Aromatic planar conjugated system

Aromatic compounds are planar, conjugated systems which have 4n + 2 electrons (Huckel s rule) (see Section 7.1). If, on deprotonation, the anion is part of an aromatic Tt-system, then the negative charge will be stabilised. Aromaticity will therefore increase the acidity of the compound. 

In considering its aromaticity, if one excludes the double bonds between C3 and Ci, and C7 and Ce, one may envision a closed, cyclic, planar, conjugated system which contains 18 it electrons. This agrees with Huckel s Rule for the determination of aromaticity. Electron delocalization over the porphyrin ring system can be depicted by the following resonance structures ... 

The H- C 2-D HMBC spectra of aromatic and conjugated compounds often allow us to distinguish between C signals that are cis versus trans to a H three bonds away Because the entire network of atoms in an aromatic or conjugated system is often planar (at least locally, barring steric constraints), there are only two vicinal bond angles from which to choose (0° and 180°). Thus, there are only two values likely to be encountered between a H on an sp -hybridized carbon and a C three bonds away the cis- I is smaller (5-6 Hz) than the trans- ] (8-10 Hz). Consequendy, the cis- J generates a weaker HMBC cross peak than the trans- ] does. 

According to Huckel s rule, monocyclic, planar, conjugated systems containing (4n + 2)n electrons are associated with extra stability and is known as aromatic stability. 

The concept of aromaticity is not restricted only to planar conjugated systems. The An + 2 interstitial electron rule extends the concept of aromaticity to three-dimensional delocalized systems 452a systems jjave marked thermodynamic stability and... 

Indole is planar with 10 TT-electrons in a completely conjugated system. The ring is classified as a TT-excessive he tero aromatic compound because of the electron-donating character of the pyrrole-type nitrogen atom. The TT-system is relatively electron-rich, particularly at C-3, as represented by resonance stmcture (lb). 

The deviation from planarity that is present in a structure such as 1 raises the question of how severely a conjugated system can be distorted from the ideal coplanar alignment of p orbitals and still retain aromaticity. This problem has been analyzed by determining the degree of rehybridization necessary to maximize p orbital overlap in 1. It is found that rehybridization to incorporate fractional amounts of s character can improve orbital alignment substantially. Orbitals with about 6% s character are suggested to be involved... 

Thus, the stabilization of the 7r-orbital due to the inclusion of an X=Y fragment (X,Y = C,Si,Ge,Sn) into the conjugated (aromatic) system will lead to an increase in the difference between the energies of the tt- and cr -orbitals and, ultimately, the planar structure will be stable to the pyra-midalization of the Si,Ge,Sn-centered bonds. Thus, the inclusion of the double bond X=Y (X,Y = C, Si,Ge,Sn) into a conjugated system affording an aromatic molecule is one of the ways of stabilizing this bonding, particularly, with respect to the pyramidalization of these centers. 

Eight-membered rings with two O, S or N atoms or combinations of these heteroatoms in a 1,2- or 1,4-relationship and three double bonds possess conjugated tt-electron frameworks and can be designated as dihetera[8]annulenes (78AHC(23)55). These 1,2- or 1,4-diheterocins are isoelectronic with the cyclooctatetraene dianion, and if planar represent potentially aromatic IOtt-electron systems. Considerable interest has attached to the degree of aromaticity of these compounds from both theoretical and experimental standpoints. Most theoretical treatments have led to the conclusion that 1,2-diheterocins, because of... 

This widely used criterion is manifested in (a) bond length equalization due to cyclic delocalization and (b) planarity of the cyclic systems13-16. The planarity of cyclic conjugated systems is regarded as a manifestation of aromaticity, while the non-planarity of cyclic conjugated systems is often taken as an indication of antiaromaticity. 

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