Molecular orbitals of ethylene

Which molecular orbital of ethylene (tt or tt ) is the most impor 1 tant one to look at in a reaction in which ethylene is attacked by an electrophile J... 

Fig. 1.23. Rcprcacntation of the molecular orbitals of ethylene. (From W. L. Jorgensen and L. Salem, The Organic Chemist s Book of Orbitals, Academic Press, New York, 1973. Reproduced with permission.)...

Figure 1.4. The it molecular orbitals of ethylene formed from the atomic p orbitals and the tt- -tt electronic transition.

Which molecular orbital of ethylene (tt or tt ) is the most impor-... 

Figure 3.7 shows the four molecular orbitals of ethylene which involve the C atoms. (The GH a orbitals are not shown here.) The 77 orbital is the HOMO, the 77 orbital the LUMO. 

Let us now apply this method to a specific example. Consider the ethylene molecule with D2h symmetry. As can be seen from the character table of the L 2h point group (Table 6.4.2), this group has eight symmetry species. Hence the molecular orbitals of ethylene must have the symmetry of one of these eight representations. In fact, the ground electronic configuration for ethylene is... 

Some molecular orbitals of ethylene (point group Z>2h)- All are completely filled except l 2g in the ground state. The x axis points into the paper. 

So the eight pairs of electrons of this molecule occupy delocalized molecular orbitals lag to 1 3U, while the first vacant orbital is l g- Note that the names of these orbitals are simply the symmetry species of theZ)2h point group. In other words, molecular orbitals are labeled by the irreducible representations of the point group to which the molecule belongs. So for ethylene there are three filled orbitals with Ag symmetry the one with the lowest energy is called lag, the next one is 2ag, etc. Similarly, there are two orbitals with Z iu symmetry and they are called lb u and 2bi . All the molecular orbitals listed above, except the first two, are illustrated pictorially in Fig. 6.4.2. By checking the >2h character table with reference to the chosen coordinate system shown in Fig. 6.4.2, it can be readily confirmed that these orbitals do have the labeled symmetry. In passing, it is noted that the two filled molecular orbitals of ethylene not displayed in Fig. 6.4.2, lag and l iu, are simply the sum and difference, respectively, of the two carbon Is orbitals. 

The pi molecular orbitals of ethylene. The pi bonding orbital is formed by constmctive overlap of unhybridized p orbitals on the sp2 hybrid carbon atoms. Destructive overlap of thesep orbitals forms the antibonding pi orbital. Combination of two atomic orbitals must give exactly two molecular orbitals. 

In the pi bonding molecular orbital of ethylene, the lobes that overlap in the bonding region between the nuclei are in phase that is, they have the same sign (+ overlaps with +, and — overlaps with —). We call this reinforcement constructive overlap. Constructive overlap is an important feature of all bonding molecular orbitals. 

Q Show how to construct the molecular orbitals of ethylene, butadiene, and the allylic Problems 15-35 and 36 system. Show the electronic configurations of ethylene, butadiene, and the allyl cation, radical, and anion. 

Fig. 1.32 Energies of the n molecular orbitals of ethylene and butadiene by orbital...

TT molecular orbitals whose energies depend on the number of nodes they have between nuclei. Those molecular orbitals with fewer nodes are lower in cnerg than the isolated/ atomic orbitals and are boiuHngMOs those molecular orbitals with more nodes are higher in energy than the isolated p orbitals and are anti-bondifig MOs. Pi molecular orbitals of ethylene and 1,3-butadiene are shown in Figure 30.1. 

If the wavefunctions for the two p orbitals are added, we form a bonding orbital (n) if the wavefunctions are subtracted, we form an antibonding orbital (it ). Molecular tt orbitals are commonly represented by the atomic orbitals that combine to form them. In the accompanying diagram, the orbitals on the left are the atomic orbitals. These are used to form the molecular orbitals of ethylene, which appear on the right. The atomic orbitals that are combined to form the molecular orbitals are called the basis set. 

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