Conjugated hydrocarbons, aromaticity

The Hiickel description of aromaticity was based in part on benzene, a cyclic fully conjugated hydrocarbon having (4n -l- 2) -electrons (ff = I) in the closed shell (ring). 

Aromaticity is usually described in MO terminology. Cyclic structures that have a particularly stable arrangement of occupied 7t molecular orbitals are called aromatic. A simple expression of the relationship between an MO description of stmcture and aromaticity is known as the Hiickel rule. It is derived from Huckel molecular orbital (HMO) theory and states that planar monocyclic completely conjugated hydrocarbons will be aromatic when the ring contains 4n + 2 n electrons. HMO calculations assign the n-orbital energies of the cyclic unsaturated systems of ring size 3-9 as shown in Fig. 9.1. (See Chapter 1, Section 1.4, p. 31, to review HMO theory.)... 

Saturated hydrocarbons are stable. Only cycloalkanes with a tight ring are unstable. Alkenes and alkynes have a strong endothermic character, especially the first homologues and polyunsaturated conjugated hydrocarbons. This is also true for aromatic compounds, but this thermodynamic approach does not show up their real stability very well. Apart from a few special cases, the decomposition of unsaturated hydrocarbons requires extreme conditions, which are only encountered in the chemical industry. 

Very little work has been done on reactions involving nucleophiles formed from hydrocarbons.124-142 The limitation on basicity of the carbanion, so that it does not react with solvent, has led to use of conjugated hydrocaibons, such as dienes or alkenes conjugated with aromatic rings. When initiated by dissolving alkali metal in liquid ammonia, complex mixtures are often produced on account of reduction processes,124 and regiochemistry and multiplicity of arylation in conjugated systems also create prob-... 

The it energy of a non-classical conjugated hydrocarbon can be compared directly with that of a classical analogue by the PMO method.14 Consider an even monocyclic polyene. This can be formed by fusion of methyl with an odd AH with one atom less. These components can also be fused to form an acyclic polyene. Comparison gives the aromatic energy of the cyclic system by difference. In this way we find that rings with An + 2 atoms are more stable, and those with An atoms less stable, than analogous acyclic compounds. The same method can be used for the bicyclic systems XVII, XIX, XXI, XXII, XXIII. The procedure is indicated below... 

There are numerous variations on the general mechanism outlined in Figure 7.10. Glutathione forms conjugates with a wide variety of xenobiotic species, including alkenes, alkyl epoxides (1,2-epoxyethylbenzene), arylepoxides (1,2-epoxynaphthalene), aromatic hydrocarbons, aromatic halides, alkyl halides (methyl iodide), and aromatic nitro compounds. The glutathione transferase enzymes required for the initial conjugation are widespread in the body. 

The reduction process of polycycles by lithium metal converts the neutral atoms to anions. The electron transfer is best achieved in ethereal solvents. This enables the stabilization of the lithium cation by coordination to the oxygen atoms of the solvent. The hydrocarbon anion and the cation are linked together by electrostatic forces in which the solvent molecules are also involved, therefore the ion-solvation equilibrium should be considered8. The limiting cases in this equilibrium are free ions and contact ion-pairs (CIP), and in between there are several forms of solvent separated ion-pairs (SSIP)9. In reality, anionic species of aromatic hydrocarbons in ethereal solvents exist between CIP and SSIP. Four major factors influence the ion-solvation equilibrium of lithium-reduced 7T-conjugated hydrocarbons, as observed by H and 7Li NMR spectroscopies8,10. 

There are several types of aromatic systems in addition to the ones described above. On example is azulene, a member of a class of nonalternant conjugated hydrocarbons.28 As a result of the different bridging pattern as compared to benzene, the n-energy levels are considerably shifted leading to light absorption in the visible region, and a purple color, as well as reduced n-electron stabilization. 

Jenks et al. studied the effects of conjugation and aromaticity on the sulfoxide bond by means of ab initio computation <1996JOC1275>. They calculated S-O bond dissociation energies (BDEs) and found that, in a formally aromatic system such as thiophene sulfoxide, the SO BDE is decreased by as much as 25kcalmoP relative to the BDE of DMSO. Although the BDE of the formally antiaromatic thiirene sulfoxide increased by about 15 kcal moP the authors concluded, on the basis of calculated geometries and isodesmic reactions with pure hydrocarbons, that cyclic unsaturated sulfoxides are neither significantly aromatic nor antiaromatic. 

The preparation of adducts of conjugated hydrocarbons, especially aromatic hydrocarbons, by reaction with alkali metals, M ... 

Randic, M. (1989). Aromaticity in Polycyclic Conjugated Hydrocarbons Dianions. J.Mol.Struct (Theochem), 185, 249-274. 

We have seen so far that MOs resulting from the LCAO approximation are delocalized among the various nuclei in the polyatomic molecule even for the so-called saturated a bonds. The effect of delocalization is even more important when looking to the n electron systems of conjugated and aromatic hydrocarbons, the systems for which the theory was originally developed by Hiickel (1930, 1931, 1932). In the following, we shall consider four typical systems with N n electrons, two linear hydrocarbon chains, the allyl radical (N = 3) and the butadiene molecule (N = 4), and two closed hydrocarbon chains (rings), cyclobutadiene (N = 4) and the benzene molecule (N = 6). The case of the ethylene molecule, considered as a two n electron system, will however be considered first since it is the reference basis for the n bond in the theory. 

Zamani-Khamiri, O., Hameka, H.F. Polarizability calculations with the SCF method. 1. Linear and dynamic polarizabilities of conjugated hydrocarbons and aromatics. J. Chem. Phys. 71, 1607-1610 (1979)... 

Randic, M. (2003a) Aromaticity of polycyclic conjugated hydrocarbons. Chem. Rev., 103, 3449-3605. 

Biphenylene and trisdehydro[12]annulene are representatives of conjugated hydrocarbons with a 4 -membered ring. The pattern of their absorption spectra is completely different from that of benzenoid aromatics. Their HOMO and LUMO are derived from the two NBMOs of an ideal perimeter and both the lowest excited singlet and triplet state can be described by the configuration A ho lu- The transition is symmetry forbidden in molecules of D2h symmetry or higher. Nonradiative decay usually dominates their photophysical properties. Quantum yields of fluorescence and intersystem crossing are low.307 The LCAO version of Platt s perimeter model has been extended to treat conjugated systems with AN jt-electrons derived from [ ]annulenes.308,309... 

Induced dipole-induced dipole and dipole-induced dipole interactions (van der Waals forces or dispersion forces) are the weakest of solvent-solute interactions and are predominant in solutions of nonpolar solutes in nonpolar or polar solvents and/or polar molecules in nonpolar solvents. These interactions, which are closely related to the polarizabilities of the solute and solvent, account for the small shifts to lower frequency of the absorption spectra of molecules upon going from the gas phase to solutions in nonpolar media. Presumably, the excited states of the nonpolar aromatic hydrocarbons and other conjugated hydrocarbons are more polarizable than the ground state, leading to stabilization of the excited states relative to the ground state and shifts of the absorption bands to the red. 

A second class of conjugated hydrocarbons that react with Mg metal is bi- and polynuclear aromatics. Anthracene, chrysene and pyrene react in THF (catalyzed by n-C4H9Cl and CH3I ) the Mg-anthracene adduct can be isolated ... 

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