Electronic theory aromaticity

For the electronic theory of organic chemistry 1926 was the annus mirabilis, and, particularly, as they applied to aromatic substitution, the... 

The electronic theory of organic chemistry, and other developments such as resonance theory, and parallel developments in molecular orbital theory relating to aromatic reactivity have been described frequently. A general discussion here would be superfluous at the appropriate point a brief summary of the ideas used in this book will be given ( 7- )-... 

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. 

The electronic theory provides by these means a description of the influence of substituents upon the distribution of electrons in the ground state of an aromatic molecule as it changes the situation in benzene. It then assumes that an electrophile will react preferentially at positions which are relatively enriched with electrons, providing in this way an isolated molecule theory of reactivity. 

However, the electronic theory also lays stress upon substitution being a developing process, and by adding to its description of the polarization of aromatic molecules means for describing their polarisa-bility by an approaching reagent, it moves towards a transition state theory of reactivity. These means are the electromeric and inductomeric effects. 

In this initial section the reactivities of the major types of azole aromatic rings are briefly considered in comparison with those which would be expected on the basis of electronic theory, and the reactions of these heteroaromatic systems are compared among themselves and with similar reactions of aliphatic and benzenoid compounds. Later in this chapter all the reactions are reconsidered in more detail. It is postulated that the reactions of azoles can only be rationalized and understood with reference to the complex tautomeric and acid-base equilibria shown by these systems. Tautomeric equilibria are discussed in Chapter 4.01. Acid-base equilibria are considered in Section 4.02.1.3 of the present chapter. 

The first paper of the frontier-electron theory pointed out that the electrophilic aromatic substitution in aromatic hydrocarbons should take place at the position of the greatest density of electrons in the highest occupied (HO) molecular orbital (MO). The second paper disclosed that the nucleophilic replacement should occur at the carbon atom where the lowest unoccupied (LU) MO exhibited the maximum density of extension. These particular MO s were called "frontier MO s . In homolytic replacements, both HO and LU.were shown to serve as the frontier MO s. In these papers the "partial" density of 2 pn electron, in the HO (or LU) MO, at a certain carbon atom was simply interpreted by the square of the atomic orbital (AO) coefficient in these particular MO s which were represented by a linear combination (LC) of 2 pn AO s in the frame of the Huckel approximation. These partial densities were named frontier-electron densities . 

The following presentation is limited to closed-shell molecular orbital wave-functions. The first section discusses the unique ability of molecular orbital theory to make chemical comparisons. The second section contains a discussion of the underlying basic concepts. The next two sections describe characteristics of canonical and localized orbitals. The fifth section examines illustrative examples from the field of diatomic molecules, and the last section demonstrates how the approach can be valuable even for the delocalized electrons in aromatic ir-systems. All localized orbitals considered here are based on the self-energy criterion, since only for these do the authors possess detailed information of the type illustrated. We plan to give elsewhere a survey of work involving other types of localization criteria. 

Burkhardt, "Arthur Lapworth and Others," 143. C. K. Ingold, "The Significance of Tautomerism and of the Reactions of Aromatic Compounds in the Electronic Theory of Organic Reactions," JCS (1933) 1120 and C. 

Busy with the move from Leeds to London in 1930, Ingold afterward was in residence at Stanford University in California in 1932, with some leisure to work out generalizations of the results already at hand. He soon published two widely read pieces "Significance of Tautomerism and of the Reactions of Aromatic Compounds in the Electronic Theory of Organic Reactions," in the Journal of the Chemical Society, and the essay review, "Principles of an Electronic Theory of Organic Reactions," in Chemical Reviews.58... 

The Principles of Aromatic Substitution from the Standpoint of the Electronic Theory of Valence." Recueil des Travaux Chimiques 48 (1929) 797812. 

The K-region of a polycyclic aromatic hydrocarbon is typified by the 9,10-bond of phenanthrene. According to the Schmidt-Pullman electronic theory, an unsubstituted K-region is a requirement for carcinogenic activity see A. Pullman and B. Pullman, La Cancerisation par les Substances Chimiques et la Structure Moleculaire, Masson, Paris, 1955. 

A. F. Holleman (1859-1953)175 played a key role in the development of our knowledge and understanding of aromatic substitution. He was the first to realize the importance of measuring rates of substitution and proportions of isomers formed. Much of his work came before the development of electronic theories of organic reactivity. In 1910, he published a book entitled Die direkte EinfUhrung von Substituenten in den Benzolkern, which contained a systematic description of all the then known facts about... 

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