Hybrid Orbitals and the Structure of Acetylene

We know that hydrogen forms one covalent bond, carbon forms four, and oxygen forms two. Trial and error, combined willi intuition, is needed to fit the atoms togetlier. 

Solution There is only one way that two hydrogens, one carbon, and one oxygen can combine  

Like the carbon atoms in etliylene, the carbon atom in formaldehyde is in a double bond and therefore. sp -hybridizcd. 

Problem 1.10 Draw a line-bond structure lor propene, C1L. CH=CM2 indicate the hybridization 

Problem 1.11 Draw a line-bond structure for 1,3-butadiene, H2C=CH—CH=Cll2 indicate the 

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Atomic Structure The Nucleus Atomic Structure Orbitals 4 Atomic Structure Electron Configurations 6 Development of Chemical Bonding Theory 7 The Nature of Chemical Bonds Valence Bond Theory sp Hybrid Orbitals and the Structure of Methane 12 sp Hybrid Orbitals and the Structure of Ethane 13 sp2 Hybrid Orbitals and the Structure of Ethylene 14 sp Hybrid Orbitals and the Structure of Acetylene 17 Hybridization of Nitrogen, Oxygen, Phosphorus, and Sulfur 18 The Nature of Chemical Bonds Molecular Orbital Theory 20 Drawing Chemical Structures 21 Summary 24... 

Now consider the alkynes, hydrocarbons with carbon-carbon triple bonds. The Lewis structure of the linear molecule ethyne (acetylene) is H—O C- H. To describe the bonding in a linear molecule, we need a hybridization scheme that produces two equivalent orbitals at 180° from each other this is sp hybridization. Each C atom has one electron in each of its two sp hybrid orbitals and one electron in each of its two perpendicular unhybridized 2p-orbitals (43). The electrons in the sp hybrid orbitals on the two carbon atoms pair and form a carbon—carbon tr-bond. The electrons in the remaining sp hybrid orbitals pair with hydrogen Ls-elec-trons to form two carbon—hydrogen o-bonds. The electrons in the two perpendicular sets of 2/z-orbitals pair with a side-by-side overlap, forming two ir-honds at 90° to each other. As in the N2 molecule, the electron density in the o-bonds forms a cylinder about the C—C bond axis. The resulting bonding pattern is shown in Fig. 3.23. 

Stang and co-workers have reported a series of tetranuclear Pt-Pt and Pt-Pd squares 41 and 42, which bind Ag. In these molecules, ethynyl Pt units form two opposite corners of a square with a bite angle of 88° between the acetylenes. The complexation of 2 equiv. of Ag was observed by P, G, and H NMR as well as by UV/VIS and IR spectroscopy. The results indicate that the cations are held in the corners of the molecule by the two acetylene units as a result of the V-tweezer effect giving structures 43 and 44. Briefly, electron density from a filled acetylene TT-orbital is donated into a vacant i/)-hybridized orbital on the metal and backdonation occurs from filled metal... 

Once again, orbital hybridization provides an explanation for the bonding of the carbon atoms. Structurally, the hydrogen and carbon atoms of acetylene molecules lie in a straight line. This same linearity of the triple bond and the two atoms attached to the triple-bonded carbons is found in all alkynes. These characteristics are explained by mixing a 2s and a single 2p orbital of each carbon to form a pair of sp hybrid orbitals. Two of the 2p orbitals of each carbon are unhybridized (see > Figures 2.9 and 2.10). 

Arynes present structural features of some interest. They clearly cannot be acetylenic in the usual sense as this would require enormous deformation of the benzene ring in order to accommodate the 180° bond angle required by the sp1 hybridised carbons in an alkyne (p. 9). It seems more likely that the delocalised 7i orbitals of the aromatic system are left largely untouched (aromatic stability thereby being conserved), and that the two available electrons are accommodated in the original sp2 hybrid orbitals (101) ... 

Comparison of the electronic structures of acetonitrile and propyne (methylacetylene). In both compounds, the atoms at the ends of the triple bonds are sp hybridized, and the bond angles are 180°. In place of the acetylenic hydrogen atom, the nitrile has a lone pair of electrons in the sp orbital of nitrogen,... 

New parameters of cyclopropene (155) have been calculated from existing MW data. A near-equilibrium structure has also been derived from scaled moments of iner-tia (Table 16). The lengths of the C—C single bond and the methylene C—H bond and H—C—H angle are similar to those in 1 (Table 1). The C=C bond is, however, considerably shorter than in ethene 1.337 (2) A, and (=)C—H is between C—H in ethene (Section II. A) and in acetylene, 1.0586 and 1.0547 A. Bond-length relations indicate that the methylene carbon in 155 uses approximately the same hybrid orbitals as 1, sp" and sp (Section II.A), to form bonds within the ring and to substituents, while the —CH= carbon in 155 is characterized by sp and sp hybrids, respectively ... 

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