Aromatic compounds structural features

We saw in Chapter 12 that aromaticity reveals itself in various ways. Qualitatively, aromatic compounds are more stable and less reactive than alkenes. Quantitatively, their heats of hydrogenation are smaller than expected. Theory, especially Huckel s rule, furnishes a structural basis for aromaticity. Now let s examine some novel features of their NMR spectra. 

The most commonly employed routes for the preparation of the / -sulfatoethylsulfone group, which is the essential structural feature of vinylsulfone reactive dyes, are illustrated in Scheme 8.5. One method of synthesis involves, initially, the reduction of an aromatic sulfonyl chloride, for example with sodium sulfite, to the corresponding sulfinic acid. Subsequent condensation with either 2-chloroethanol or ethylene oxide gives the / -hydroxyethylsulfone, which is converted into its sulfate ester by treatment with concentrated sulfuric acid at 20 30 °C. An alternative route involves treatment of an aromatic thiol with 2-chloroethanol or ethylene oxide to give the /Miydroxyethylsulfonyl compound which may then be converted by oxidation into the /Miydroxyethylsulfone. 

Extensive molecular dissection of the morphine molecule over the past several decades led to a host of molecules which showed narcotic analgesic activity even though they possessed but faint suggestion of the structural features present in morphine itself. Thus, both cyclic molecules such as meperidine (70) and alphaprodine (71), and acyclic Compounds such as methadone (72) were found to be effective analgesics. Common features of these compounds were formalized by the Beckett-Casy rule, which states as minimal required structural features (a) an aromatic ring attached to... 

Compounds 1 and 2 were identified by FTIR and 13C-NMR. The 13C proton decoupled spectra for 1 and 2 are dominated by signals ranging from 62 to 195 ppm. The 13C chemical shift assignments were made based on comparisons with 4,4 -(hexafluoroisopropylidene)diphenol and from calculations based on substituted benzenes and naphthalenes.15 The 13C-NMR spectrum clearly showed that the Friedel-Crafts acylation of 1 by 4-fluorobenzoyl chloride yielded the 1,4-addition product exclusively. The 13C chemical shifts for 2 are listed in Table 8.1. The key structural features in the FTIR spectrum of2 include the following absorptions aromatic C-H, 3074 cnr1, ketone C=0, 1658 cm-1, aromatic ether Ar—0—Ar, 1245 cm-1, and C—F, 1175 cm-1. 

Aromaticity, defined as a structural feature, was used as a predictive tool for compounds that had not been prepared previously, whereas reactivity, bond length, or magnetic criteria had to await the isolation of a compound and its experimental investigation. Only recently has the development of quantum-chemical methods reached the point where one can predict with sufficient accuracy the magnetic properties, the bond lengths, and the reactivity patterns of aromatics. The multidimensional character and the definition and measurement of aromaticity generated confusion and conflicts.43 A recent review discussed the multidimensional character of aromaticity and theoretical and experimental approaches to aromatic structures and their predictions, and references are indicated extensively.66... 

A particularly interesting group of compounds in combustion effluents are those with a vinylic bridge such as acenaphthylene (peak 4) and cyclopenteno[cd]pyrene (peak 32). Peak 23, although not labeled, has been positively identified as acephenanthrylene, a compound which also has a vinylic bridge. We emphasize this structural feature because of its chemical reactivity (compared to the fully aromatic portions of the PAH). We shall see later that this reactivity is important when considering the fate of PAH in the atmosphere. 

The discovery of 1 -aryl-1 -dimethylamino-cyclohexanes resulted from a surrey of compounds in which aromatic and basic features, both critical structural requirements of opioid analgesics, but usually separated by two or three carbon atoms, are linked to the same quaternary carbon. The synthesis of these compounds yielded a series of highly potent opioids (e.g. 1-Benzyl-4-(4-bromo-phenyl)-4-dimethylamino-cyclohexanol), however none of them are in clinical use (Lednicer et al., 1981). 

Mass spectral data have frequently been used in the structural determination of boron heterocycles. One paper has been devoted to the mass spectra of some six-membered boron-nitrogen systems. It was concluded that the spectra could be interpreted analogously to their hydrocarbon counterparts. In all cases the molecular peak was the base peak of the spectrum (68T6755). Doubly charged molecular ions, a feature typical of aromatic compounds, are often encountered. It should be noted, however, that some certainly non-aromatic aminoboranes give such doubly charged ions as well. 

The GCM approach has been applied to the estimation of Tm for organic compounds containing functional groups with O, S, N, and halogen atoms [10], for rigid aromatic compounds [11], and for organic polymers with various possible substituents [12]. The latter method employs various corrections that account for special structural features in the polymer molecule. The first two methods are described below. 

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