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This chapter discusses pericyclic reactions. These are reactions that occur as a result of the cyclic reorganization of electrons. These reactions were very puzzling to chemists because they are generally not sensitive to any kind of catalyst or to a change in solvent. And what was even more puzzling was that some pericyclic reactions take place only under thermal conditions, some only under photochemical conditions, yet some could be successfully carried out under both thermal and photochemical conditions. It seems rather simple now, but it takes great scientific insight to come up with a simple theory to explain what appeared to be a diverse set of reactions. We now know that all these reactions can be explained if bonding overlap of pi orbitals is maintained during the course of the reaction (a theory known as the conservation of orbital symmetry theory). In this chapter, you will learn how to use this theory to explain the relationships among reactant, product, and reaction conditions in a pericyclic reaction. There are three kinds of pericyclic reactions and each is governed by a set of selection rules. Fortunately, all these rules can be summarized in one word: TE-AC. This makes them much easier to remember. The last section in this chapter explains how to use TE-AC.
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