Acetylene half-hydrogenation

Palladium is good for hydrogenations of most unsaturations except benzenes. It is frequently used by synthetic organic chemists for hydrogenolyzing off protecting groups. It is especially useful for the half-hydrogenation of acetylenes. 

The kind of surface site catalyzing alkyne half-hydrogenation has been the subject of some speculation. Two possibilities are shown in Fig. 2.2. Certainly such a site should fit the evidence Burwell and associates obtained from the hydrogenation of di-tert-butyl acetylene (2,2,5,5-tetramethyl-3-hexyne). 31 They proposed that the molecule dislocated a surface atom, pulling it up out of the plane of the surface (Fig. 2.3). That work concluded that hydrogenation... 

The mechanism of the hydropolymerisation of acetylene is not too clear. It has been suggested [9,169] that in the hydrogenation of acetylene to ethylene, the half-hydrogenated state, an adsorbed vinyl species, may exist in either a normal or free radical form, viz. 

The mechanism proposed (84, 95) is as follows. The formation of cis-2-butene is adequately described by steps (1), (2), and (3) proposed above for the palladium-catalyzed reaction. Since the initial cisitrana ratio in the butenes is almost independent of initial reactant pressures and temperature, and since the distribution of deuterium in these two isomers is similar, it has been concluded that produced directly from 2-butyne, and not by the subsequent isomerization of cis-2-butene. Steps (6), (7), (8), and (9) describe the simplest route which satisfies the experimental observations and involves the addition of hydrogen to a free radical form of the half-hydrogenated state, which is envisaged to be in equilibrium with the normal form. An analogous equilibrium was postulated in the mechanism for acetylene... 

The hydrogenation of acetylene may therefore be explained by a mechanism very similar to that already discussed for ethylene, and of course the two reactions can go forward simultaneously on the different spacings of the same catalyst. The formation of higher hydrocarbons can also be explained by similar postulates, as follows. The characteristic feature of the adsorbed acetylene molecule is that it is still unsaturated, whereas adsorbed ethylene is not. Moreover, it is important to note that the polymerization reaction with acetylene takes place when hydrogen is present but does not occur in its absence it therefore seems likely that the half-hydrogenated state of acetylene plays a vital part in the process. 

The total yield of hydrogen under the conditions of these measurements was about 1.6 molecules/100 e.v. If one-half resulted from the primary dissociation also leading to acetylene ion, a yield of 0.8 acetylene ions/100 e.v. may be estimated. This value is a minimum since acetylene ion production can also be accompanied by hydrogen atom formation and is highly uncertain but consistent with the mass spectral fragmentation pattern of acetylene and W which lead to an estimate of ca. 0.94 acetylene ions/100 e.v. 

Metal Hydrides. Metal hydrides generally react readily with acetylenes, often by an insertion mechanism. Cobalt hydrocarbonyl gives complicated mixtures of compounds with acetylenes. The only products which have been identified so far are dicobalt hexacarbonyl acetylene complexes (34). Greenfield reports that, under conditions of the hydroformy lation reaction, acetylenes give only small yields of saturated monoaldehydes (30), probably formed by first hydrogenating the acetylene and then reacting with the olefin. Other workers have identified a variety of products from acetylene, carbon monoxide, and an alcohol with a cobalt catalyst, probably cobalt hydrocarbonyl. The major products observed were succinate esters (74,19) and succinate half ester acetals (19). 

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. 

However, conversion of the acetylenic into the olefinic bond makes special preparative demands. This half-reduction can be effected by chemical reagents as well as by catalytically activated hydrogen the stable fra .s-ethylenic stereoisomers are usually obtained by the first method, whereas the second of these methods gives predominantly the metastable cis-isomers of higher energy content. 

Acetylene and other compounds which contain the =CH group react with sodium and potassium and form metallic derivatives. When acetylene is passed over gently-heated sodium one-half of the hydrogen in the hydrocarbon is replaced by the metal and a sodium acetylide, C2HNa, is formed. At red heat the disubstituted-product C2Na2 results. 

The hydrogenation of dimethylethynylcarbinol (DMEC), an alcohol containing an acetylene bond was conducted with the most active catalyst, P4VP-Pd(NaBH4) [70]. The rate of reduction of the triple bond of DMEC is lower than that of the double bond. This is clearly seen from the kinetic curve in Fig. 26a i . the reaction rate sharply increases after the absorption of a half of the calculated quantity of hydrogen. The chromatographic analysis of reaction products shows relatively selective hydrogenation of the acetylenic bond (Fig. 26b). 

For a vibrationally highly excited acetylene, the isomerization process from acetylene to vinylidene is of interest (see Fig. 3.20). In this process, both of the bending modes and should be excited where one of the hydrogen atoms goes half-way round the two carbon atoms. Therefore, the reaction coordinate of the isomerization is chosen to be the sum -H v. However,... 

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