Hybridization sp Orbitals and the Structure of Acetylene

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, 

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. 

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 

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 

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