Acetylene molecule

Vibrational spectroscopy has been, and will continue to be, one of the most important teclmiques in physical chemistry. In fact, the vibrational absorption of a single acetylene molecule on a Cu(lOO) surface was recently reported [ ]. Its endurance is due to the fact that it provides detailed infonnation on structure, dynamics and enviromnent. It is employed in a wide variety of circumstances, from routine analytical applications, to identifying novel (often transient) species, to providing some of the most important data for advancing the understanding of intramolecular and intemiolecular interactions. 

Repeat the analysis of the G2 calculation in the section on G2 and G3 in this chapter for the acetylene molecule. 

Because it was not possible to explain the differences in the effectiveness of hydrogen as compared to other gases on the basis of differences in their physical properties, ie, thermal conductivity, diffusivity, or heat capacity differences, their chemical properties were explored. To differentiate between the hydrogen atoms in the C2H2 molecules and those injected as the quench, deuterium gas was used as the quench. The data showed that although 90% of the acetylene was recovered, over 99% of the acetylene molecules had exchanged atoms with the deuterium quench to form C2HD and... 

To extend the study of the apparent decomposition recombination reaction, and specifically to determine if the carbon atoms exchange with other atoms in other acetylene molecules, tests using carbon isotopes were conducted. A mixture of 50% regular acetylene, C2H2, and 50% heavy acetylene. 

The presence of other functional groups ia an acetylenic molecule frequendy does not affect partial hydrogenation because many groups such as olefins are less strongly adsorbed on the catalytic site. Supported palladium catalysts deactivated with lead (such as the Liadlar catalyst), sulfur, or quinoline have been used for hydrogenation of acetylenic compound to (predominantiy) cis-olefins. 

When two sp-hybridized carbon atoms approach each other, sp hybrid orbitals on each carbon overlap head-on to form a strong sp-sp a bond. In addition, the pz orbitals from each carbon form a pz-pz it bond by sideways overlap and the py orbitals overlap similarly to form a py-py tt bond. The net effect is the sharing of six electrons and formation of a carbon-carbon triple bond. The two remaining sp hybrid orbitals each form a bond with hydrogen to complete the acetylene molecule (Figure 1.16). 

The VSEPR model applies equally well to molecules in which there is no single central atom. Consider the acetylene molecule, C2H2. Recall that here the two carbon atoms are joined by a triple bond ... 

Nakamura, T., Ohno, K., Kotani, M., and Hijikata, K., Progr. Theoret. Phys. Kyoto) 8, 387, Interaction of -electrons in the acetylene molecule/ Calculations using both MO-CI and HL with ionic-homopolar resonance. 

The orbital phase theory was applied to the conformations of alkenes (a- and P-substituted enamines and vinyl ethers) [31] and alkynes [32], The conformational stabilities of acetylenic molecules are described here. 

Figure 4.37 Synthesis trees for Reppe synthesis of cyclooctatetraene (a) treating each molecule of acetylene as a separate input (b) treating all 4 acetylene molecules as a single input.

Photoexcitation of an acetylene molecule results in either dimerization or dissociation of the molecule (C2 + H2 or C2H + H). In the mass spectrometer, the major positive ions are C2H2+ (75%) and C2H+ (15%). However, at STP no gaseous products are seen under radiolysis. There are only two major products, benzene and a polymer, cuprene with an empirical formula QJH. The detailed mechanisms are still debatable. However, the following remarks may be made ... 

Both carbon atoms in the acetylene molecule undergo sp hybridization. Two p orbitals remain unhybridized. So, one sp hybrid orbital from each carbon atom overlaps with the s orbital of a hydrogen atom and two C — Ho bonds result. Also, between the two adjacent C atoms a C—Co bond is formed as a result of end to end overlap of the sp hybrid orbitals. So in the C2H2 molecule there are three o bonds in total. 

The original rnanufacturing route to vinyl chloride (VC) didn t involve ethylene dichloride (EDC) but was the reaction of acetylene with hydrochloric acid. This process was commercialized in the 1940s, but like most acetylene-based chemistry in the United States, it gave way to ethylene in the 1950s and 1960s. The highly reactive acetylene molecule was more sensitive, hazardous,... 

This makes the acetylene molecule linear, i.e. bond angles of 180°, and there are two n bonds with electron density either side of this axis. The properties of an alkyne, like acetylene, are also special in that the Jt bonds are again much more reactive than the a bond. 

Note how we have resorted to another form of representation of the ethane, ethylene, and acetylene molecules here, representations that are probably familiar to you (see Section 1.1). These line drawings are simpler, much easier to draw, and clearly show how the atoms are bonded - we use a line to indicate the bonding molecular orbital. They do not show the difference between a and rr bonds, however. We also introduce here the way in which we can represent the tetrahedral array of bonds around carbon in a two-dimensional drawing. This is to use wedges and dots for bonds instead of lines. By convention, the wedge means the bond is coming towards you, out of the plane of the paper. The dotted bond means it is going away from you, behind the plane of the paper. We shall discuss stereochemical representations in more detail later (see Section 3.1). 

For the acetylene molecule, the sp hybrid orbital is formed by combining one 2s orbital and one 2p orbital to form two sp orbitals. Each C atom uses one sp orbital to bond to one Is from H, and the other sp orbital to bond to the other C. The C=C triple bond is formed by the addition of two tt bonds by the overlap of two 2p orbitals from each C. Some of the properties of these hybrid bonds are shown in table 4.13. 

This hydride then may add an acetylene molecule to form the vinyl derivative. A carbon monoxide insertion will produce the acrylyl nickel compound which can yield acrylate esters by either of two routes. Direct alcoholysis of the acyl nickel group may take place, as occurs with acylcobalt compounds (42) or, an acyl halide (or other acyl derivative, e.g., acyl alkanoate) may be eliminated. Alcoholysis of the acyl halide would then complete the catalytic cycle (35). 

The frequently observed peak at m/z 65 results from elimination of a neutral acetylene molecule from the tropylium ion. 

Terminal acetylenes and Ru3(CO)j2 yield complexes of the type [57] (9,190, 336), whereas internal acetylenes form either complexes [56] or acetylene-substituted RU4 complexes (229). Alternatively, two acetylene moieties are incorporated with formation of metallacyclopentadienes (229), a class of compounds more familiar in osmium cluster chemistry (cf. Chapter 3.4.). Instead of two acetylene molecules, one molecule of an arylbutadiene may be the precursor of the metallacycle (382). 

In many cases the reaction of osmium carbonyls and acetylenes does not stop at the first stages as in [56], [57], or [57]. Instead, two or more acetylene molecules are incorporated, and in some cases acetylene trimerization to benzenes takes place (182, 371, 379). Incorporation of two acetylene molecules can lead to metallacyclo-pentadiene clusters like [55] (126,168,171,182,184, 223, 371), or to metallacyclo-hexadienone clusters hke [59] (126, 223). And the complex [90], another intermediate, is related to [55] by an intramolecular oxidative addition reaction (168,169). 

Incorporation of a third acetylene molecule takes place by CO replacement and without interference with the metallacyclopentadiene ring (170, 371, 379). In the cluster, then, the three acetylene ligands rearrange to a triacetylene ligand of unknown structure before the benzene is liberated (371, 379). 

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