The tt Molecular Orbitals of Ethylene and 1,3-Butadiene

The Alder rule is not perfect. It correctly predicts the major product from the Diels-Alder reaction of 1,3-cyclopentadiene with methyl acrylate, but is incorrect in the case of methyl methacrylate. On the basis of these facts, give the structure of the major product isolated in each case. 

The mechanism of the Diels-Alder reaction is best understood on the basis of a molecular orbital approach. To understand this approach we need to take a more detailed look at the tt orbitals of alkenes and dienes. 

Both the TT and tt molecular orbitals of ethylene are antisymmetric with respect to the plane of the molecule. The bonding tt orbital has no nodes other than this plane, whereas the antibonding tt orbital has a nodal plane between the two carbons. The more nodes an orbital has, the higher is its energy. 

As is true for all orbitals, a tt orbital may contain a maximum of two electrons. Ethylene has two tt electrons, and these occupy the bonding tt molecular orbital, which is the HOMO. The antibonding tt molecular orbital is vacant, and is the LUMO. 

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

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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. 

To analyze whether these reactions are allowed or forbidden, chemists focus on the frontier molecular orbitals of the reactants. The frontier molecular orbitals consist of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs). The terms highest and lowest refer to the energy of the orbitals, and the terms occupied and unoccupied refer to whether the orbital is populated with two electrons or is empty. For example, the HOMO and LUMO of butadiene would be orbitals 2 and 3 in Figure 20.2, respectively. The HOMO and LUMO of ethylene are simply the tt and tt orbitals given in Figure 1.21. 

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