Physical Organic Chemistry

Author: N.S. Isaacs
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Publication Date: 1997-01-01
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Fragment: An understanding of chemical reactivity begins with an understanding of chemical bonding, the forces which render certain aggregates of atoms (i.e. the familiar molecules) more stable than others. 1-3 It is on this basis that chemical reactions—changes in bonding—may be approached and a rational and consistent theory of organic chemistry be devised. Two milestones in the understanding of bonding may be quoted. The first, the recognition of the electron-pair covalent bond by Lewis 4 and by Langmuir 5 in 1919, still provides a model for the description of molecular structure adequate for most purposes and will be extensively employed in the following text. According to this concept, valence electrons are shared so as to create filled shell configurations and are regarded as essentially localized in the inter nuclear space. For the first row elements of which organic compounds are almost entirely composed, this is the octet (2s2, 2p6); for hydrogen, 1s1. The second leap in understanding was made by the introduction of quantum mechanics to chemistry following the molecular orbital description of bonding in the hydrogen molecule by Heitler and London, 6 in 1929. This approach superseded the concept of localized electrons and paved the way to quantitative understanding of bonding, the satisfactory calculations of bonding energies, optimum bond lengths and geometries. It will be necessary to turn to these methods, despite the necessity of somewhat lengthy computation, when the need arises to consider specific molecular orbital properties (for example, in the theory of pericyclic reactions, Chapter 14). Nonetheless, quantum concepts permeate any description of chemical bonding, though rather distinct models may be used which will now be briefly described. I.I.I The valence bond (VB) model We know the structure of a molecule in that it contains defined atoms located precisely in space. One begins with this determinate part of molecular structure (which can be obtained accurately, by X-ray crystal diffraction for instance) by setting all nuclei in their correct spatial positions. The indeterminate part, the disposition of the bonding electrons, 12SS1534 is then accomplished by adding these in pairs such that no atom exceeds its closed shell number. There are inevitably many ways in which this can be achieved, each localized structure (known as a 'contributing' structure or, in the older literature, a 'canonical' structure) being regarded as contributing, in some measure determined by its energy, to the true structure. The molecule is conceived as a 'resonance hybrid' of many contributing structures whose contribution can be expressed as a fraction entering into the whole. The relationship between contributing structures, which differ only in the distribution of valence electrons, is expressed by the double arrow,. Although for the purposes of exact calculations of molecular energies, for instance, many contributing structures are needed even for a molecule such as H2, it is frequently found that a single VB structure suffices to describe the structure adequately for qualitative purposes. For example, methane may be represented as 1 and contributions from such structures as 2 ignored for the interpretation of reaction mechanisms.