Hybridization sp Orbitals and the Structure of Acetylene

In addition to forming single and double bonds by sharing two and four electrons, respectively, carbon also can form a triple bond by sharing six electrons. To account for the triple bond in a molecule such as acetylene, C2H2, we need a third kind of hybrid orbital, an sp hybrid. 

When two sp-hybridized carbon atoms approach each other, sp hybrid orbitals from each carbon overlap head-on to form a strong sp-sp a bond. In addition, the orbitals from each carbon form a tt bond by sideways overlap, and the Py orbitals overlap similarly to form a Py-py n bond. 

An sp-hybridized carbon atom. The two sp hybrid orbitals (green) are oriented 180 away from each other, perpendicular to the two remaining p orbitals (blue). 

The structure of acetylene. The two sp-hybridized carbon atoms are joined by one sp sp V bond and two p-p n bonds. 

TABLE 13 Comparison of C-C and C-H Bonds in Methane, Ethane, Ethylene, and Acetylene 

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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... 

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). 

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. 

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 ... 

When a carbon atom is attached to two other atoms, as in acetylene or carbon dioxide, there is sp hybridization and the bonds lie in a straight line. The electronic structure of carbon dioxide is shown in Figure 1.19. Two % bonds are now formed in addition to the a bonds. In the figure, the unhybridized p orbitals above and below the plane overlap with the p orbitals of the right-hand oxygen atom, and the unhybridized p orbitals in the plane overlap with the p orbitals of the left-hand oxygen atom. 

Historically, the application of molecular orbital theory to the electronic structures of isoelec-tronic 14-electron molecules such as acetylene, HCN, N2, and O2 was an excellent pioneering demonstration of the value of quantum chemistry. Within the framework of molecular orbital theory, the C - C bond in acetylene is a triple bond involving one a-bond, and two orthogonal 7t-bonds. The a-bond is formed by two sp-hybrid orbitals from each carbon, and the two 71-bonds are formed from the perpendicular p-orbitals. Alternatively, the so-called bent or banana bonds have been invoked to describe the multiple C-C bonds in acetylene (Fig. 1-1) [3-5]. This creates a conceptual dilemma, though one bonding model can be transformed to the other by appropriate linear combinations. It is now realized that both... 

Figure 1.22 shows a Lewis structure and an orbital overlap diagram for acetylene, C2H2. A carbon-carbon triple bond consists of one sigma bond and two pi bonds. The sigma bond is formed by the overlap of sp hybrid orbitals. One pi bond is formed by the overlap of a pair of parallel 2p atomic orbitals. The second pi bond is formed by the overlap of a second pair of parallel 2p atomic orbitals. 

The two j -hybridized orbitals are available to form ct bonds (one on either side), and the two p orbitals are available to form it bonds, giving the bonding structure for acetylene shown in Figure 1.33. A triple bond between two carbon atoms is therefore the result of three separate bonding interactions one ct bond and two it bonds. The a bond results from the overlap of sp orbitals, while each of the two it bonds result from overlapping p orbitals. As shown in Figure 1.33, the geometry of the triple bond is linear. 

Bonding in acetylene, C2H2, is similar to that in C2H4, but with these differences The Lewis structure of C2H2 features a triple covalent bond, H—C=C—H. The molecule is linear, as found by experiment and as expected from VSEPR theory. A hybridization scheme to produce hybrid orbitals in a linear orientation is sp. The valence-shell orbital diagrams representing sp hybridization are... 

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