Acetylenes, hydrogenation

Mono-olefins. Stoicheiometric CoS and C0S2 were inactive in the isomerization and isotopic (H/D) exchange of olefins. Cobalt metal partially sulphided with H2S catalysed the isomerization of n-butenes and exchange 

Tanaka, K. Tanaka, and K. Miyahara, Nippon Kagaku Kaishi, 1978, 1338 (Chem. 

Over TaS2 and intercalates MTa3Se (M = Fe, Co, Ni) cyclohexene (373-673 K) in hydrogen isomerized selectively to 1-methylcyclopentene and in nitrogen dehydrogenated to benzene. Intercalation of Fe and Ni increased activity without changing selectivity, but Co caused activity to decrease. The intercalated metals also decreased the rate of deactivation of the catalyst. 

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Acetylenic hydrogens are unusual in that they are more shielded than we would expect for protons bonded to sp hybridized carbon This is because the rr electrons circulate around the triple bond not along it (Figure 13 9a) Therefore the induced magnetic field is parallel to the long axis of the triple bond and shields the acetylenic proton (Figure 13 9b) Acetylenic protons typically have chemical shifts near 8 2 5... 

Rea.ctlons, Propargyl alcohol has three reactive sites—a primary hydroxyl group, a triple bond, and an acetylenic hydrogen—making it an extremely versatile chemical intermediate. 

Methylbutynol. 2-Methyl-3-butyn-2-ol [115-19-5] prepared by ethynylation of acetone, is the simplest of the tertiary ethynols, and serves as a prototype to illustrate their versatile reactions. There are three reactive sites, ie, hydroxyl group, triple bond, and acetylenic hydrogen. Although the triple bonds and acetylenic hydrogens behave similarly in methylbutynol and in propargyl alcohol, the reactivity of the hydroxyl groups is very different. 

Hypohahtes replace the acetylenic hydrogen with chlorine, bromine, or iodine (203). 

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. 

Since both complete hydrogenation of acetylene or any hydrogenation of the ethylene results in the production of a less valuable product such as ethane, conditions must be chosen carefiiUy and a catalyst must be used that is both sufficiently active for acetylene hydrogenation and extremely selective to avoid ethylene hydrogenation. Since hydrogenation of acetylenic bonds proceeds stepwise and since acetylene is more strongly adsorbed on the catalytic... 

W. K. Lam, "Acetylene Hydrogenation," paper presented at Pacific Mrea Chemical Engineering Congress, Acapulco, Mexico, Oct. 19—22,1988. 

Troublesome amounts of C and Q acetylenes are also produced in cracking. In the butadiene and isoprene recovery processes, the acetylenes in the feed are either hydrogenated, polymerized, or extracted and burned. Acetylene hydrogenation catalyst types include palladium on alumina, and some non-noble metals. 

Flammable gases and vapors include acetylene, hydrogen, butadiene, ethylene oxide, propylene oxide, acrolein, ethyl ether, ethylene, acetone, ammonia, benzene, butane, cyclopropane, ethanol, gasoline, hexane, methanol, methane, natural gas, naphtha, and propane. 

Protection of an acetylenic hydrogen is often necessary because of its acidity. The bulk of a silane can protect an acetylene against catalytic hydrogenation because of rate differences between an olefin (primary or secondary) vs. the more hindered protected alkyne. Trialkylsilylacetylenes are often used as a convenient method for introducing an acetylenic unit because they tend to be easily handled liquids or solids, as opposed to gaseous acetylene. 

An a priori choice of an optimum acetylene hydrogenation catalyst is not always easy. For instance, hydrogenation of octadeca-3,6-diynol over P-2 nickel gave the corresponding (Z,Z) dienol satisfactorily (37), but when the... 

Amines (7a,12a), especially pyridine (75), have also been used as solvents in the hydrogenation of acetylenes. Hydrogenation of 3 over 5% Pd-on-BaS04 in pyridine gave df-cis-jasmanate (4) quantitatively (40). The authors comment that this combination for reduction of acetylenes was superior to the Lindlar catalyst in all cases examined. (See also Refs. 12 and 24 for similar conclusions.)... 

Replacement, of acetylenic hydrogen atom by bromine, 46, 86 of acyl chlorine atom by fluoride atom using hydrogen fluoride, 46, 3... 

Segregate stocks of chlorine from acetylene, hydrogen, ammonia and fuel gases and ensure no accidental contact with ethers, hydrocarbons and other organics and finely divided metals. Never mix chlorine with another gas in the cylinder. 

Acetylene as well as its compounds have an acetylenic hydrogen atom with an acid character that can be substituted by a chlorine atom. Thus,with acetylene, calcium hypochlorite and sodium hypochlorite in an acid medium form (this can be formed by calcium carbide hydrolysis) chloroacetylene and dichloro-acetylene, which combust spontaneously in air. 

The dangerous reactions of halogenation, which affect either the acetylenic hydrogen atom or the triple bond have just been described. There are also reactions which lead to accidents and affect one of the acetylenic sites. 

Chlorine dioxide Copper Fluorine Hydrazine Hydrocarbons (benzene, butane, propane, gasoline, turpentine, etc) Hydrocyanic acid Hydrofluoric acid, anhydrous (hydrogen fluoride) Hydrogen peroxide Ammonia, methane, phosphine or hydrogen sulphide Acetylene, hydrogen peroxide Isolate from everything Hydrogen peroxide, nitric acid, or any other oxidant Fluorine, chlorine, bromine, chromic acid, peroxide Nitric acid, alkalis Ammonia, aqueous or anhydrous Copper, chromium, iron, most metals or their salts, any flammable liquid, combustible materials, aniline, nitromethane... 

A new process for the manufacture of acetylene has been proposed. The process will involve the dehydrogenation of ethane over a suitable catalyst (yet to be found). Pure ethane will be fed to a reactor and a mixture of acetylene, hydrogen, and unreacted ethane will be withdrawn. The reactor will operate at 101.3 kPa total pressure and at some as yet unspecified temperature T. 

The intrinsic rate expressions for these reactions are both first-order in hydrogen and zero-order in acetylene or ethylene. If there are diffusional limitations on the acetylene hydrogenation reaction, the acetylene concentration will go to zero at some point within the core of the catalyst pellet. Beyond this point within the central core of the catalyst, the undesired hydrogenation of ethylene takes place to the exclusion of the acetylene hydrogenation reaction. 

The acetylenic hydrogen is weakly acidic (pKa 25) and can be removed with a strong base (e.g. NaNH2) to give an anion (called an alkynide anion or acetylide ion). 

Replacement of the Acetyleneic Hydrogen Atom of Terminal Alkynes... 

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