Ethylene orbital mixing diagram

Orbital mixing diagram for the formation of ethylene by mixing two CH2 groups. The computed MOs are shown in Figure 1.15. 

In our rules for orbital mixing, rule 5 states that similar molecules have similar MO diagrams. This is essentially true, but there are important differences that we must consider. Formaldehyde and ethylene are isoelectronic they have the same number of valence electrons and the same types of valence orbitals. Thus, we can expect similar MOs for formaldehyde and ethylene, but with some changes (more properly termed "perturbations") introduced by the oxygen. Experience has shown that the primary consequence of introducing heteroatoms into a hydrocarbon system is to alter orbital energies, as stated in rule 12. 

The mixing diagram for creating the tt MOs of butadiene from ethylene group orbitals and the calculated orbitals for the tt system of butadiene. 

Analysis of olefin substituent effects relevant to the Diels-Alder reaction. A. The familiar mixing diagram for the formation of the tt and ir orbitals of ethylene, which serves as a reference for the rest of the figure. 

The reason for this is outlined in Fig. 22, which shows in the center of the diagram, the it orbitals of ethylene, and the it donor and acceptor orbitals of a generalized substituent. As shown at the left of the diagram, donor substituents cause polarization like that of propene, since the low-lying orbital causes the tt to mix in... 

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