Vinyl acetylene, hydrogenation

Vinyl fluoride (fluoroethene), is manufactured from the cataly2ed addition of hydrogen fluoride to acetylene. It is used to prepare poly(vinyl fluoride) which has found use in highly weather-resistant films (Tedlar film, Du Pont). Poly(vinyhdene fluoride) also is used in weather-resistant coatings (see Eluorine compounds, organic). The monomer can be prepared from acetylene, hydrogen fluoride, and chlorine but other nonacetylenic routes are available. 

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). 

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. 

High-temperature flow-reactor studies [60,61] on benzene oxidation revealed a sequence of intermediates that followed the order phenol, cyclopentadiene, vinyl acetylene, butadiene, ethene, and acetylene. Since the sampling techniques used in these experiments could not distinguish unstable species, the intermediates could have been radicals that reacted to form a stable compound, most likely by hydrogen addition in the sampling probe. The relative time order of the maximum concentrations, while not the only criterion for establishing a mechanism, has been helpful in the modeling of many oxidation systems [4,13]. 

Acetylene has yet another use. About half of all acetylene produced today goes towards the production of other organic chemicals. Adding hydrogen cyanide to acetylene, for example, yields acrylonitrile, which is used in the production of acrylic fibers. Acetylene can also be converted into vinyl acetylene, which is the raw material needed for the manufacture of neoprene, one of the most useful synthetic rubbers. 

The chemical properties and uses of propargyl alcohol has three potentially reactive sites (1) a primary hydroxyl group (i.e., CH2OH), (2) a triple bond (-C=C-), and (3) an acetylenic hydrogen (-C=CH) that makes the alcohol an extremely versatile chemical intermediate. The hydroxyl group can be esterified with acid chlorides, anhydrides, or carboxylic acids, and it reacts with aldehydes or vinyl ethers in the presence of an acid catalyst to form acetals. At low temperatures, oxidation with chromic acid gives propynal or propynoic acid ... 

This is illustrated by the products of addition of hydrogen bromide to vinyl acetylene (177) (Traynard, 1962) (equation 35). Only in the case of halogen addition have products formally arising from electrophilic attack to the double bond been isolated in substantial amounts (Petrov et al., 1960). 

Application Increase the value of steam cracker C4 cuts via low-temperature selective hydrogenation and hydroisomerization catalysis. Several options exist removal of ethyl and vinyl acetylenes to facilitate butadiene extraction processing downstream conversion of 1, 3 butadiene to maximize 1-butene or 2-butene production production of high-purity isobutylene from crude C4 cuts total C4 cut hydrogenation and total hydrogenation of combined C3/C4 and C4C5 cuts for recycle to cracking furnaces or LPG production. 

Green oil is a mixture of oligomers and partially hydrogenated oligomers of vinyl acetylene, D. Seddon unpublished results. 

Acetylene hydrogenation is widely practiced and efficient. However, a green-oil which comprises vinyl acetylene oligomers is also produced and in some instances can foul the unit. In large cracking operations, the acetylene may be recovered by absorption processes based on copper salts, which selectively absorb the acetylene. 

A more effective acetylene hydrogenation catalyst to ethylene would also facilitate the development of coal to ethylene via the acetylene route, which is at present restricted to the use of acetylene for the production of vinyl chloride. 

The fruitfulness of the idea of a stepwise addition with an independent variation of the addends was brilliantly illustrated by Normant s studies, which resulted in the elaboration of a general method of alkene synthesis based on the reaction of alkyne carbometallation. Basically this reaction represents a case of the well-known nucleophilic addition to a carbon-carbon triple bond. In the Normant reaction, however, the initial addition of a nucleophile (an organome-tallic reagent) across the triple bond results in the formation of a stabilized carbanion-like intermediate equivalent to a vinyl carbanion. This intermediate can similarly be further reacted with an external electrophile. Most typically, copper-modified Mg or Li reagents, which are unable to react with acidic acetylenic hydrogens, are used in this sequence. 

A brief study by Jackson et at 1470 A gave results very different from those reported above. The photolysis was conducted at pressures of 200 n and 10 torr of benzene. No hydrogen, allene, cyclohexadienes, biphenyl or dihydrobiphenyls was observed, while acetylene, ethylene, methylacetylene and vinyl acetylene were found. These authors conclude that neither the atomic nor molecular elimination of hydrogen occurs, while Hentz and Rzad conclude that both maybe operative. Thus the situation is confused at present, but both studies agree that polymer formation is extensive at 1470 A. 

Significant deactivation was observed at the same temperature for vinyl acetylene but, at 200°Ct complete conversion was observed for about 7 hours. After this time, activity dropped rapidly (Figure 4) suggesting that similar rapid deactivation might be observed for methylacetylene at the higher temperatures after longer times on line. However, hydrogenation of an... 

Acetylenic hydrogens (C—H,. sp-lj) appear anomalously at 2 to 3 ppm owing to anisotropy (to be discussed in Section 3.12). On the basis of hybridization alone, as already discussed, one would expect the acetylenic proton to have a chemical shift greater than that of the vinyl proton. An sp carbon should behave as if it were more electronegative than an sp carbon. This is the opposite of what is actually observed. 

Hydrogen fluoride Protonated methane Acetylene Vinyl cation Hydrogen cyanide Carbon monoxide Nitrogen... 

Ghosh, A.K., and Agnew, J.B., Adsorption of acetylene, hydrogen chloride and vinyl chloride on activated carbons Kinetics and thermodynamics, using transient response technique, Chem. Eng. Commun., 40, 169-182 (1986). 

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