1,3-Dehydrochlorination

Chlorinated rubber resins tend to undergo dehydrochlorination that is, a hydrogen atom on one segment of the polymer molecule joins with a chlorine atom on an adjacent segment to form hydrogen chloride. When they split off from the polymer molecule, a double bond forms in their place. In the presence of heat and light, this 

Dehydrochlorination is increased by exposnre to heat and light. Therefore, chlorinated rubber coatings are not suitable for use in high-temperatnre applications. Sensitivity to light, however, can be nullified by pigmentation. 

The disposal and destruction of chlorinated compounds is a subject of great importance. In fact, in 1993, some environmental groups had proposed the need for a chlorine-free economy. The cost of complete elimination of chlorinated compounds is quite staggering with the latest estimate as high as 160 billion/year.46 The most common method to destroy chlorocarbons is by high-temperature thermal oxidation (incineration).47 The toxic chlorinated compounds seem to be completely destroyed at high temperatures however, there is concern about the formation of toxic by-products such as dioxins and furans.48 

Oxidative catalysis over metal oxides yields mainly HC1 and C02. Catalysts such as V203 and Cr203 have been used with some success.49 50 In recent years, nanoscale MgO and CaO prepared by a modified aerogel/hypercritical drying procedure (abbreviated as AP-CaO) and AP-MgO, were found to be superior to conventionally prepared (henceforth denoted as CP) CP-CaO, CP-MgO, and commercial CaO/MgO catalysts for the dehydrochlorination of several toxic chlorinated substances.51 52 The interaction of 1-chlorobutane with nanocrystalline MgO at 200 to 350°C results in both stoichiometric and catalytic dehydrochlorination of 1-chlorobutane to isomers of butene and simultaneous topochemical conversion of MgO to MgCl2.53-55 The crystallite sizes in these nanoscale materials are of the order of nanometers ( 4 nm). These oxides are efficient due to the presence of high concentration of low coordinated sites, structural defects on their surface, and high-specific-surface area. 

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Aldrin is obtained from the Diels-Alder addition product of cyclopentadiene and vinyl chloride by dehydrochlorination followed by condensation with hexachlorocyclopenta-diene. 

Trichloroethylene is manufactured by the dehydrochlorination of tetrachloroelhane derived from the chlorination of ethyne with lime or by vapour phase cracking. 

CH2 CCl2- Colourless liquid, b.p. 32°C, manufactured by the dehydrochlorination of trichloroethane. In the presence of light and air, it decomposes with the evolution of HCI, phosgene, and methanal and deposition of some polyvinylidene chloride. Consequently it must be stored away from light and in the presence of dissolved inhibitors (such as phenols and amines). Under the influence of... 

Once the principal route to vinyl chloride, in all but a few percent of current U.S. capacity this has been replaced by dehydrochlorination of ethylene dichloride. A combined process in which hydrogen chloride cracked from ethylene dichloride was added to acetylene was advantageous but it is rarely used because processes to oxidize hydrogen chloride to chlorine with air or oxygen are cheaper (7) (see Vinyl polymers). 

In similar fashion, vinylidene chloride [75-35-4], C2H2CI2, has been prepared by successive chlorination and dehydrochlorination of vinyl chloride... 

C with low conversion (10—15%) to limit dichloroalkane and trichloroalkane formation. Unreacted paraffin is recycled after distillation and the predominant monochloroalkane is dehydrochlorinated at 300°C over a catalyst such as nickel acetate [373-02-4]. The product is a linear, random, primarily internal olefin. 

Ultimately, as the stabilization reactions continue, the metallic salts or soaps are depleted and the by-product metal chlorides result. These metal chlorides are potential Lewis acid catalysts and can greatiy accelerate the undesired dehydrochlorination of PVC. Both zinc chloride and cadmium chloride are particularly strong Lewis acids compared to the weakly acidic organotin chlorides and lead chlorides. This significant complication is effectively dealt with in commercial practice by the co-addition of alkaline-earth soaps or salts, such as calcium stearate or barium stearate, ie, by the use of mixed metal stabilizers. 

The direct reaction of other alkyl chlorides, such as butyl chloride, results in unacceptably low overall product yields along with the by-product butene resulting from dehydrochlorination. AH alkyl haHdes having a hydrogen atom in a P- position to the chlorine atom are subject to this complication. 

Minor amounts of acetylene are used to produce chlorinated ethylenes. Trichloroethylene (trichloroethene) and perchloroethylene (tetrachloroethene) are prepared by successive chlorinations and dehydrochlorinations (see Chlorocarsons and chlorohydrocarsons). The chlorinations take place in the Hquid phase using uv radiation and the dehydrochlorinations use calcium hydroxide in an aqueous medium at 70—100°C. Dehydrochlorination can also be carried out thermally (330—700°C) or catalyticaHy (300—500°C). 

At one time, the only commercial route to 2-chloro-1,3-butadiene (chloroprene), the monomer for neoprene, was from acetylene (see Elastomers, synthetic). In the United States, Du Pont operated two plants in which acetylene was dimeri2ed to vinylacetylene with a cuprous chloride catalyst and the vinyl-acetylene reacted with hydrogen chloride to give 2-chloro-1,3-butadiene. This process was replaced in 1970 with a butadiene-based process in which butadiene is chlorinated and dehydrochlorinated to yield the desired product (see Chlorocarbonsandchlorohydrocarbons). 

