Molecular orbitals of ethane

Fig. 1.31. Molecular orbitals of ethane revealing 7c character of Hy, and n y orbitals. Only the filled orbitals are shown.

Figure 1.17 The hypothetical formation of the bonding molecular orbitals of ethane from two sp -hybridized carbon atoms and six hydrogen atoms. All of the bonds are sigma bonds. (Antibonding sigma molecular orbitals — are called a orbitals — are formed in each instance as well, but for simplicity these are not shown.)... 

Figure 1.22 A model for the bonding molecular orbitals of ethane formed from...

Fig. 1.34. Molecular orbitals of ethane revealing 7T character of tt, and ttJ, orbitals.

Athene is chemically more interesting than ethane because of the 7t bond. In fact, the n bond is the most important feature of ethene. In the words of Chapter 5, the C-C 7t orbital is the HOMO (Highest Occupied Molecular Orbital) of the alkene, which means that the electrons in it are more available than any others to react with something that wants electrons (an electrophile). Since this orbital is so important, we will look at it more closely. 

The diagram shows only the imeraciion with the energy state of ethane (the C-K bond). Other molecular orbitals of the ethane also distort. [Courtesy of R. Masel, Chemical Kinerics (McGraw Hill. 2002), p. 594 ]... 

The bond angles at the carbon atoms of ethane, and of all alkanes, are also tetrahedral like those in methane. A satisfactory model for ethane can be provided by ry) -hybridized carbon atoms. Figure 1.19 shows how we might imagine the bonding molecular orbitals of an ethane molecule being constructed from two ry) -hybridized carbon atoms and six hydrogen atoms. 

Table 8.4. The LCAO coefficients of the localized orbitals of ethane in the antiperiplanar conformation [P. Millie, B. Levy, G. Berthier, in Localization and Delocalization in Quantum Chemistry , ed. O. Chalvet, R. Daudel, S. Diner, J.P. Malrieu, Reidel Publish. Co., Dordrecht (1975)]. Only the nonequivalent atomic orbitals have been shown in the table (four significant digits) for the CC and one of the equivalent CH bonds [with the proton H(l), Fig. 8.21). The z axis is along the CC bond. The localized molecular orbitals corresponding to the carbon inner shells Is are not listed...

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. 

Hence we have two molecular orbitals, one along the line of centres, the other as two sausage-like clouds, called the n orbital or n bond (and the two electrons in it, the n electrons). The double bond is shorter than a single C—C bond because of the double overlap but the n electron cloud is easily attacked by other atoms, hence the reactivity of ethene compared with methane or ethane. 

For a molecule as simple as Fl2, it is hard to see much difference between the valence bond and molecular orbital methods. The most important differences appear- in molecules with more than two atoms. In those cases, the valence bond method continues to view a molecule as a collection of bonds between connected atoms. The molecular- orbital method, however, leads to a picture in which the sane electron can be associated with many, or even all, of the atoms in a molecule. We ll have more to say about the similarities and differences in valence bond and molecular- orbital theory as we continue to develop their principles, beginning with the simplest alkanes methane, ethane, and propane. 

The lowest unoccupied molecular orbital (LUMO) of ethane, propene, and 2-methylpropene ... 

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