Hybrid Orbitals and the Structure of Ethane

The same kind of orbital hybridization that accoimts for the methane stmcture also accounts for the bonding together of carbon atoms into chains and rings to make possible many millions of organic compoimds. Ethane, C2H6, is the simplest molecule containing a carbon-carbon bond. 

Problem 1.8 Draw a line-bond structure for propane, Cl I3CH2CHJ. Predict the value of each bond 

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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... 

The three-dimensional structure of ethane, C2H6, has the shape of two tetrahedra joined together. Each carbon atom is sp3 hybridized, with four sigma bonds formed by the four sp3 hybrid orbitals. Dashed lines represent bonds that go away from the viewer, wedges represent bonds that come out toward the viewer, and other bond lines are in the plane of the page. All the bond angles are close to 109.5°. 

We can account for the structure of ethyne on the basis of orbital hybridization as we did for ethane and ethene. In our model for ethane (Section 1.12B) we saw that the carbon orbitals are sp hybridized, and in our model for ethene (Section 1.13) we saw that they are sp hybridized. In our model for ethyne we shall see that the carbon atoms are sp hybridized. 

From the standpoint of bonding, the important thing to remember is that there are four bonds for each carbon, composed of sp hybrid orbitals. All the bonds are covalent, and there are no rmshared electrons on carbon. The two molecular models of ethane are shown for comparison with the two-dimensional structures, and the electron density potential map is also shown. The ball-and-stick model (5d) shows the relative position of the atoms, and the space-filling model (5e) shows the relative size of the atoms. Note the concentration of electron density between the two carbon atoms and the carbon and hydrogen atoms in 5f, which is consistent with the position of the covalent bonds. 

Ethylene is a planar molecule, as the structural representations of Figure 1.24 indicate. Because sp hybridization is associated with a tetrahedral geometry at carbon, it is not appropriate for ethylene, which has a trigonal planar geometry at both of its carbons. The hybridization scheme is determined by the number of atoms to which the carbon is directly attached. In ethane, four atoms are attached to carbon by a bonds, and so four equivalent sp hybrid orbitals are required. In ethylene, three atoms are attached to each carbon, so three equivalent hybrid orbitals are required. As shown in Figure 1.25, these three orbitals are generated by mixing the carbon 2s orbital with two of the 2p orbitals and are called sp hybrid orbitals. One of the 2p orbitals is left unhybridized. 

Ethane has a similar structure, each carbon being sp3 hybridised, there being a central link between the two carbon atoms by means of one sp3 orbital of one atom overlapping with the other. The three remaining sp3 hybrid orbitals on each carbon atom are used to overlap with three hydrogen Is orbitals. Both methane and ethane are typical saturated hydrocarbons, with the carbon exhibiting a full valency state of four. The bonds all result from end-on overlap between the orbitals, and are [Pg.39]

The simplest member of the saturated hydrocarbons, which are also called the alkanes, is methane (CH4). As discussed in Section 14.1, methane has a tetrahedral structure and can be described in terms of a carbon atom using an sp-J hybrid set of orbitals to bond to the four hydrogen atoms (see Fig. 22.1). The next alkane, the one containing two carbon atoms, is ethane (C2H6), as shown in Fig. 22.2. Each carbon in ethane is surrounded by four atoms and thus adopts a tetrahedral arrangement and sp3 hybridization, as predicted by the localized electron model. 

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