Vinylic chlorine, displacement

The rather unreactive chlorine of vinyl chloride can be displaced with nucleophiles by the catalytic action of PdCb. The conversion of vinyl chloride to vinyl acetate (797) has been studied extensively from an industrial standpoint[665 671]. DMF is a good solvent. 1,2-Diacetoxyethylene (798) is obtained from dichloroethylene[672]. The exchange reaction suffers steric hindrance. The alkenyl chloride 799 is displaced with an acetoxy group whereas 800 and 801 cannot be displaccd[673,674]. Similarly, exchange reactions of vinyl chloride with alcohols and amines have been carried out[668]. 

Because of its relatively high, price, there have been continuing efforts to replace acetylene in its major appHcations with cheaper raw materials. Such efforts have been successful, particularly in the United States, where ethylene has displaced acetylene as raw material for acetaldehyde, acetic acid, vinyl acetate, and chlorinated solvents. Only a few percent of U.S. vinyl chloride production is still based on acetylene. Propjiene has replaced acetylene as feed for acrylates and acrylonitrile. Even some recent production of traditional Reppe acetylene chemicals, such as butanediol and butyrolactone, is based on new raw materials. 

This route has been completely displaced, first by chlorination and dehydro-chlorination of ethylene or vinyl chloride, and more recendy by oxychlorination of two-carbon raw materials (2) (see Chlorocarbonsandchlorohydrocarbons). 

Alternatives to oxychlorination have also been proposed as part of a balanced VCM plant. In the past, many vinyl chloride manufacturers used a balanced ethylene—acetylene process for a brief period prior to the commercialization of oxychlorination technology. Addition of HCl to acetylene was used instead of ethylene oxychlorination to consume the HCl made in EDC pyrolysis. Since the 1950s, the relative costs of ethylene and acetylene have made this route economically unattractive. Another alternative is HCl oxidation to chlorine, which can subsequently be used in dkect chlorination (131). The SheU-Deacon (132), Kel-Chlor (133), and MT-Chlor (134) processes, as well as a process recently developed at the University of Southern California (135) are among the available commercial HCl oxidation technologies. Each has had very limited industrial appHcation, perhaps because the equiHbrium reaction is incomplete and the mixture of HCl, O2, CI2, and water presents very challenging separation, purification, and handling requkements. HCl oxidation does not compare favorably with oxychlorination because it also requkes twice the dkect chlorination capacity for a balanced vinyl chloride plant. Consequently, it is doubtful that it will ever displace oxychlorination in the production of vinyl chloride by the balanced ethylene process. 

Haloacetyl groups have also a synthetic potential. Thus, pesticidal (alkylthio)-vinyl esters of phosphorus acid derivatives have been prepared by the introduction and subsequent displacement of two chlorine atoms in the acetyl moiety attached to the furazan ring (Scheme 69) [73GEP(0)2144393]. 

However, if both fluorine and chlorine are attached to the same vinylic carbon, the elimination stage of the mechanism becomes more important and, consequently, chlorine is selectively displaced, reflecting the greater leaving-group ability of chlorine compared with fluorine [45, 46] (Figure 5.23). 

The reaction of acrylonitrile with ethanolic RhCls yields a product of approximate composition (CH2=CH—CN)2RhCl2 in which one of the acrylonitrile moieties is cr-bonded to Rh(III) and the other perhaps 7r-bonded as indicated by the chlorine-bridged structure (175) since it can be displaced by pyridine to yield (CH3CHCN)RhCl2(C5H5N)3 (168). The reaction is thought to involve an intermediate Rh(III) hydride. The comparable reaction with crotononitrile, methacrylonitrile, cinna-monitrile, methyl vinyl ketone, methyl vinyl sulfone, and isoprene does not occur (168). 

Vinylic complexes of iron have been prepared by displacement of vinylic fluorine from perfluorocyclopentene to give (17), or of chlorine from what was thought to be 3,4-dichlorotetrafluorocyclobutene, but was in fact... 

A last example shows the reaction of a vinyl chloride such as 3.30 with ethylamine (see reaction 7). Addition of the amine generated the usual enolate anion intermediate, which displaced the P-chlorine to give a new conjugated ester. In this case, the product was ethyl 3-(N,N-diethylamino)but-2-enoate (3.57).23 The yield was relatively poor, but this example illustrates that conjugate addition of amines can lead to either the normal saturated product or, as in this case, an unsaturated amino acid. 

Chemical modification of polymers is the only way to obtain materials derived from natural macromolecular compounds (cellulose, starch) or polymers with no available monomer (poly(vinyl alcohol). Even when the monomer is available, chemical modification can be the only way to obtain its polymer thus, attempts at direct preparation of poly(epiiodohydrin) were unsuccessful but partial success was achieved by nucleophilic displacement of the chlorine atom of poly(epichlorohydrin) by sodium iodide in the presence of a small amount of methyltributylammonium in butanol. However, chemical modifications are increasingly used to synthesize new polymers with specific properties. 

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