The Structural Theory

It is from the concept of isomerism that we can trace the origins of the structural theory—the idea that a precise arrangement of atoms uniquely defines a substance. Ammonium cyanate and urea are different compounds because they have different structures. To some degree the structural theory was an idea whose time had come. Three scientists stand out, however, in being credited with independently proposing the elements of the structural theory. These scientists are August Kekule, Archibald S. Couper, and Alexander M. Butlerov. 

Shortly thereafter, but independently of Kekule, Archibald S. Couper, a Scot working in the laboratory of Charles-Adolphe Wurtz at the Ecole de Medicine in Paris, and Alexander Butlerov, a Russian chemist at the University of Kazan, proposed similar theories. 

How can we even begin to study a subject of such enormous complexity Is organic chemistry today as Wohler saw it a century and a half ago The jungle is still there—largely unexplored—and in it are more remarkable things than Wohler ever dreamed of. But, so long as we do not wander too far too fast, we can enter without fear of losing our way, for we have a chart the structural theory. 

The structural theory is the basis upon which millions of facts about hundreds of thousands of individual compounds have been brought together and arranged in a systematic way. It is the basis upon which these facts can best be accounted for and understood. 

The structural theory is the framework of ideas about how atoms are put together to make molecules. The structural theory has to do with the order in which atoms are attached to each other, and with the electrons that hold them together. It has to do with the shhpes and sizes of the molecules that these atoms form, and with the way that electrons are distributed over them. 

Any consideration of the structure of molecules must begin with a discussion of chemical bonds, the forces that hold atoms together in a molecule. 

We shall discuss chemical bonds first in terms of the theory as it had developed prior to 1926, and then in terms of the theory of today. The introduction of quantum mechanics in 1926 caused a tremendous change in ideas about how molecules are formed. For convenience, the older, simpler language and pictorial representations are often still used, although the words and pictures are given a modem interpretation. 

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The development of the structural theory of the atom was the result of advances made by physics. In the 1920s, the physical chemist Langmuir (Nobel Prize in chemistry 1932) wrote, The problem of the structure of atoms has been attacked mainly by physicists who have given little consideration to the chemical properties which must be explained by a theory of atomic structure. The vast store of knowledge of chemical properties and relationship, such as summarized by the Periodic Table, should serve as a better foundation for a theory of atomic structure than the relativity meager experimental data along purely physical lines. ... 

In 1866 only a few years after publishing his ideas concerning what we now rec ognize as the structural theory of organic chemistry August Kekule applied it to the structure of benzene He based his reasoning on three premises... 

The structural theory requires that there be four bonds to each carbon... 

The article A History of the Structural Theory of Benzene—The Aromatic Sex tet and Huckel s Rule in the February 1997 issue of the Journal of Chemical Educa tion (pp 194-201) IS a rich source of additional informa tion about this topic... 

Phenolic resins were the first totally synthetic plastics invented. They were commercialized by 1910 [I]. Their history begins before the development of the structural theory of chemistry and even before Kekule had his famous dreams of snakes biting their tails. It commences with Gerhardt s 1853 observations of insoluble resin formation while dehydrating sodium salicylate [2]. These were followed by similar reports on the behavior of salicylic acid derivatives under a variety of reaction conditions by Schroder et al. (1869), Baeyer (1872), Velden (1877), Doebner (1896 and 1898), Speyer (1897) and Baekeland (1909-1912) [3-17]. Many of these early reports appear to involve the formation of phenolic polyesters rather than the phenol-aldehyde resins that we think of today. For... 

There are numerous families of organic compounds, with structures analogous to hydrocarbons, that contain other atoms (e.g., O, N, S, Cl) besides C and H. Classification is done in accordance with the structural theory on the basis of functional groups present. The atom or atomic grouping that characterizes a particular family and also determines the properties of its members is called a Junctional group. Table 2-42 contains a selected list of common functional groups and examples of... 

The structure theory of inorganic chemistry may be said to have been bom only fifty years ago, when Werner, Nobel Laureate in Chemistry in 1913, found that the chemical composition and properties of complex inorganic substances could be explained by assuming that metal atoms often coordinate about themselves a number of atoms different from their valence, usually four atoms at the comers either of a tetrahedron or of a square coplanar with the central atom, or six atoms at the comers of an octahedron. His ideas about the geometry of inorganic complexes were completely verified twenty years later, through the application of the technique of x-ray diffraction. 

