using catalysts containing melts of cuprous and cupric chloride. The details of the mechanism and kinetics of many of these reactions are still unresolved. It appears, though, that copper chloride can function effectively as a catalyst for chlorination and dehydrochlorination as well as being able to participate in os chlorination reactions. 

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