Nonbenzenoid aromatic hydrocarbons

Azulene. The absorption spectrum of azulene, a nonbenzenoid aromatic hydrocarbon with odd-membered rings, can be considered as two distinct spectra, the visible absorption due to the 1Lb band (0-0 band near 700 nm) and the ultraviolet absorption of the 1L0 band.29 This latter band is very similar to the long wavelength bands of benzene and naphthalene CLb) and shows the same 130 cm-1 blue shift when adsorbed on silica gel from cyclohexane.7 As in the case of benzene and naphthalene, this blue shift is due to the fact that the red shift, relative to the vapor spectra, is smaller (305 cm"1) for the adsorbed molecule than in cyclohexane solution (435 cm"1). Thus it would appear that the red shifts of the 1La band are solely due to dispersive forces interacting with the aromatic molecule, in agreement with Weigang s prediction,29 and dipole-dipole interaction is negligible. 

Andersen Jr, A. G. Masada, G. M. 1974 Polarographic reduction potentials of some nonbenzenoid aromatic hydrocarbons. J. org. Chem. 39, 572-573. 

Spectra of nonbenzenoid aromatic hydrocarbons show considerable resemblance to spectra of benzenoid compounds. Tropolone and its derivatives show absorption in the region 220 - 250 nm ( ca 30,000) and 340 - 375 nm ca 8,000) the latter absorption is characterized by the group of fine structure bands typical of aromatic systems. 

This mode of cyclization opened a new access74 to the stable, tetracyclic peri-condensed, nonbenzenoid aromatic hydrocarbon dicyclopenta(e,f—k,l)heptalene (.159), also named73 azupyrene. The 4,5-cyclopentenoazulene (156) needed was formed in 28% yield by a Hafner synthesis from the pentamethinium salt (155)75 and sodium cyclopentadienide ... 

The same group138 has applied the method to fluorobenzenes, and Bertelli and Golino139 have independently used solvent shift criteria for an examination of jr-electron delocalization in various nonbenzenoid aromatic hydrocarbons. 

Another example of a nonbenzenoid aromatic hydrocarbon is the compound azulene. Azulene has a resonance energy of 205 kj moP There is substantial separation of charge between the rings in azulene, as is indicated by the electrostatic potential map for azulene shown in Fig. 14.18. Factors related to aromaticity account for this property of azulene (see Practice Problem 14.12). 

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]. 

Part of the folklore of nonbenzenoid hydrocarbons suggests fulvenes are on the nonaro-matic/aromatic border. It is thus not obvious whether these species really belong in this chapter. Yet, because their aromaticity is so much less than that found for their isomeric benzenoid derivatives we feel confident to proceed. Other than the parent hydrocarbon species 103 [i.e. 127 wherein (R, R ) = (H, H) most of the other thermochemically char-acterized fulvenes have substimtion on the exomethylene carbon cf (R R ) = (H, Me), (Me, Me) and (Ph, Ph) for reference, the suggested enthalpies of formation of the (H, H), (H, Me), (Me, Me) and (Ph, Ph) species are 224, 185, 144 and 402 kJmor respectively. Were all differences in steric interactions and contributions from the dipolar resonance structures of the generic type 128 negligible, then AH (127, R R ) and A//f(CH2=CR R ) would be linearly related. We find that a nearly perfect straight line... 

While benzenoid hydrocarbons are the subject of long-standing studies (many of their derivatives are applied in all fields of human activities), nonbenzenoid hydrocarbons are a subject of rather moderate interest. Undoubtedly, some of these systems have substantial applications and create a significant theoretical interest. Hence, many of them have been subject to thorough structural studies. Their molecular geometries may serve as a good source of information to study their aromaticity. 

The first group of alternant nonbenzenoid hydrocarbons are annulenes. Except for benzene and cyclobutadiene, all others are nonplanar. Table 19 presents the aromaticity indices for some annulenes and their derivatives calculated either from the experimental or ab initio calculated geometries. 

Additional aromatic compounds that, however, are not benzenoid (i.e., lack the fundamental six-carbon hexagonal structure characteristic of benzene and its derivatives ) are also known. The bright blue crystalline, planar, 4n -e 2 Ti-electron nonbenzenoid hydrocarbon azulene (CioHg), while not enjoying the same diminished reactivity characteristic of its benzenoid isomer naphthalene (CioHg), nonetheless clearly enjoys a similar ring current (Figure 6.31). 

Azulene is one of the few completely conjugated nonbenzenoid hydrocarbons that appears to have appreciable aromatic stabilization. There is some divergence on this point between the SCF-MO and HMO treatments. The latter estimates a resonance energy about half that for the isomeric naphthalene, whereas the SCF-MO method assigns a resonance energy which is only about one seventh that of naphthalene. The parent hydrocarbon and many of its derivatives have been well characterized and are stable compounds. The structure of azulene itself has been... 

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