MO Theory for Conjugated Molecules

Determine whether these reactions occur by conrotation or disrotation  

You will find a set of handheld models invaluable in helping to visualize the stereochemistry of the reactions in this chapter. Build a model of (2/s,4/s)-hexadiene and examine both conrotation to give trons-3,4-dimethylcyclobutene and disrotation to give cis-3,4-dimethylcyclobutene. 

These apparently simple reactions were poorly understood by organic chemists until 1965, when R. B. Woodward and R. Hoffmann developed a theoretical explanation for the selectivities. A related theory was developed by K. Fukui at about the same time. (Fukui and Hoffmann shared the 1981 Nobel Prize in chemistry for this work. Woodward died in 1979 and so did not share in this award because the Nobel Prize is not given posthumously. However, he had already won a Nobel Prize in 1965 for his work in the area of organic synthesis.) 

Both of these theories explain whether the transition state for the reaction under consideration is favorable or not by examining how the reactant MOs are converted to the product MOs. To understand and use these theories, we need to first discuss the MOs for the conjugated molecules that participate in these reactions. 

A model for localized MOs—that is, MOs that extend over only two atoms—was developed in Chapter 3. To describe conjugated molecules, MOs that extend over more than two atoms, delocalized MOs, are needed. Some aspects of the theory for delocalized MOs were presented in Chapters 15 and 16 all of the important aspects are reiterated here. 

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