Frontier Orbitals and Organic Chemical Reactions [antikvár]

CHAPTER 1 Introduction Molecular orbital theory is a powerful and versatile asset to the practice of organic chemistry. As a theory of bonding it has almost completely superseded the valence bond theory: it has proved, in the long run, to be just as amenable to pictorial, non-mathematical expression; it has given the right answers to some decisive questions; it is the theory most theoreticians prefer; and quite importantly, organic chemists find it less misleading in everyday use. It is widely known today, not only as a theory of bonding but also as a theory capable of giving some, insight into the forces involved in the making and breaking of chemical bonds. Most conspicuously, it was used by Woodward and Hoffmann1 to explain the pattern of reactivity in pericyclic reactions; indeed, most of the other theories explaining the Woodward-Hoffmann rules are also based on molecular orbital theory. More recently, molecular orbital theory has provided a basis for explaining many other aspects of chemical reactivity besides the allowedness or otherwise of pericyclic reactions. The new work is based on the perturbation treatment of molecular orbital theory, introduced by Coulson and Longuet-Higgins,2 and is most familiar to organic chemists as the frontier orbital theory of Fukui.3 Earlier molecular orbital theories of reactivity concentrated on the product-like character of transition states: the concept of localization energy in aromatic substitution is a well-known example. The perturbation theory concentrates instead on the other side of the reaction coordinate. It looks at how the interaction of the molecular orbitals of the starting materials influences the transition state. Both influences on the transition state are obviously important, and it is therefore important to know about both of them, not just the one, if we want a better understanding of transition states, and hence of chemical reactivity. In this book, I have presented the theory in a much simplified, and, in particular, an entirely non-mathematical language. I shall assume only that the reader is familiar with the concept of a molecular orbital and its expression as a linear combination of atomic orbitals. I have simplified the treatment in order to make it accessible to every practising organic chemist, whether student or research worker, whether mathematically competent or not. In order to reach such a wide audience, I have frequently used very simple arguments and assumed relatively little knowledge. I hope that experienced organic chemists can recognize and skip these sections. Also, an undergraduate reader may, in many other places, wonder why so much space is devoted to a particular topic. The general importance of some of these topics lies in the history of the subject and the importance the problem had at one time. In other cases it may simply