Polynuclear Aromatic Hydrocarbons and Heterocyclic Compounds

Researchers have observed that the primary and secondary bands in the spectra of polynuclear aromatic hydrocarbons shift to longer wavelength. In fact, even the second primary band, which appears at 184 nm for benzene, is shifted to a wavelength within the range of most UV spectrophotometers. This band lies at 220 nm in the spectrum of naphthalene. As the extent of conjugation increases, the magnitude of the bathochromic shift also increases. 

The ultraviolet spectra of the polynuclear aromatic hydrocarbons possess characteristic shapes and fine structure. In the study of spectra of substituted polynuclear aromatic derivatives, it is common practice to compare them with the spectrum of the unsubstimted hydrocarbon. The nature of the chromophore can be identified on the basis of similarity of peak shapes and fine structure. This technique involves the use of model compounds. Section 10.15 will discuss it fiuther. 

FIGURE 10.19 Ultraviolet spectra of naphthalene and anthracene. (From Friedel, R. A., and M. Orchin, Ultraviolet Spectra of Aromatic Compounds, John Wiley and Sons, New York, 1951. Reprinted by permission.) 

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Heterocyclic molecules have electronic transitions that include combinations of n — 7t and transitions. The spectra can be rather complex, and analysis of the transitions involved will be left to more advanced treatments. The common method of studying derivatives of heterocyclic molecules is to compare them to the spectra of the parent heterocyclic systems. Section 10.15 will further describe the use of model compounds in this fashion. 

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In terms of composition, the compoimds positively identified as pitch components consist predominantly of condensed polynuclear aromatic hydrocarbons or heterocyclic compounds containing from three to six rings. Some methyl and hydroxyl substituent groups have also been observed. 

Means, J.C., Hassett, J.J., Wood, S.G., Banwart, W.L., Ali, S., Khan, A. (1980) Sorption properties of polynuclear aromatic hydrocarbons and sediments Heterocyclic and substituted compounds. In Polynuclear Hydrocarbons Chemistry and Biological Effects. Bjorseth, A., Dennis, A.J., Editors, pp. 395 404, Ann Arbor Sci. Publishers, Ann Arbor, MI. 

Recognize fused aromatic systems such as polynuclear aromatic hydrocarbons and fused heterocyclic compounds, and use the theory of aromatic compounds to explain their properties. 

Binuclear aromatic hydrocarbons are found in heavier fractions than naphtha. Trinuclear and polynuclear aromatic hydrocarbons, in combination with heterocyclic compounds, are major constituents of heavy crudes and crude residues. Asphaltenes are a complex mixture of aromatic and heterocyclic compounds. The nature and structure of some of these compounds have been investigated. The following are representative examples of some aromatic compounds found in crude oils ... 

Sawicki, E., S. P. McPherson, T. W. Stanley, J. Meeker, and W. C. Elbert. Quantitative composition of the urban atmosphere in terms of polynuclear aza heterocyclic compounds and aliphatic and polynuclear aromatic hydrocarbons. Int. J. Air Water Pollut. 9 515-524, 1%5. 

Crude and refined oils are known to contain the following aromatic compounds aromatic and polynuclear aromatic hydrocarbons (PAHs), phenols and cresols, heterocyclics (such as pyridine, quinoline), benzoic acid, esters and ethers. Many are quite water-soluble (Table IV) and are expected to be found in the dissolved fraction. 

Polycyclic organic matter, derived from the total exhaust emission, is an extremely complex mixture. It includes a large number of compounds such as polynuclear aromatic hydrocarbons (PAH), derivations of PAH such as nitro-PAH and amino-PAH, oxygenated PAH such as phenols and quinones, and heterocyclic aromatic compounds containing sulfur and oxygen. In order to assist in the identification of classes of toxic compounds it is possible to fractionate the exhaust emissions into vapor and... 

As an illustration, crude benzene extracts of test materials (e.g., carbonaceous shale samples) can be preseparated by TLC into neutral polynuclear aromatic hydrocarbons (PAHs) and polar compounds (e.g., heterocyclic nitrogen compounds, aza-arenes, phenols) and other nonpolar hydrocarbons. This type of separation can be accomplished with a silica-gel plate in a onedimensional run with, for example, benzene-cyclohexane. Plates are air-dried and observed under UV light. Saturated hydrocarbons run with the solvent to... 

Spectra-structure correlation charts showing the probable positions of the characteristic absorption frequencies of some 1000 aliphatic, aromatic, and heterocyclic compounds [ ], together with various classes of aliphatic and organometallic derivatives in the 15-to 35-/1 region, are shown in Figs. 3 and 4. Some of the important classes of compounds studied and summarized in these correlation charts are alkanes, alkenes, cyclopropanes, cyclopentanes, cyclohexanes, substituted benzenes, polynuclear hydrocarbons, heterocyclics. 

A wide assortment of electron transfer photosensitizers for diaryiiodonium salts have been described in the journal and patent literature. Among these are polynuclear aromatic hydrocarbons with three or more rings such as anthracene, alkoxyanthracenes, pyrene, and perylene heterocyclic compounds of low basicity such as carbazoles " and phenothiazines aromatic ketones such as benzophenone, Michler s ketone, and thioxanthone and substituted thiox-anthones coumarins phenanthrene-9,10-quinone Mannich bases and (dimethylamino)benzylidyne compounds. In addition, the use of dyes such as eosine, acridine orange, acridine red, and benzoflavin has been employed to provide photosensitization in the visible region of the spectrum. 

Between 1961 and 1967 the electrochemical generation method has been used to obtain, identify, and investigate free radicals derived from about 400 different organic compounds, such as unsaturated acyclic and alicyclic hydrocarbons, condensed and noncondensed polynuclear aromatic hydrocarbons, heterocyclic compounds, quinones, carbonyl compounds, nitriles, nitroso and nitro derivatives, and carboxylic esters. 

Aromatic compounds are unsaturated cyclic molecules that possess additional stability as a result of the arrangement of 7i-electrons associated with the unsaturation of the ring system. This book will concentrate on the chemistry of benzene (4) and its derivatives and related polynuclear hydrocarbons. Aromatic compounds are also known as arenes they can becarbocydic, indicating that the ring skeleton contains only carbon atoms, or heterocyclic, with at least one atom other than carbon in the ring. These heteroatoms are typically N, O or S. Heterocyclic compounds, which can be aromatic or alicyclic, are covered in another book in this series. 

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