Abstract
Valence is a concept to quantify the covalent bonding between atoms in molecules as opposed to ionic bonding. Chemical reactivity provided the driving force for a quantitative fixation in terms of a convenient valence index. It is most interesting that the earliest manifestation of a theory of valence started with a consideration of electronic distribution in condensed unsaturated hydrocarbons [1]. It was thought that positions with greater or lesser reactivity may appear in the molecule if the π electrons are more unevenly distributed than in benzene, as in anthracene. Svartholm also recognized that there are two different competing kinds of reactivity in these molecules, namely reactivity at atoms and reactivity at bonds. His analysis allowed him to compare different regions in a single molecule only, but not several molecules among themselves. Several years passed before improvement of this simple picture came from two sides. Daudel and Pullman [2] normalized the portions of electrons that were attached to bonds and the portions of nonbonding electrons providing reactive sites at atoms, so that the sum of all portions was equal to the number of π electrons. This simple but powerful procedure allowed a comparison between different molecules and provided trends in reactivity from benzene to anthracene. From this analysis it was concluded that the a position in naphthalene is more reactive than the carbon positions in benzene. This was the intuitive birth of the idea of free valence for both different molecular sites and different molecular systems. The expression free valence was actually used five years later by Daudel et al. [3] in a paper on static and dynamic indices based on molecular diagrams. A formalism was proposed to relate the activation energy of a substitution reaction in an unsaturated hydrocarbon and the free valence at the atomic site. The attack of the α position of naphthalene by a radical was discussed in some detail. More general applications to substitution and addition reactions were presented.
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© 1989 Kluwer Academic Publishers, Dordrecht, The Netherlands
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Jug, K. (1989). A Valence Concept for Molecular Structure and Reactions. In: Maruani, J. (eds) Molecules in Physics, Chemistry, and Biology. Topics in Molecular Organization and Engineering, vol 3. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-2853-4_7
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DOI: https://doi.org/10.1007/978-94-009-2853-4_7
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