Resonance energy in aromatics

The reason for this becomes apparent when one compares the shapes of the localized it orbitals with that of the ethylene 7r orbital. All of the former have a positive lobe which extends over at least three atoms. In contrast, the ethylene orbital is strictly limited to two atoms, i.e., the ethylene 7r orbital is considerably more localized than even the maximally localized orbitals occurring in the aromatic systems. This, then, is the origin of the theoretical resonance energy the additional stabilization that is found in aromatic conjugated systems arises from the fact that even the maximally localized it orbitals are still more delocalized than the ethylene orbital. The localized description permits us therefore to be more precise and suggests that resonance stabilization in aromatic molecules be ascribed to a "local delocalization of each localized orbital. One infers that it electrons are more delocalized than a electrons because only half as many orbitals cover the same available space. It is also noteworthy that localized it orbitals situated on joint atoms (n 2, it23, ir l4, n22 ) contribute more stabilization than those located on non-joint atoms, i.e. the joint provides more paths for local delocalization. 

The material presented in Section II warrants, apparently, the conclusion that the main test of aromaticity and antiaromaticity is represented by the energetic criterion realizable within the framework of various schemes for calculating resonance energies. In most cases it correlates with structural and magnetic criteria moreover, it often accords well with a manifestation of numerous properties of compounds, which, being regarded as attributes of aromaticity, make its very concept substantially broader. Indeed, the concept of aromaticity claims an increasing number of types of compounds and requires a more and more sophisticated classification. 

One can predict that singlet 1-naphthylnitrene will cyclize more readily than phenylnitrene because the resonance energy per aromatic ring is lower in naphthalene than benzene, but by the same token, 1-naphthylnitrene should cyclize more slowly than vinylnitrene. 

To explore this possibility further, quantum chemical calculations were carried out for the isodesmic reactions of the model compounds 66 and 67 with dihydrogen to give the corresponding dihydrides (Scheme 17). These calculations showed that the reaction of 66 with H2 is about 14 kcalmol-1 less exothermic than that of 67. This difference may reflect aromatic resonance energy in the unsaturated molecule 66, reducing the enthalpy of the hydrogenation reaction. 

Ichikawa, H. and Ebisawa, Y., Hartree-Fock MO theoretical approach to aromaticity. Interpretation of Hiickel resonance energy in terms of kinetic energy of ti electrons, J. Am. Chem. Soc. 107, 1161-1165 (1985). 

Naphthalene undergoes electrophilic aromatic substitution at C-1 more easily than at C-2. There is a smaller loss of resonance energy in forming the intermediate for reaction at C-1 and reaction takes place more rapidly at this centre. However, the products of aromatic substitution at C-1 suffer interactions with C-8 (peri interactions) and are less stable than the corresponding products of substitution at C-2. Hence those aromatic substitution reactions that are carried out under conditions that allow equilibration between isomers (thermodynamic control) lead to substitution ai C-2, but reactions that are carried out under conditions... 

An essential part of the driving force of the elimination step is the recovery of the aromatic resonance energy in going from the cyclohexadienyl to the benzene system. 

When one estimates the heats of formation of aromatic nitrenes and carbenes from those of NH and CH2 with the aid of group additivity, the nitrenes will automatically become 10—15 kcal/mol more stable than the isomeric carbenes. These estimates are of necessity for the triplet species we do not know the resonance energies in the singlets. Nor do we know the singlet-triplet splittings, which may be different in carbenes and nitrenes. We have, therefore, also performed semi-... 

In our view, the explanation for this high reactivity is not to be ascribed to a lack of aromaticity these compounds have resonance energies in excess of that of benzene (Section IV,A). Rather, the reactivity is thought to be due to the ease with which this rc-excessive heteroaromatic system can undergo one-electron oxidation or 1,3-addition to generate another aromatic (benzenoid) system, as shown at structures 67 and 68, respectively. In terms of more familiar systems, the reactivity of the 1,3-positions of isoindole may be thought of as a compounding of the reactivity at the a-positions of pyrrole with that at the meso positions of anthracene. 

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