Using Group Orbitals to Make Ethylene

The standard bonding picture for ethylene is viewed as being made from two sp hybridized carbons, and it contains a C-C double bond comprised of a ct bond and a it bond. In MOT, however, we don t hybridize, and we don t presume bonding arrangements. Instead, we build ethylene with no assumptions by just combining two CH2 groups. Let s see how well this construction turns out. 

The hybrid (r(out) orbital is strongly directional, pointing along the C-C bond. We expect a strong interaction. The p orbital does not point across the C-C bond. As such, the (p + p) interaction should be weaker than [o-(out) + cr(out)], because of poorer overlap. The combination of these interactions produces the C-C double bond of ethylene. Thus, [o(out) + o-(out)] is the major o bond component of the double bond. The (p -I- p) mixing produces the TT bond of ethylene. 

The computed MOs of ethy e Note the molecule is shown in different orientations for 

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. 

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