Structural: formulas 156 theory

The structural formulas used to represent molecules are based on valence bond theory. Double and triple bonds are just additional... 

We are here picturing the production of real states [mesomeric forms] from unreal states [i.e., the ordinary structural formulas] and not the deformation of real states by some external disturbance, as is the case in most of the physical problems to which perturbation theory is commonly applied.. . . There can be no physical separation. . . between resonance vibrations and other electronic vibrations it follows that the unperturbed structures. .. are only of the nature of intellectual scaffolding, and that the actual state is the mesomeric state. Chemical evidence in support of these ideas is extensive.43... 

In Chapter 1 we have stated that the classical structural theory is the only way to "visualise" the synthesis of a more or less complex organic compound. However, all or most of the information given by a structural formula can also be expressed.by a matrix (see also Appendix A-1). There are different kinds of matrices for example, the adjacency matrix J, which originates in graph theory and indicates only which atoms are bonded, or the connectivity matrix C, whose off-diagonal entries are the formal covalent bond orders. For instance, the corresponding matrices of hydrogen cyanide are ... 

There were already many interconversion reactions of organic compounds known at the time that valence theory, structural formulas, and the concept of the tetrahedral carbon came into general use. As a result, it did not take long before much of organic chemistry could be fitted into a concordant whole. One difficult problem was posed by the structures of a group of substitution... 

The molecule can now be fitted together. If more than one Lewis structural formula can be written down and can be plausibly described in terms of VB theory, resonance is invoked. 

Many researchers tried to explain the secret of the Clar s aromatic sextet theory, or hypothesis from quantum-chemical points of view. However, those trials have been failing until the graph and combinatorial theories came to be applied to this challenging problem [9,10]. In the following discussion it will be shown how various techniques and concepts of the graph theory are useful for realizing and formulating not only the fantastic theory of Clar but also the mathematical beauty of the structural formula of aromatic hydrocarbons. 

The classical rules of valency do not apply for complex ions. To explain the particularities of chemical bonding in complex ions, various theories have been developed. As early as 1893, A. Werner suggested that, apart from normal valencies, elements possess secondary valencies which are used when complex ions are formed. He attributed directions to these secondary valencies, and thereby could explain the existence of stereoisomers, which were prepared in great numbers at that time. Later G. N. Lewis (1916), when describing his theory of chemical bonds based on the formation of electron pairs, explained the formation of complexes by the donation of a whole electron pair by an atom of the ligand to the central atom. This so-called dative bond is sometimes denoted by an arrow, showing the direction of donation of electrons. In the structural formula of the tetramminecuprate(II) ion... 

The structural formulas used to represent molecules are based on valence bond theory. Double and triple bonds simply represent additional pairs of shared valence electrons. But structural formulas, while useful, don t tell the whole story about the nature of the bonds between atoms in a molecule. Valence bond theory falls flat when it tries to explain delocalized electrons and resonance structures. To get at what is really going on inside molecules, chemists had to dig deeper. 

In this respect, chemistry does not differ from other sciences. Contemporary chemical research is organized around a hierarchy of models that aid its practitioners in their everyday quest for the understanding of natural phenomena. The building blocks of the language of chemistry, including the representations of molecules in terms of structural formulae [1], occupy the very bottom of this hierarchy. Various phenomenological models, such as reaction types and mechanisms, thermodynamics and chemical kinetics, etc. [2], come next. Quantum chemistry, which at present is the supreme theory of electronic structures of atoms and molecules, and thus of the entire realm of chemical phenomena, resides at the very top. 

The only information that structural formulas provide is how a given atom is bonded (if at all) to others, or, in the words of Butlerov, how the chemical interaction is distributed . Moreover, when chemists began using the graph theory, they simplified the representation of a molecule as compared to the classical one. In distinction to a structural formula a topological graph, as a rule, has no... 

Thus in the benzene molecule, one will be able to speak of neither single, nor double, nor diagonal (para) bonds in the ordinary sense of the word. Of the structural formulas [proposed] for benzene up to now, the concept which comes closest to this one is that which von Baeyer designated as the centric formula, of course, in terms of the usual theory. ... 

Theory of van t Hoff-LeBel.—Two men independently of each other advanced a theory which explains these facts. One, a Dutch chemist by the name of van t Hoff, and the other a French chemist, LeBel. On examining the structural formulas of optically active compounds these men each saw that they differed in a common way from all optically inactive compounds which were not possible of being split into optical components. Taking as an illustration the alcohol with which we are dealing, viz., active amyl alcohol or 2-methyl butanol-1 we see by examining its formula that one of the carbon atoms is characteristically different from all of the others. 

Asymmetric Carbon.—Now van t Hoff and LeBel found that all optically active compounds contained at least one such carbon atom. They ascribed the existence of two optically active forms to the presence in the compound of this uns3anmetrically related or asymmetric carbon atom. The asymmetry of the compounds, in that one form is dextrorotatory the other levo-rotatory, is due to this asymmetric arrangement of the molecule in space. We emphasized the fact that our structural formulas as we have been using them are simply plane representations of relationships, and indicate nothing as to the arrangement in space of the atoms or groups in a molecule. The theory of van t Hoff and LeBel considers the molecule as it is arranged in space. The isomerism so explained is known as stereo-isomerism or space isomerism. 

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