Chlorination and Chlorination—Dehydrochlorination of Paraffins. Linear internal olefins were produced by Shell at Geismar from 1968 to 1988, using the dehydrochlorination of chlorinated linear paraffins, a process that also yields hydrogen chloride as a by-product. To avoid the production of dichloroparaffins, which are converted to diolefins by dehydrochlorination, chlorination of paraffins is typically limited to 10% conversion. 

The chlorohydrin process involves reaction of propylene and chlorine in the presence of water to produce the two isomers of propylene chlorohydrin. This is followed by dehydrochlorination using caustic or lime to propylene oxide and salt. The Dow Chemical Company is the only practitioner of the chlorohydrin process in North America. However, several companies practice the chlorohydrin process at more than 20 locations in Germany, Italy, Bra2il, Japan, Eastern Europe, and Asia. 

Epoxid tion. Epoxidation, also referred to as saponification or dehydrochlorination, of propylene chlorohydrin (both isomers) to propylene oxide is accompHshed using a base, usually aqueous sodium hydroxide or calcium hydroxide. 

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. 

The degradation of VDC polymers in nonpolar solvents is comparable to degradation in the soHd state (101,125,129,130). However, these polymers are unstable in many polar solvents (131). The rate of dehydrochlorination increases markedly with solvent polarity. In strongly polar aprotic solvents, eg, hexamethylphosphoramide, dehydrochlorination proceeds readily (129,132). This reaction is cleady unlike thermal degradation and may well involve the generation of ionic species as intermediates. 

Chloroprene (qv), 2-chloro-1,3-butadiene, [126-99-8] is produced commercially from butadiene in a three-step process. Butadiene is first chlorinated at 300°C to a 60 40 mixture of the 1,2- and 1,4-dichlorobutene isomers. This mixture is isomeri2ed to the 3,4-dichloro-l-butene with the aid of a Cu—CU2CI2 catalyst followed by dehydrochlorination with base such as NaOH (54). 

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 1,1,2-trichloroethane [25323-89-1] produces vinyHdene chloride (1,1-dichloroethylene). Addition of hydrogen chloride to vinyHdene chloride in the presence of a Lewis acid, such as ferric chloride, generates 1,1,1-trichloroethane. Thermal chlorination of 1,2-dichloroethane is one route to commercial production of trichloroethylene and tetrachloroethylene. 

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). 

Dehydrochlorination of 1,1,2-trichloroethane at 500°C in the presence of a copper catalyst gives a different product, ie, cis- and /n7 j -l,2-dichloroethylene. Addition of small amounts of a chlorinating agent, such as chlorine, promotes radical dehydrochlorination in the gas phase through a disproportionation mechanism that results in loss of hydrogen chloride and formation of a double bond. The dehydrochlorination of 1,2-dichloroethane in the presence of chlorine, as shown in equations 19 and 20, is a typical example. 

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). 

Carbon tetrachloride can be reduced to chloroform using a platinum catalyst (10) or zinc and acid. With potassium amalgam and water, carbon tetrachloride can be totally reduced to methane. It is widely employed as an initiator in the dehydrochlorination of chloroethanes at 400—600°C ... 

Ethyl chloride can be dehydrochlorinated to ethylene using alcohoHc potash. Condensation of alcohol with ethyl chloride in this reaction also produces some diethyl ether. Heating to 625°C and subsequent contact with calcium oxide and water at 400—450°C gives ethyl alcohol as the chief product of decomposition. Ethyl chloride yields butane, ethylene, water, and a soHd of unknown composition when heated with metallic magnesium for about six hours in a sealed tube. Ethyl chloride forms regular crystals of a hydrate with water at 0°C (5). Dry ethyl chloride can be used in contact with most common metals in the absence of air up to 200°C. Its oxidation and hydrolysis are slow at ordinary temperatures. Ethyl chloride yields ethyl alcohol, acetaldehyde, and some ethylene in the presence of steam with various catalysts, eg, titanium dioxide and barium chloride. 

Pyrolysis. Pyrolysis of 1,2-dichloroethane in the temperature range of 340—515°C gives vinyl chloride, hydrogen chloride, and traces of acetylene (1,18) and 2-chlorobutadiene. Reaction rate is accelerated by chlorine (19), bromine, bromotrichloromethane, carbon tetrachloride (20), and other free-radical generators. Catalytic dehydrochlorination of 1,2-dichloroethane on activated alumina (3), metal carbonate, and sulfate salts (5) has been reported, and lasers have been used to initiate the cracking reaction, although not at a low enough temperature to show economic benefits. 

Oxychl orin ation of ethylene has become the second important process for 1,2-dichloroethane. The process is usually incorporated into an integrated vinyl chloride plant in which hydrogen chloride, recovered from the dehydrochlorination or cracking of 1,2-dichloroethane to vinyl chloride, is recycled to an oxychl orin a tion unit. The hydrogen chloride by-product is used as the chlorine source in the chlorination of ethylene in the presence of oxygen and copper chloride catalyst ... 

Unreacted 1,1-dichloroethylene exits the reactor as vapor and can be condensed and recycled to the reactor. Product 1,1,1-trichloroethane exits the reactor as a Hquid, along with the Lewis acid catalyst, and can be removed from the catalyst by flash distillation. Selectivity is high however, some dehydrochlorination of the product can occur in the distillation step. 

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