Hydrogenation of ethyne

Write a balanced chemical equation for (a) the hydrogenation of ethyne (acetylene, C2H2) to ethene (C2H4) by hydrogen (give the oxidation number of the carbon atoms in the reactant and product) (b) the shift reaction (sometimes called the water gas shift reaction, WGSR) (c) the reaction of barium hydride with water. 

Some positive charge is clearly evident on the hydrogens of ethyne. 

Figure 11-4 Generalized valence bond (GVB) orbitals for one hydrogen of ethyne (left) and of ethane (right) see Section 6-7. The hydrogen and carbon nuclei are located in the X, V plane of the coordinate system at the positions indicated by crosses, the hydrogen nucleus being on the left.

Determine the reaction enthalpy for the hydrogenation of ethyne to ethane, C2H2(g) +... 

O Sodium amide, a strong base, was used to remove the acidic hydrogen of ethyne. The resulting acetylide ion was used as a nucleophile in an SN2 substitution (see Section 10.8). 

There are a number of other qualities that catalysts possess which should be introduced at this point. It is not only different types of catalyst that afford different products a single catalyst can also do this, so for example the hydrogenation of ethyne can lead with a platinum catalyst to a mixture of ethene and ethane, and ethene once formed can reant further to ethane. The extent to which the intermediate product, which is often the desired one, is formed is measured by the selectivity S, where... 

Scheme 9.4 Mechanism of hydrogenation of ethyne over a metal catalyst.

There have been two recent studies of the hydrogenation of ethyne on gold catalysts. With 1% Au/TiC>2 made by thermally decomposing the supported complex [Au2(PPh3)6](BF4)2 and having particles of average size 4.6 nm, reaction occurred at 453 K with about 90% selectivity to ethene,... 

Reprinted from Chemical Engineering and Processing, 32, A.N.R. Bos et. al., A kinetic study of the hydrogenation of ethyne and ethene on a commercial Pd/AljO, catalyst, 53-63,1993, with kind permission from Elsevier Science S.A., P.O. Box 564,1001 Lausanne, Switzerland. 

Bos ANR, Westerterp K (1978) Mechanism and kinetics of the selective hydrogenation of ethyne and ethene. Chem Eng Process 32 1... 

Borodzinski A, Bond GC (2006) Selective hydrogenation of ethyne in ethene-rich streams on palladium catalysts. Part 1. Effect of changes to the catalyst during reaction. Catal Rev - Sci Eng 48 91... 

Houzvicka J, Pestman R, Ponec V (1995) The role of carbonaceous deposits and support impurities in the selective hydrogenation of ethyne. Catal Lett 30 289... 

Taking the hydrogenation of ethyne as a case study, it is well established now that the interaction of ethyne and/or ethene with a metal surface leads to a wide variety of adsorbed species, the proportion of which depends on the hydrocarbon pressure, the temperature, and the nature of the metal [76]. Earlier identihcations of such species—in particular, the ethylidyne =C-CH3—have been reported hrst on Pt(l 11) single crystals [77,78], then on Pd/Al203 [79]. With all available spectroscopic and kinetic data at various conditions, a nearly complete reaction scheme for the hydrogenation of ethyne on Pd(lll) has recently been proposed [80] and illustrated in Figure 6. The choice between the two routes through ethylidyne or Ti-bonded adsorbed species obviously depends on the nature of the metal, but also on some... 

Figure 6 A proposed reaction scheme for the hydrogenation of ethyne on Pd and Pt surfaces.

The selective semi-hydrogenation of ethyne in ethene is also an industrial process of vital importance, used in both laboratory practice and relevant to fine chemicals and polymer production [445,462]. The reaction is generally performed over low loaded Pd catalysts. Systematic investigations have been performed over Pd-Ag, Pd-Cu and Pd-Au supported catalysts that are superior to mono-metallic Pd catalysts [445,461,463]. The presence of Au decreases the carbon coverage and improves the ethene selectivity [445]. 

The catalytic hydrogenation of ethyne (acetylene, C2H2) to ethene proceeds through a surface reaction between adsorbed ethyne and hydrogen atoms. We use vibrational spectroscopy to learn about the interaction between adsorbed ethyne and the catalyst and to identify intermediates. 

Figure 9.2. Hydrogenation of ethyne over Pd/pumice at 293 K in a constant volume system pressure fall versus time (Pfe = 50 Torr Pp =150 Torr). ...

TABLE 9.1. Kinetic Parameters for the Hydrogenation of Ethyne in the Absence of... 

Figure 9.3. Hydrogenation of ethyne over Ni/pumice at 486 K in a constant volume system dependence of initial rate on Ph (Pe = 30.5 Totr). ...

Figure 9.4. Hydrogenation of ethyne over AhOs-supported metals effect of P on selectivity (Pe 50Torr).27...

Figure 9.5. Hydrogenation of ethyne over Al203-supported metals effect of temperature on selectivity (Pe = 50 Torr Ph = 200 Torr). ...

HYDROGENATION OF ETHYNE 2, IN DYNAMIC SYSTEMS WITH ADDED ETHENE ... 

Table 9.6 was proposed. Addition of copper to nickel in powder form improved selectivity, but oligomerisation also increased " it was also especially marked with cobalt. Simultaneous hydrogenation of ethyne and propyne shows that the former must be somewhat more strongly adsorbed on nickel and platinum this is understandable, since the C—C single bond in propyne is shorter (146 pm) than that in ethane (154 pm) due to hyperconjugation with the triple bond, which is thereby weakened. The relief of strain upon adsorption is therefore less. 

It is interesting to compare the mechanisms proposed for the hydrogenations of ethyne and benzene. With the former, there is no suggestion of a role for spillover catalysis with the latter there is no role for carbonaceous deposits in creating active centres or in acting as vehicles for hydrogen atom transfer. 

The effect of hybridization and effective electronegativity on acidity is borne out in the calculated electrostatic potential maps for ethyne, ethene, and ethane shown in Fig. 3.4. Some positive charge (indicated by blue color) is clearly evident on the hydrogens of ethyne pK = 25), but almost no positive charge is present on the hydrogens of ethene and ethane (both having pK values more than 20 units greater than ethyne). 

To obtain the chloroethane required for this reaction, we could substitute a Cl atom for an H atom in CH3CH3 which, in turn, can be produced by the hydrogenation of ethyne. 

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