Aromatic n-electron systems

Reaction occurs in two steps addition of an electrophile to the aromatic n electron system followed by loss of a ring proton. 

Heteroatoms in five-membered 7i-excessive heteroaromatic compounds are responsible for the chemical behaviour of the molecules as a whole. The heteroatoms not only give electrons to form an aromatic n-electron system but also determine the direction of the attack of electrophilic or nucleophilic agents. In fused 7i-excessive heterocycles containing two or more heteroatoms, the reactivity of compounds and their physical properties are substantially affected by both the mutual arrangement of the heteroatoms and the electronic effects associated with their nature. 

Reactions That Lead to the Formation of an Extended Aromatic n-Electron System... 

In the literature discussing these results, the coincidence of the NN bond lengths in diazonium ions with that in dinitrogen seems always to be regarded with complete satisfaction. In the opinion of the present author this close coincidence is somewhat surprising, firstly because of the fact that in diazonium ions one of the nitrogen atoms is bonded to another atom in addition to the N(2) atom, and secondly because work on dual substituent parameter evaluations of dediazoniation rates of substituted benzenediazonium ions clearly demonstrates that the nx orbitals of the N(l) nitrogen atom overlap with the aromatic 7t-electron system (see Sec. 8.4). 

Fluorene (FI) is a polycyclic aromatic compound, which received its name due to strong violet fluorescence arising from its highly conjugated planar n-electron system (Chart 2.44). 

The absorption and emission of radiation in the near ultraviolet (UV) and visible regions of the electromagnetic spectrum are associated with electronic (and associated vibronic) transitions involving n- and/or n-electron systems of molecules. Synthetic and natural polymers absorb in the UV region and particularly strong absorption spectra are recorded for polymers containing aromatic and heteroaromatic groups (e.g., poly(styrenes), poly(vinyl naphthalenes), poly(vinyl carbazoles)). 

Benzene conforms to Hiickel s ruie, which predicts that planar cyclic polyenes containing 4 -I- 2 7t electrons show enhanced stability associated with aromaticity (see Section 2.9.3). Pyridine is also aromatic nitrogen contributes one electron in a orbital to the Jt electron system, and its lone pair is located in an sp orbital that is in the plane of the ring and perpendicular to the n electron system. It also conforms to HtickeTs rule, in that we still have an aromatic sextet of Jt electrons. 

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... 

For the purposes of this review it is convenient to focus attention on that class of molecules in which the valence electrons are easily distinguished from the core electrons (e.g., -n electron systems) and which have a large number of vibrational degrees of freedom. There have been several studies of the photoionization of aromatic molecules.206-209 In the earliest calculations either a free electron model, or a molecule-centered expansion in plane waves, or coulomb functions, has been used. Only the recent calculation by Johnson and Rice210 explicitly considered the interference effects which must accompany any process in a system with interatomic spacings and electron wavelength of comparable magnitude. The importance of atomic interference effects in the representation of molecular continuum states has been emphasized by Cohen and Fano,211 but, as far as we know, only the Johnson-Rice calculation incorporates this phenomenon in a detailed analysis. 

A linear correlation between 13C chemical shifts and local n electron densities has been reported for monocyclic (4n + 2) n electron systems such as benzene and nonbenzenoid aromatic ions [76] (Section 3.1.3, Fig. 3.2). In contrast to theoretical predictions (86.7 ppm per n electron [75]), the experimental slope is 160 ppm per it electron (Fig. 3.2), so that additional parameters such as o electron density and bond order have to be taken into account [381]. Another semiempirical approach based on perturbational MO theory predicts alkyl-induced 13C chemical shifts in aromatic hydrocarbons by means of a two-parameter equation parameters are the atom-atom polarizability nijt obtained from HMO calculations, and an empirically determined substituent constant [382]. 

Aromatic carbenium ions such as cyclopropenium and cycloheptatrienium ion (Fig. 3.2) [76, 495] are efficiently stabilized as the positive charge is delocalized within the (An + 2) n electron system. Carbon-13 shifts are smaller than 180 ppm. 

The fully delocalized n electron system of the benzene ring remains intact during electrophilic aromatic substitution reactions. However, in the Birch reduction, this is not the case. In the Birch reduction, benzene, in the presence of sodium metal in liquid ammonia and methyl alcohol, produces a nonconjugated diene system. This reaction provides a convenient method for making a wide variety of useful cyclic dienes. 

The mechanism for this reaction begins with the generation of a methyl carbocation from methylbromide. The carbocation then reacts with the n electron system of the benzene to form a nonaromatic carbocation that loses a proton to reestablish the aromaticity of the system. 

Conversion of the amino group into the acetamido group by acetylation modifies the interaction of the nitrogen lone pair with the n-electron system of the aromatic ring so that the ring is less powerfully activated towards electrophilic attack. 

For an assemblage of two identical molecules spaced d nm apart, the HOMO and LUMO energies split into four levels, each split by 2t eV apart ("dimer splitting") [26] here t is akin to the Hiickel69 resonance integral (i of Section 3.15 Indeed, chemists will remember Eq. (8.6.10) from the simple Hiickel molecular orbital theory for aromatic 7r-electron systems. As the number of molecules N increases, the energy levels become spaced more closely, until they form a quasi-continuous band of bandwidth W, where... 

Because of its n electron system and its polarizability, liquid S02 is a good solvent for aromatic hydrocarbons. Aliphatic compounds are less soluble in liquid S02, so it is possible to devise a solvent extraction process utilizing liquid S02 to separate aliphatic and aromatic hydrocarbons. 

Azulene is aromatic because it has a conjugated cyclic n electron system containing ten n electrons (a Hiickel number). 

Protonation of 4-pyrone gives structure A, which has resonance forms B, C, D, E and F. In E and F, a lone pair of electrons of the ring oxygen is delocalized into the ring to produce a six n electron system, which should be aromatic according to Hdckel s rule. 

The side chain nitrogen atom of AW-dimethyltryptamine is more basic than the ring nitrogen atom because its lone electron pair is more available for donation to a Lewis acid. The aromatic nitrogen electron lone pair is part of the ring n electron system. 

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