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21.1: Prelude to Resonance and Molecular Orbital Methods

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    The structural theory of organic chemistry originated and developed from the concepts of valence and the tetrahedral carbon atom. It received powerful impetus from the electronic theory of bonding, as described in Chapter 6. We now express the structures of many organic compounds by simple bond diagrams which, when translated into three-dimensional models, are compatible with most observed molecular properties. Nonetheless, there are many situations for which ordinary structure theory is inadequate. An example is benzene (Section 1-1G), which does not behave as would be expected if it were a cyclic polyene related to alkatrienes, such as \(\ce{CH_2=CH-CH=CH-CH=CH_2}\).

    There are many other substances that do not behave as predicted from the properties of simpler compounds. Some substances are more stable, some more reactive, some more acidic, some more basic, and so on, than we would have anticipated. In this chapter we shall look at the theories that explain some of these apparent anomalies. These theories will be based on quantum-mechanical arguments (Section 1-5).

    There are two popular approaches to the formulation of the structures and properties of organic compounds based on quantum mechanics - resonance and molecular-orbital methods. In the past, there has been great controversy as to which of these methods actually is more useful for qualitative purposes and, indeed, the adherents to one or the other could hardly even countenance suggestions of imperfections in their choice. Actually, neither is unequivocally better and one should know and use both - they are in fact more complementary than competitive.

    We have used the concepts of the resonance methods many times in previous chapters to explain the chemical behavior of compounds and to describe the structures of compounds that cannot be represented satisfactorily by a single valence-bond structure (e.g., benzene, Section 6-5). We shall assume, therefore, that you are familiar with the qualitative ideas of resonance theory, and that you are aware that the so-called resonance and valence-bond methods are in fact synonymous. The further treatment given here emphasizes more directly the quantum-mechanical nature of valence-bond theory. The basis of molecular-orbital theory also is described and compared with valence-bond theory. First, however, we shall discuss general characteristics of simple covalent bonds that we would expect either theory to explain.

    Contributors and Attributions

    John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."

    21.1: Prelude to Resonance and Molecular Orbital Methods is shared under a not declared license and was authored, remixed, and/or curated by John D. Roberts and Marjorie C. Caserio.