PAHs pyrolytic

A number of papers report investigations of the pyrolytic cleavage of aromatic hydrocarbons. The oxidation and pyrolysis of anisole at 1000 K have revealed first-order decay in oxygen exclusively via homolysis of the O—CH3 bond to afford phenol, cresols, methylcyclopentadiene, and CO as the major products.256 A study of PAH radical anion salts revealed that CH4 and H2 are evolved from carbene formation and anionic polymerization of the radical species, respectively.257 Pyrolysis of allylpropar-gyltosylamine was studied at temperatures of 460-500 °C and pressures of 10-16 Torr. The product mixture was dominated by hydrocarbon fragments but also contained SO2 from a proposed thermolysis of an intermediate aldimine by radical processes.258... 

Most PAHs enter into the environment by atmospheric emissions. They may therefore be atmospherically transported over long distances, especially when they are bound to aerosols. In this way, they have become ubiquitous contaminants reaching remote areas [29]. There is a direct relationship between pyrolytic PAH... 

Fernandez P, Vilanova RM, Martinez C, Appleby P, Grimalt JO (2000) The historical record of atmospheric pyrolytic pollution over Europe registered in the sedimentary PAH from remote mountain lakes. Environ Sci Technol 34 1609-1913... 

PAHs are released to the environment from a number of sources pyrogenic sources including fossil fuel combustion and pyrolytic processes of organic matter such as incineration and petrogenic sources such as oils spills. Direct oil spilled from stationary sources and accidents cause contamination of land. Oil spills at US Army bases in South Korea is reported as a source of soil and groundwater contamination to nearby area... 

Therefore, Ph/An> 15 may point to petrogenic sources and Ph/An< 10 to pyrolytic sources. Due to the wide range of values for this index found in the literature, values between 10 and 15 are considered indeterminate relative to source. In petroleum-derived PAHs, pyrene is more abundant than fluoranthene, while at higher combustion temperatures a predominance of fluoranthene over pyrene is characteristic. So the Fl/Pyr values greater than 1 are obviously related to pyrolytic sources, whereas values less than 1 are attributed to petrogenic sources. In order to avoid erroneous conclusions, the two ratios are often combined. When Ph/An> 15 and Fl/Pyr < 1, the PAH input is mainly from crude oil sources and when Ph/An< 10 and Fl/Pyr > 1, the major input may be related to combustion (Wang et ah, 2004b). 

Sources of PAHs can be distinguished by using select ratios. For example, ratios of total methylphenanthrenes to phenanthrene is used to identify PAHs as pyrolytic or petrogenic sources. Other work has resorted to the application of compound-specific isotope analysis (CSIA), to constrain better anthropogenic sources of PAHs. 

PAFIs and their alkylated derivatives are also used to distinguish between pyrolytic and petrogenic inputs to sediments and soils and are often used in conjunction with aliphatic indices to discriminate hydrocarbon sources. The lower molecular weight 2- to 4-ringed PAHs and their alkylated versions comprise a small, but significant, component of crude oils and liquid fuels. Ratios of parent PAHs (non-alkylated) as well as parent to alkylated versions are frequently used to distinguish... 

The mechanisms by which pyrolytic production of PAHs occur are complex, and have been widely studied since the 1950s (Badger et al., 1958 Howsam and Jones, 1998). Pyrolytic production of PAHs is generally believed to occur through a free... 

A further important reaction class refers to the substitutive addition reactions which were already discussed in the pyrolytic mechanism of deposit formation. During PAH formation and growth, there are alkyl substitutions (mainly methyl) on the edge surface too. Active radicals add on the aromatic structures with a simultaneous dealkylation. This results in an increase in the molecular weight and dehydrogenation of the aromatics. 

Fig. 7.3 Major pyrolytic polycyclic aromatic hydrocarbons (PAH) found in sediments (abbreviations refer to Fig. 7.4).

Fig. 7.4 Major pyrolytic polycyclic aromatic hydrocarbon (PAH) distributions in Recent and ancient sediments, normalized to pyrene abundance (from molecular ion responses during GCMS analysis see Box 4.3 after Killops Massoud 1992). See Fig. 7.3 for key to names and structures.

It appears that a recognizable pyrolytic PAH fingerprint can survive over geological time-scales. Wild fires, primarily initiated by lightning strikes, are the most likely source of these PAHs in ancient sediments. Such fires have probably been a feature of terrestrial ecosystems from at least the Late Devonian (see Section 1.4.2), as suggested by the occurrence in sediments of fusinites and semifusinites (see Section 4.3.1b), the proposed products of vegetation fires (Chaloner 1989). Observed PAH distributions in ancient sediments are often like those seen in Recent sediments and are consistent with... 

Pyrolytic PAH distributions in ancient sediments sometimes differ from those typical of Recent sediments in exhibiting enhanced levels of the more highly peri-condensed structures, especially benzofe]pyrene, benzo[gfe]perylene and coronene (Fig. 7.4 Killops Massoud 1992). The reasons are as yet unknown but may reflect the effects of either different formation conditions or varying geochemical processes over geological time periods. 

It can be seen that there are natural inputs of pyrolytic PAHs to the environment as there are for atmospheric carbon dioxide. However, unlike C02, anthropogenic PAH contributions in contemporary sediments exceed natural inputs. This increase in environmental burden may, therefore, be significant in its effects on organisms. 

Aromatic compounds have not only been of academic interest ever since organic chemistry became a scientific discipline in the first half of the nineteenth century but they are also important products in numerous hydrocarbon technologies, e.g. the catalytic hydrocracking of petroleum to produce gasoline, pyrolytic processes used in the formation of lower olefins and soot or the carbonization of coal in coke production [1]. The structures of benzene and polycyclic aromatic hydrocarbons (PAHs) can be found in many industrial products such as polymers [2], specialized dyes and luminescence materials [3], liquid crystals and other mesogenic materials [4]. Furthermore, the intrinsic (electronic) properties of aromatic compounds promoted their use in the design of organic conductors [5], solar cells [6],photo- and electroluminescent devices [3,7], optically active polymers [8], non-linear optical (NLO) materials [9], and in many other fields of research. 

Beside the practical importance of aromatic compounds, there has always been an interest in more or less theoretical problems like the scope, limitation and effects of electron delocalization in aromatic compounds (the aromaticity problem ). These investigations were strongly encouraged by the discovery of fullerene formation in a carbon plasma [18], in fuel-rich flames [24] or by the pyrolytic transformation of PAHs [25] together with a variety of the as yet potential application of these aromatic carbon cage compounds [18]. New selective C-C bond formation reactions as well as mechanisms of the rearrangement in carbon skeletons have been studied. 

Other masked ethynyl groups including dihaloethenyls [54b],ethenyl ethers [58] and terminally substituted acetylenes [59] have also been applied in the pyrolytic preparation of PAHs, presumably also via carbene intermediates [60]. Furthermore, the combination of the different synthetic strategies is also possible, as demonstrated by the first successful synthesis of benzocorannulene (28, Scheme 10, [54 g]) and other PAHs. 

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