Electronic Theories of Structure and Reactivity

Once chemists gained an appreciation of the fundamental principles of bonding, a logical next step became the understanding of how chemical reactions occurred. Most 

The University of Kazan was home to a number of prominent nineteenth-century organic chemists. Their contributions are recognizee) in two articles publishecJ in the January ancJ February 1994 issues of the Journal of Chemical Education (pp. 39-42 and 93-98). 

A 1968 German stamp combines a drawing of the structure of benzene with a portrait of Kekule. 

Linus Pauling is portrayed on this 1977 Volta stamp. The chemical formulas depict the two resonance forms of benzene, and the explosion in the background symbolizes Pauling s efforts to limit the testing of nuclear weapons. 

The discoverer of penicillin, Sir Alexander Fleming, has appeared on two stamps. This 1981 Hungarian issue includes both a likeness of Fleming and a structural formula for penicillin. 

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

The diazo-compounds and corresponding aromatic carbenes that form the basis for our dissection of structure and reactivity are shown in Table 1. The carbenes in this group are carefully chosen so that the variation in structure is systematic the theory identifies the carbene bond angle and certain electronic factors as controlling chemical and physical properties, and as far as possible, these two features are varied independently of each other for these carbenes. Table 2 lists some other aromatic carbenes that have been studied. In general, the structures of these carbenes are not simply related to each other. Nevertheless, the principles uncovered by analysis of the compounds of Table 1 can be readily extended to those of Table 2. 

The theory as presented so far is clearly incomplete. The topology of the density, while recovering the concepts of atoms, bonds and structure, gives no indication of the localized bonded and non-bonded pairs of electrons of the Lewis model of structure and reactivity, a model secondary in importance only to the atomic model. The Lewis model is concerned with the pairing of electrons, information contained in the electron pair density and not in the density itself. Remarkably enough however, the essential information about the spatial pairing of electrons is contained in the Laplacian of the electron density, the sum of the three second derivatives of the density at each point in space, the quantity V2p(r) [44]. 

The acetylene molecule with its unique six-electron chemical bond, strength, high energy, and at the same time its vulnerability to diverse transformation hardly fits the Procrustean bed of modern theories of valency and reactivity. Being a steady challenge to theorists [121,122], it stimulates the development of fundamental works in the field of structure of matter and energy transformation. 

The most notable studies are those of Ingold, on the orienting and activating properties of substituents in the benzene nucleus, and of Dewar on the reactivities of an extensive series of polynuclear aromatic and related compounds ( 5.3.2). The former work was seminal in the foundation of the qualitative electronic theory of the relationship between structure and reactivity, and the latter is the most celebrated example of the more quantitative approaches to the same relationship ( 7.2.3). Both of the series of investigations employed the competitive method, and were not concerned with the kinetics of reaction. 

Quantum mechanics provide many approaches to the description of molecular structure, namely valence bond (VB) theory (8-10), molecular orbital (MO) theory (11,12), and density functional theory (DFT) (13). The former two theories were developed at about the same time, but diverged as competing methods for describing the electronic structure of chemical systems (14). The MO-based methods of calculation have enjoyed great popularity, mainly due to the availability of efficient computer codes. Together with geometry optimization routines for minima and transition states, the MO methods (DFT included) have become prevalent in applications to molecular structure and reactivity. 

The reactivities of the diene and dienophile in a Diels-Akier reac tion are highly dependent on their electronic structures.15 In the case of a Dieh-Alder reaction with normal electron demand, the dienophile is substituted with an electron acceptor Z, whereas the dienophile carries an electron donor X. The reaction in question follows this pattern. Increased reactivity in such a case can be rationalized with the frontier orbital theory of hukui and HouL according to which the energy difference between the (HOMO) of the diene and the lowest unoccupied molecular orbital (LUMO) of the dienophile is reduced in a favorable way by the substituents. 

Jhe development of chemistry in the 20th century has been dominated and motivated by the electronic theory of the chemical bond and the role of electrons in chemical reactivity. The electronic structure of the chemical bond could be deduced by more or less direct methods, such as electronic excitation spectra, dipole moments, or paramagnetism but there was no direct indication for the transfer of electrons in chemical reactions. Using isotopic techniques it has been possible to demonstrate bond cleavage and atom transfer reactions, but it is impossible to label an electron and trace its transfer from one molecule to another. It was not until the discovery of the radiolytically produced solvated electron that electron transfer processes could be examined directly and unambiguously. 

Primas, H. Separability in many-electron systems. In Modem quantum chemistry. Istanbul lectures. Sinanoglu, O. (ed.). New York Academic Press 1965 s°) Paldus, J., Ctzek, J. Relation of coupled-pair theory, Cl and some other many-body approaches. In Energy, Structure, and Reactivity. Proceedings of the 1972 Boulder Summer Research Conference on Theoretical Chemistry. Smith, D. W., McRae, W. B. (eds.). New York Wiley 1973, pp. 198-212... 

Most chemists still tend to think about the structure and reactivity of atomic and molecular species in qualitative terms that are related to electron pairs and to unpaired electrons. Concepts utilizing these terms such as, for example, the Lewis theory of valence, have had and still have a considerable impact on many areas of chemistry. They are particularly useful when it is necessary to highlight the qualitative similarities between the structure and reactivity of molecules containing identical functional groups, or within a homologous series. Many organic chemistry textbooks continue to use full and half-arrows to indicate the supposed movement of electron pairs or single electrons in the description of reaction mechanisms. Such concepts are closely related to classical valence-bond (VB) theory which, however, is unable to compete with advanced molecular orbital (MO) approaches in the accurate calculation of the quantitative features of the potential surface associated with a chemical reaction. 

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