Naphthalene, aromaticity orbitals

The aromaticity of naphthalene is explained by the orbital picture in Figure 15.12. Naphthalene has a cyclic, conjugated it electron system, with p orbital overlap both around the ten-carbon periphery of the molecule and across the central bond. Since ten 77 electrons is a Hiickel number, there is tt electron delocalization and consequent aromaticity in naphthalene. 

The requirements necessary for the occurrence of aromatic stabilisation, and character, in cyclic polyenes appear to be (a) that the molecule should be flat (to allow of cyclic overlap of p orbitals) and (b) that all the bonding orbitals should be completely filled. This latter condition is fulfilled in cyclic systems with 4n + 2n electrons (HuckeVs rule), and the arrangement that occurs by far the most commonly in aromatic compounds is when n = 1, i.e. that with 6n electrons. IO71 electrons (n = 2) are present in naphthalene [12, stabilisation energy, 255 kJ (61 kcal)mol-1], and I4n electrons (n = 3) in anthracene (13) and phenanthrene (14)—stabilisation energies, 352 and 380 kJ (84 and 91 kcal) mol- respectively ... 

Aromatic substitution reactions are often complicated and multistep processes. A correlation, however, in many cases can be found between the charged attacking species and the electron density distribution in the molecule attacked during electrophilic and nucleoph c substitution. No such correlation is expected in radical substitution where the attacking particles are neutral, rather a correlation between the reactivities of separate bonds and a free valency index of the bond order. This allows the prediction of the most reactive bonds. Such an approach has been used by researchers who applied quantum calculations to estimate the reactivities of the isomeric thienothiophenes and to compare them with thiophene or naphthalene. " Until recently quantum methods for studying reactivities of aromatics and heteroaromatics were developed mainly in the r-electron approximation (see, for example, Streitwieser and Zahradnik ). The M orbitals of a sulfur atom were shown not to contribute substantially to calculations of dipole moments, polarographic reduction potentials, spin-density distribution, ... 

It appears that the inhibition by both H2S and aromatic hydrocarbons involves competition between the inhibitor and the reactive sulfur compound for adsorption on the active site. However, inhibition could also be the result of occupancy of one or more of the bonding orbitals of the Co(Ni) by some nonreacting molecule, such as H2S or naphthalene. This would prevent the oxidative addition of the thiophene ring to the Co(Ni) in a mechanistic sequence such as that described in Figs. 

The electron spin resonance (ESR) spectra of the radical ions of 230 indicate there are no large deviations from the free-electron g value that would have been expected had the 3d orbitals of the sulfur atom played an important part in influencing the spin density of the molecule. Consequently, structure 230 may not be the main contributor to the electronic structure of the compound. Such stability in this compound could be attributed to the inertness of the NSN group and the presence of the aromatic naphthalene ring. However, the H-NMR chemical shifts (8 = 4.45 ppm) suggest the compound is antiaromatic. The compound is therefore referred to as an ambiguous aromatic compound (78JA1235). 

The cyclic peralkylsilane oligomers, (R2Si) with n = 4-6, manifested especially strong electron delocalization.5 These rings are structurally analogous to those of the cycloalkanes, since the silicon atoms form four sigma bonds. However, the electronic properties of the cyclosilanes more nearly resemble those of aromatic hydrocarbons such as benzene. One example of such behavior is their reduction to anion radicals. Aromatic hydrocarbons such as naphthalene can be reduced, electrolytically or with alkali metals, to deeply colored anion radicals in which an unpaired electron occupies the lowest unoccupied molecular orbital (LUMO) of the hydrocarbon (equation (2)). 

Parallel with this work went the development of quantum theory, with the valence bond and molecular orbital treatment of aromatic molecules. Theoretical predictions were verified by accurate bond length measurements and excellent agreement was finally obtained, particularly in the much studied naphthalene-anthracene series. At this stage it could be claimed that the structure of planar aromatic systems was well understood. 

The presence of overcrowding in triphenylene has been demonstrated by Clar (1950) from an examination of the absorption spectra at 18°C and — 170°C. At — 170°C the / -band spectra of such aromatic hydrocarbons as benzene, naphthalene, anthracene, and pyrene become more distinct, showing much more fine structure than at 18°C. This is explained by the cessation at low temperature of thermal collisions which produce molecular deformations, thereby improving the definition of the molecular electronic orbitals. Where this change in spectra does not occur, permanent deformation at both low and high temperatures... 

Clear theoretical evidence is presented to show that the u-electron systems of benzenoid aromatic molecules are described well in terms of localized, non-orthogonal, singly-occupied orbitals. The characteristic properties of molecules such as benzene or naphthalene arise from a profoundly quantum mechanical phenomenon, namely the mode of coupling of the electron spins, rather than from any supposed delocalization of the orbitals. Other systems considered include azobenzenes, such as pyridine, five-membered rings, such as furan, and inorganic heterocycles, such as borazine ( inorganic benzene ). 

The most vexed subject in this field is the site of radical attack on substituted aromatic rings. Some react cleanly where we should expect them to. Phenyl radicals add to naphthalene 7.36, to anthracene 7.37 and to thiophene 7.38, with the regioselectivity shown on the diagrams. In all three cases, the frontier orbitals are clearly in favour of this order of reactivity (because of the symmetry in these systems, both HOMO and LUMO have the same absolute values for the coefficients). 

Electron transfer from alkali metals to aromatics is very easy in suitable solvents [182]. The radical anions produced in this way do not dimerize. The formation of a covalent C—C bond would be accompanied by the loss of resonance stability in the aromatic system. Paul et. al. [188] have shown that the unpaired electron is placed in the lowest unoccupied orbital of the molecule, and that the stability of the radical anion increases in the order diphenyl < naphthalene < phenanthrene < anthracene. 

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