Aromatic compounds electron delocalization

A compound that has a continuous ring of p orbitals, as in an aromatic compound, but delocalization of the pi electrons over the ring increases the electronic energy, (p. 722)... 

The authors conclude that [2.2.3]cyclazine (219) is intermediate in aromaticity between the polynuclear aromatic hydrocarbons and peripheral conjugated aromatic compounds involving delocalized 10 tt or 14 tc electrons. They also state that the resulting [2.2.3]cyclazines and benzo[2.2.3]-cyclazines are strongly dlatropic and claim that further theoretical study is required before the details of these novel conjugated systems can be fully understood. 

Arylamines contain two functional groups the amine group and the aromatic ring they are difunctional compounds The reactivity of the amine group is affected by its aryl substituent and the reactivity of the ring is affected by its amine substituent The same electron delocalization that reduces the basicity and the nucleophilicity of an arylamme nitrogen increases the electron density in the aromatic ring and makes arylamines extremely reactive toward electrophilic aromatic substitution... 

Aromatic compound (Section 113) An electron delocalized species that is much more stable than any structure wntten for It in which all the electrons are localized either in cova lent bonds or as unshared electron pairs... 

The stability of isothiazole derives from the fact that it has an aromatic delocalized ir-electron system. The NMR chemical shifts, which depend, inter alia, on ring currents, and the high stability of the molecular ions in mass spectrometry, are typical of aromatic compounds, and X-ray measurements confirm the partial double bond character of all the bonds of the ring. 

I Aromatic compounds have 4n + 2 tt electrons, which are delocalized over the ring. 

In contrast to the 1,4-dithiocin system, 1,4-dioxocin (1) is well-known and has been characterized as an olefinic compound by its spectra as well as its chemical behavior.5-6 The reason why 1,4-dioxocin in contrast to 1.4-dihydro-1.4-diazocine (see Section 1.4.) and 4//-l,4-oxazocinc (sec Section 1.12.), does not qualify as a 107r-aromatic species, is the less pronounced tendency of oxygen atoms for 7t-electron delocalization. An X-ray analysis of the 6-substituted 1,4-dioxocin 2 confirms the presumed nonplanar conformation of the 1,4-dioxocin structural element.9 The eight-membered ring exhibits a twisted boat-chair confirmation. 

The observed planarity and bond length equalization in 1,3,2-diazaphospholenium cations likewise suggest that these compounds have substantial n-electron delocalization and possess possibly aromatic character. Several studies were undertaken to quantify the degree of n-delocalization by computational calculations using the interpretation of population analyses, ELF calculations, evaluation of magnetic criteria [nucleus independent chemical shift (NICS) values], and the... 

Generalized valence bond interaction energies were computed for mono/poly-nitrogenous five- and six-membered heterocycles.203 Results that diverged from those obtained by other methods were obtained only for poly-nitrogenous systems such as pyridazine, benzotriazole, and tetrazole, which may confirm Bird s earlier finding123 204 that electron delocalization is not a stand-alone and direct measure of aromaticity for nitrogenous heterocyclic compounds. 

Carbon and nitrogen are the most common elements from the first row of the periodic table to form aromatic compounds, characterized by cyclic electron delocalization. The bonding of these elements in the conjugated systems shows a large variety. Carbon can be a divalent (carbene), sp carbon with one jT-electron, but also sp carbon can be part of hyperconjugate aromatic systems, provided that it is properly substituted. The pyrrole- and pyridine-type nitrogens also allow the formation of cyclic electron delocalization in a large variety of aromatic systems. 

The reason for this behavior in the case of the double-bonded structures ( A -type phosphorus) is the easy dimerization of the P=C bond. Tricoordinate planar (or nearly planar) phosphorus ( B - and C -type bonding) can be stabilized by repyramidalization when the cyclic electron delocalization is disturbed or lost (e.g., in a chemical reaction). The fine balance between these energetic effects cannot easily be predicted by using analogies or other simple models. Such predictions, however, can be made by using the sophisticated methods of computational chemistry, leaving the field of the chemistry of the aromatic phosphorus compounds an interesting research area with unexpected results in the future. 

Resonance-stabilized systems include car-boxylate groups, as in formate aliphatic hydrocarbons with conjugated double bonds, such as 1,3-butadiene and the systems known as aromatic ring systems. The best-known aromatic compound is benzene, which has six delocalized k electrons in its ring. Extended resonance systems with 10 or more 71 electrons absorb light within the visible spectrum and are therefore colored. This group includes the aliphatic carotenoids (see p.l32), for example, as well as the heme group, in which 18 k electrons occupy an extended molecular orbital (see p. 106). 

Spectroscopy provides many clues to the identity of a compound. Aromatic compounds, because of the delocalization of the electrons, have unique features in their spectra. In fact, spectral evidence can indicate what atoms or functional groups are attached to the aromatic ring or whether the ring itself contains an atom other than carbon. 

A The answer to this question is based upon assuming at first the ring to be planar, then counting the number of all the electrons that could contribute to a delocalized TU-sysiem. For planar aromatic compounds the number should conform to 4 i +2. If it does not then the ring is either non-planar or anti-aromatic ... 

The definition of aromaticity conceived by Hiickel strictly applies to monocyclic ring systems, but indole, constructed from the fusion of benzene and pyrrole, behaves as an aromatic compound, like quinoline and isoquinoline. The ring fusion, however, affects the properties of both components. This is reflected in the valence bond description of indole, shown in Scheme 7.1, where one canonical representation shows electron density shared between N-1 and C-3 in the pyrrole unit (implying enamine character). Note that although other canonical forms can be drawn, where the lone-pair electrons are delocalized into the benzenoid ring, their energy content is relatively high and they are of limited importance. 

The term charge tranter refers to a succession of interactions between two molecules, ranging from very weak donor-acceptor dipolar interactions to interactions that result in the formation of an ion pair, depending on the extent of electron delocalization. Charge transfer (CT) complexes are formed between electron-rich donor molecules and electron-deficient acceptors. Typically, donor molecules are p-electron-rich heterocycles (e.g., furan, pyrrole, thiophene), aromatics with electron-donating substiments, or compounds... 

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