Aromatic theory

Effects other than angular distortions can lead to bond fixation. A simple model based on Hiickel aromatic theory accounts for a large number of such cases (e.g., starphenylene and triphenylene). Ideas that electronegative groups in the annelation are responsible for the bond fixation are shown to be inconsistent when tested against a significant sample of data. 

Explain why dehydroannulenes, which have some of the double bonds of the annulene replaced by triple bonds, can be considered in aromaticity theory as equivalent to the parent annulene. What advantages might dehydroannulenes have over annulenes in the study of aromaticity ... 

The shortness and equality of the P-N bonds in the trimeric and in the tetrameric chloride have been demonstrated in Section IV,B. The thermochemical evidence for a bond order greater than unity in the trimeric chloride is confirmed by the high value of the observed P-N stretching frequency, which is maintained or even increased as the series is ascended. The effect of mass on ring vibration frequencies is not large m, 25), so that the higher and lower infra-red absorption frequencies characteristic of the phosphonitrilic fluorides and bromides, respectively, are indications of their relative bond strengths, and are consistent with the aromatic theory. 

The modern practice of symbolizing substitution by placing the respective designations before the otherwise unaltered name of the parent compound grew from this approach. The field for substitution names widened beyond limits when, with the inauguration of structural chemistry and the aromatic theory, a detailed picture of the chemical formula became available and hydrocarbon radicals now competed with inorganic substituents in producing an unforeseen multiplicity of names. 

Various theories of chemical bonding such as Hiickel aromatic theory in organic chemistry and crystal field theory in inorganic chemistry are successful primarily because of the full use that they make of s)unmetry properties of molecules and complex ions. Arguments based on symmetry are very powerful since they usuaUy supply answers of a yes or no variety, compared to the maybe answers of most methods used in discussing chemical bonds. 

The first point that Kekule wanted to make is that his was the earliest such hypothesis for the aromatic nucleus that followed the rules of atomicity, especially carbon tetravalence, and that could not immediately be rejected for obvious predictive (or retrodictive) failures, ft is true that Couper and Loschmidt had each tentatively suggested structures for benzene, but Kekule was right that this was the first time that anyone had made a serious, competent, and sustained attempt to develop such a structural aromatic theory and to explore its consequences empirically. It was also the first time that anyone had proposed the CHT structure as the essence of aromaticity. 

This conjectural scenario would relieve Kekule from the suspicion of being disingenuous in his 1865-66 statements about the early origin of his theory. It is also consistent with the hints about aromatics that one finds in his 1858 paper, as well as with his 1859 and 1860 suggestions of two parallel isomeric series. However, accepting it presents us with a new conflict, namely the "dream story" he told in 1890. How could he have had the first idea of the benzene theory in a (day)dream ca. 1862 if he had already formulated the theory in 1858 But 1 believe that it is entirely possible to continue to credit the sincerity of Kekule s reminiscence, allowing a formative role for this personal experience even when that experience did not represent the first formulation of his aromatic theory. 

Baeyer concluded with some final thoughts about the relationship between Kekule s theory of chemical structure and his aromatic theory. The crucial breakthrough, he stated, was the former, which had been published seven years earlier (in 1858). Structure theory demonstrated that "the general laws of mechanics do not suffice to explain the essence of matter, that atoms possess specific properties, a knowledge of which must precede the application of mechanics. This knowledge we owe to you [Kekule] it forms the content of structural chemistry, and it has reached a preliminary conclusion with the benzene theory."... 

Three phases are discernible in the history of aromatic theory. From Kekule s perception of the cyclic nature of benzene until 1922, classical valency theory was exploited to its limits. The decade 1920-30 saw the emergence of electronic theories, and the confusion of electronic and classical ideas presented by the chemical literature of that time is difficult to resolve in retrospect. From about 1930 onwards, the significant ideas of the previous decade assumed clearer outlines and crystallized in a quantum theory of valency which to a great extent has solved the problem of aromatic stability and reactivity. 

Adopting the view that any theory of aromaticity is also a theory of pericyclic reactions [19], we are now in a position to discuss pericyclic reactions in terms of phase change. Two reaction types are distinguished those that preserve the phase of the total electi onic wave-function - these are phase preserving reactions (p-type), and those in which the phase is inverted - these are phase inverting reactions (i-type). The fomier have an aromatic transition state, and the latter an antiaromatic one. The results of [28] may be applied to these systems. In distinction with the cyclic polyenes, the two basis wave functions need not be equivalent. The wave function of the reactants R) and the products P), respectively, can be used. The electronic wave function of the transition state may be represented by a linear combination of the electronic wave functions of the reactant and the product. Of the two possible combinations, the in-phase one [Eq. (11)] is phase preserving (p-type), while the out-of-phase one [Eq. (12)], is i-type (phase inverting), compare Eqs. (6) and (7). Normalization constants are assumed in both equations ... 

HMO theory is named after its developer, Erich Huckel (1896-1980), who published his theory in 1930 [9] partly in order to explain the unusual stability of benzene and other aromatic compounds. Given that digital computers had not yet been invented and that all Hiickel s calculations had to be done by hand, HMO theory necessarily includes many approximations. The first is that only the jr-molecular orbitals of the molecule are considered. This implies that the entire molecular structure is planar (because then a plane of symmetry separates the r-orbitals, which are antisymmetric with respect to this plane, from all others). It also means that only one atomic orbital must be considered for each atom in the r-system (the p-orbital that is antisymmetric with respect to the plane of the molecule) and none at all for atoms (such as hydrogen) that are not involved in the r-system. Huckel then used the technique known as linear combination of atomic orbitals (LCAO) to build these atomic orbitals up into molecular orbitals. This is illustrated in Figure 7-18 for ethylene. 

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. 

For electrophilic substitutions in general, and leaving aside theories which have only historical interest, two general processes have to be considered. In the first, the 5 3 process, a transition state is involved which is formed from the aromatic compound, the electrophile (E+), and the base (B) needed to remove the proton ... 

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. 

QUANTITATIVE CORRELATIONS OF SUBSTITUENT EFFECTS The theories outlined above are concerned with the way in which substituents modify the reactivity of the aromatic nucleus. An alternative approach to the effects of substituents is provided by quantitative... 

In agreement with the theory of polarized radicals, the presence of substituents on heteroaromatic free radicals can slightly affect their polarity. Both 4- and 5-substituted thiazol-2-yl radicals have been generated in aromatic solvents by thermal decomposition of the diazoamino derivative resulting from the reaction of isoamyl nitrite on the corresponding 2-aminothiazole (250,416-418). Introduction in 5-position of electron-withdrawing substituents slightly enhances the electrophilic character of thiazol-2-yl radicals (Table 1-57). 

Twentieth century theories of bonding m benzene gave us a clearer picture of aromatic ity We 11 start with a resonance description of benzene... 

One of molecular orbital theories early successes came m 1931 when Erich Huckel dis covered an interesting pattern m the tt orbital energy levels of benzene cyclobutadiene and cyclooctatetraene By limiting his analysis to monocyclic conjugated polyenes and restricting the structures to planar geometries Huckel found that whether a hydrocarbon of this type was aromatic depended on its number of tt electrons He set forth what we now call Huckel s rule... 

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