1.2- dichloro-2-butene, dehydrochlorination

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). 

The vinylacetylene [689-97-4] route to chloroprene has been described elsewhere (14). It is no longer practical because of costs except where inexpensive by-product acetylene and existing equipment ate available (see Acetylene-DERIVED chemicals). In the production of chloroprene from butadiene [106-99-0], there are three essential steps, chlorination, isomerization, and caustic dehydrochlorination of the 3,3-dichloro-l-butene, as shown by the following equations Chlorination... 

In the production of chloroprene from butadiene, there are three essential steps liquid- or vapour-phase chlorination of butadiene to a mixture of 3,4-dichloro-l-butene and l,4-dichloro-2-butene catalytic isomerization of 1,4-dichloro-2-butene to 3,4-dichloro-l-butene and caustic dehydrochlorination of the 3,4-dichloro-l-butene to chloroprene. By-products in the first step include hydrochloric acid, 1-chloro-1,3-butadiene, trichlorobutenes and tetrachlorobutanes, butadiene dimer and higher-boiling products. In the second step, the mixture of l,4-dichloro-2-butene and 3,4-dichloro-l-butene isolated by distillation is isomerized to pure 3,4-dichloro-l-butene by heating to temperatures of 60-120°C in the presence of a catalyst. Finally, dehydrochlorination of 3,4-dichloro-l-butene with dilute sodium hydroxide in the presence of inhibitors gives crude chloroprene (Kleinschmidt, 1986 Stewart, 1993 DuPont Dow Elastomers, 1997). 

At low conversions (10-25%) high yields can be achieved (85-95%). 1,4-Dichloro-2-butenes formed may be used in the manufacture of other products or isomerized to give the desired 3,4-dichloro-l-butene. This is done in the presence of a catalytic amount of CuCl2 with the equilibrium shifted to the formation of 3,4-dichloro-1-butene by distilling it off the reaction mixture. Finally, 3,4-dichloro-l-butene undergoes alkaline dehydrochlorination to produce chloroprene (5-15% NaOH, 80-110°C). 

The point (a) is demonstrated by the data in Table 11 for 2-halobutanes which give three products 1-butene, cis-2-butene and frans-2-butene. The differences in selectivities can be even larger than indicated by these data. The dehydrochlorination of 1,1,2-trichloroethane yields 1,2-dichloro-ethylene (I) and trans- and ci s-l,2-dichloroethylene (II). On silica—alumina, the value of the ratio I/II was 10 3, on alumina, 0.30 and on KOH— Si02,10 [66]. 

An example of a few of these reactions that occur in our environment with several commonly used pesticides is illustrated in Figures 7-11. Fleck (15) has illustrated in Figure 7 that ultraviolet light catalyzes the decomposition of DDT. In the presence of air, one of the decomposition products is 4,4 -dichlorobenzophenone. However, when air is absent, 2,3-dichloro-l,l,4,4-tetrakis-(p-chlorophenyl)-2-butene is formed. This compound, through subsequent oxidation, may be converted into 4,4 -dichlorobenzophenone. In mammals 2,2-bis(p-chloro-phenyl) acetic acid (DDA) has been identified and shown to be excreted in the feces and urine. The mechanism of formation of DDA is believed to be an initial dehydrochlorination to DDE, which is then hydrolyzed to DDA as shown in Figure 8. Mattson et ah (29) found both DDT and DDE in most samples of human fat, and Walker et ah (44) noted low levels of these same compounds in restaurant meals. 

Ethylene glycol also serves as an efficient solvent for the production of mono-vinylacetylene by the dehydrochlorination of 1,3-dichloro-2-butene with potassium hydroxide in a flask fitted with a Friedrichs condenser and aTruebore stirrer carry-... 

Similarly, dehydrochlorination of poly(l,2,3-trichlorobutadiene) should bring about the formation of the cumulene form of carbyne as it seems to be predetermined by chemical structure of intermediate polyene, viz. poly(2,3-dichloro-2-butene-l,4-diylidene) (Scheme 12.3). 

In the first step, butadiene is chlorinated in the vapour phase at 330-420°C and atmospheric pressure. The main products are 3,4-dichloro-l-butene and l,4-dichloro-2-butene in approximately equal amounts. The latter material is then isomerized to the former by heating with a copper catalyst such as cuprous chloride. The 3,4-dichloro-l-butene is dehydrochlorinated by treatment with 10% aqueous sodium hydroxide at 85°C. Chloroprene is isolated by distillation under reduced pressure in the presence of polymerization inhibitors. 

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