Ethane molecular orbitals

With a total of fourteen valence electrons to accommodate in molecular orbitals, ethane presents a more complicated picture, and we now meet a C—C bond. We will not go into the full picture—finding the symmetry elements and identifying which atomic orbitals mix to set up the molecular orbitals. It is easy enough to see the various combinations of the Is orbitals on the hydrogen atoms and the 2s, 2px, 2py and 2pz orbitals on the two carbon atoms giving the set of seven bonding molecular orbitals in Fig. 1.19. 

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. 

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

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. 

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...

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

Note how we have resorted to another form of representation of the ethane, ethylene, and acetylene molecules here, representations that are probably familiar to you (see Section 1.1). These line drawings are simpler, much easier to draw, and clearly show how the atoms are bonded - we use a line to indicate the bonding molecular orbital. They do not show the difference between a and rr bonds, however. We also introduce here the way in which we can represent the tetrahedral array of bonds around carbon in a two-dimensional drawing. This is to use wedges and dots for bonds instead of lines. By convention, the wedge means the bond is coming towards you, out of the plane of the paper. The dotted bond means it is going away from you, behind the plane of the paper. We shall discuss stereochemical representations in more detail later (see Section 3.1). 

This weak transition is due to the promotion of an electron from the non-bonding molecular orbital n to an anti-bonding tt orbital. This transition is usually observed in molecules that contain a heteroatom as part of an unsaturated system. The most common of these bands corresponds to the carbonyl band at around 270 to 295 nm, which can be easily observed. The molar absorption coefficient for this band is weak. The nature of the solvent influences the position of absorption bands because the polarity of the bond is modified during absorption. For example, ethanal Amax = 293 nm (e = 12 in ethanol as solvent). 

Fig. l.ll. Energy levels of an AX spin system (a) and interaction of the nuclear spins A and X with I = 1/2, involving the bonding electrons (ethane molecular orbital model). 

Kao and Huang employed molecular mechanics and ab initio molecular orbital (MO) theory to determine the structure, energies, and conformations of the (Z)- (40, R = H) and ( )- (12, R1 - R4 = H) isomers. For the (Z)-isomer, an unsymmetrical conformation (67) is favored by at least 6 kcal over symmetrical chair, boat, and twist forms, which suffer from ethane-type and other H-H repulsions. In the case of ( )-isomer, the twist... 

Thus, the formal net M-M bonding is provided by two 7t-type molecular orbitals, (Scheme 7.1b). At the long distances, direct CO -CO interactions in (OC)3Fe-Fe (CO)3 are minimal, so that the eclipsed conformation with better overlap is favored. This is to be contrasted with the rotational preferences of ethane [45—47]. 

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. 

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