Molecular orbital theory, pericyclic correlation diagrams

Three levels of explanation have been advanced to account for the patterns of reactivity encompassed by the Woodward-Hoffmann rules. The first draws attention to the frequency with which pericyclic reactions have a transition structure with (An + 2) electrons in a cyclic conjugated system, which can be seen as being aromatic. The second makes the point that the interaction of the appropriate frontier orbitals matches the observed stereochemistry. The third is to use orbital and state correlation diagrams in a compellingly satisfying treatment for those cases with identifiable elements of symmetry. Molecular orbital theory is the basis for all these related explanations. 

Discuss Frontier Molecular Orbital (F.M.O.) method for pericyclic reactions. What are electrocyclic reactions Drawing correlation diagram, describe the comrotatoiy and disrotatory interconversion of cyclobutene and butadiene. Discuss Frontier Molecular Orbital (F.M.O.) method of analysing electrocyclic reactions. Derive selection rules for electrocyclic reactions. What are electrocyclic reactions Drawing correlation diagram discuss disrotatory and conrotatory interconversion of cyclobutene and butadiene. Support the results of correlation diagram by F.M.O. theory. 

From the foregoing discussion it appears that the frontier orbital method is at once a simple, concise, and accurate method for assessing the stereochemical outcome of pericyclic reactions. Furthermore, it is a method that is equally applicable to symmetrical and to unsymmetrical systems. There are some disadvantages in the theory, however. Firstly, it is necessary to derive the general phase characteristics of the HOMO and LUMO levels. Hiickel molecular calculations can be used for tliis purpose, but there are available a number of approximate methods, for example the electron-in-a-box model, which are usually satisfactory even if they are more difficult to apply to more complex systems. Nevertheless, frontier orbital analysis is quicker and more simple than the formalized correlation diagram approach, and with a little practice one can intuitively arrive at the correct relative phase relationsliips in the HOMO and LUMO levels. 

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