Organic chemistry structural theory

Interestingly, Westheimer (1965) at Harvard developed a summer short course in organic chemistry structured much like the development above. He began with the simple structural theory and then looked at cases where it failed ... 

HE STRUCTURAL THEORY of Kekulc has been the growth hormone of organic chemistry. This theory used the atom and the bond as its fundamental concepts. It assumed no knowledge of either the composition of the atom or the nature of the bond. In recent years, however, modern theodes of atoms and bonds have nearly filled these lacunae. 

FIGURE 260. Plate from Youmans Chemical Atlas (original in color) depicting the three prevailing theories of organic chemistry structure in reactivity prior to Karlsruhe. 

One of the most challenging molecules for organic electronic structure theory is cyclopropane. It is difficult to understand C-C-C angles of 60° with conventional bonding models. Also, in Section 11.5.14 we examined the unusual reactivity of the cyclopropylcarbinyl cation. Much of the rearrangement chemistry of this cation, as well as its special thermodynamic stability (Chapter 2), can be understood to arise from the uniquely strong donor character of the a bonds in cyclopropane. Here we provide a more detailed description of the bonding in cyclopropane that nicely rationalizes these observations. 

II. The Structure of Mesomeric Systems ibid., 3350. A Molecular Orbital Theory of Organic Chemistry. III. Charge Displacements and Elearomeric Substituents ibid., 3353. A Molecular Orbital Theory of Organic Chemistry. IV. Free Radicals ibid., 3355. A Molecular Orbital Theory of Organic Chemistry. V. Theories of Reactivity and the Relationship Between Them ibid., 3357. A Molecular Orbital Theory of Organic Chemistry. VI. Aromatic Substitution and Addition. 

B]onds are theoretical constructs, idealizations, which have aided chemists during the past one hundred years in developing the convenient and extremely valuable classical structure theory of organic chemistry. The theory of resonance constitutes an extension of this theory. It is based upon the use of the same idealizations — the bonds between atoms — as used in classical structure theory, with the important extension that in describing the benzene molecule two arrangements of these bonds are used, rather than only one. [Pauling, 1970,... 

THEORIES RELATING STRUCTURE AND REACTIVITY 7.2.1 The electronic theory of organic chemistry... 

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... 

Another broad approach to the description of molecular structure that is of importance in organic chemistry is molecular orbital theory. Molecular orbital (MO) theory pictures electrons as being distributed among a set of molecular orbitals of discrete... 

Several methods of quantitative description of molecular structure based on the concepts of valence bond theory have been developed. These methods employ orbitals similar to localized valence bond orbitals, but permitting modest delocalization. These orbitals allow many fewer structures to be considered and remove the need for incorporating many ionic structures, in agreement with chemical intuition. To date, these methods have not been as widely applied in organic chemistry as MO calculations. They have, however, been successfully applied to fundamental structural issues. For example, successful quantitative treatments of the structure and energy of benzene and its heterocyclic analogs have been developed. It remains to be seen whether computations based on DFT and modem valence bond theory will come to rival the widely used MO programs in analysis and interpretation of stmcture and reactivity. 

The last decade has witnessed an unprecedented strengthening of the bone between theory and experiment in organic chemistry. Much of this success may be credited to the development of widely applicable, unifying concepts, such as the symmetry rules of Woodward and Hoffmann, and the frontier orbital thee>ry of Eukui. Whereas the the ore tical emphasis had historically been on detailed structure and spectroscopy, the new methods are de signe d to solve pre)blems e>f special importance to organic chemists reactivity, stereochemistry, and mechanisms. 

A most important early addition to organic structure theory was made by the first Nobel Laureate in Chemistry, van t Hoff, who in 1874 recognized that the optical activity of carbon compounds can be explained by the postulate that the four valence bonds of the carbon atom are directed in space toward the comers of a tetrahedron. 

The convenience and usefulness of the concept of resonance in the discussion of chemical problems are so great as to make the disadvantage of the element of arbitrariness of little significance. Also, it must not be forgotten that the element of arbitrariness occurs in essentially the same way in the simple structure theory of organic chemistry as in the theory of resonance — there is the same use of idealized, hypothetical structural elements. In the resonance discussion of the benzene molecule the two Kekule structures have to be described as hypothetical it is not possible to synthesize molecules with one or the other of the two Kekule structures. In the same way, however, the concept of the carbon-carbon single bond is an idealization. The benzene molecule has its own structure, which cannot be exactly composed of structural elements from other molecules. The propane molecule also has its own structure, which cannot be composed of structural elements from other molecules — it is not possible to isolate a portion of the propane molecule, involving parts of two carbon atoms... 

Dewar, M. J. S., The Molecular Orbital Theory of Organic Chemistry, McGraw-Hill, New York, 1969 R. G. Pan, The Quantum Theory of Molecular Electronic Structure, Benjamin, New York, 1963. 

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. 

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