To conclude this section, we can state that all of the theories presented hitherto, even when starting from general principles, inevitably embody several assumptions, which in fact represent the heart of the analysis. However, the physical meaning of these assumptions usually is not known, so that no theory is able to predict in which reaction series isokinetic behavior appears and in which it does not. Neither is the structural theory of organic chemistry able to make such a prediction and to define the terms reaction series or similar reactions or small structure changes it can only afford many examples. 

Structural theory facilitates the evaluation of exergy cost and incorporation of thermoeconomics functional analysis (Erlach et al., 1999). It is a common formulation for the various thermoeconomic methods providing the costing equations from a set of modeling equations for the components of a system. The structural theory needs a productive structure displaying how the resource... 

In the second half of the nineteenth century the structural theory of organic chemistry was developed. It led to the concept that chemical, physical and biological properties of all kinds must vary with structural change. The earliest structure-property relationships (SPR) were qualitative. With the development of methods of quantitative measurement of these properties data accumulated. Attempts were then made to develop quantitative models of the structural dependence of these properties. These methods for the quantitative description of structural effects will now be described. 

As Rocke has noted, the structure theory in organic chemistry had no real rivals in the nineteenth century opponents were forced to retreat into positiv-... 

In addition to atomism, the principal chemical theories of the nineteenth century included electrochemical dualism, the radical theory, the type theory, and the structure theory, the latter strongly identified with what chemists called the "law of linking" of carbon atoms. The valence theory evolved as a way of tying together the notions of chemical equivalence and chemical structure, and it carried along the old problem that some chemical elements (e.g., nitrogen) exhibit different combining values with another element in different circumstances. 

It is remarkable that, in the space of less than two decades, the structural theory of organic chemistry should have moved from the first hesitant steps, where the chemical structure was considered to be separate and distinct from the physical structure of the molecule, and represented only the "affinities" of the atoms within the molecule, to the point where those same formulas were now viewed as representations of the actual physical locations of the atoms in the molecule (76). What was left undone at the end of the nineteenth century, by which time three-dimerrsional graphical formitlas for organic compoimds were in routine use, was, of coruse, a description of exactly what the "chemical affinities" of the atoms composing the molecules were. The answer to this problem would have to await the new century, and the development of modem theories of the atom and bonding. 

As was already pointed out, the coefficient C in the q2 dependence of the reduced first cumulant T/q2 is a valuable measure of the structure. Theory predicts an increase of C with polydispersity but a decrease with branching. Thus, the behavior resembles that of g but the dependence is different in quantitative respects102. The combination of both parameters C and g therefore, should make possible, a better differentiation between various models. At present, only a few, partly contradictory, results are known for Unear chains184-187,216,217 and no data are available for branched molecules. The discussion of C as a structure-significant parameter must be kept short, not only for this reason but also because of experimental difficulties209,218. These difficulties become evident when the experimental results from various laboratories are compared. With the exception of... 

You should reserve the term unsaturated for compounds that can, at least potentially, react by addition, and saturated for compounds that can only be expected to react by substitution. The difference between addition and substitution became much clearer with the development of the structure theory that called for carbon to be tetravalent and hydrogen univalent. Ethene then was assigned a structure with a carbon-to-carbon double bond, and ethane a structure with a carbon-to-carbon single bond ... 

In 1950s and 1960s Bradley developed the structure theory for metal alkoxides, based on the principle that metal atoms should achieve maximal coordination at minimal molecular complexity of the aggregate, [M(OR) ]m. 

Theoretical chemistry rates some special mention in this context. Nowadays this activity tends to be quite mathematical [1], but history shows us that theoretical chemistry need not be mathematical at all. From the first years of the crystallization of chemistry as a subject distinct from alchemy, chemists have utilized theory, in the sense of disciplined speculation. Nonmathematical examples are found in the structural theory of organic chemistry [2] and in most applications of the powerful Woodward-Hoffman orbital symmetry rules [3]. 

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