Overview of Polycyclic Aromatic Compounds (PAC)

Abstract

The chemical group of polycyclic aromatic compounds (PAC), including the better-known subgroup of polycyclic aromatic hydrocarbons (PAH) and the heterocyclic aromatic compounds (NSO-PAC, heterocycles), comprise several thousand individual compounds. It is hard to find a comprehensive overview in the literature of these PACs that includes a substantial amount of relevant properties. Here an attempt is made to summarize the most studied but also some less well-known PACs. In addition to basic data such as recommended names, abbreviations, CAS numbers, molecular formulas, chemical structures, and exact mono-isotopic molecular weights, physico-chemical properties taken from the literature like boiling points, vapor pressures, water solubilities, Henry's Law constants, n-octanol-water partition coefficients (log K_OW), and pK_a are summarized. Selected toxicological data are listed indicating carcinogenic and mutagenic activity or effects on different organisms. PAC nomenclature is a complex topic, so suggestions for practical use are made. Regarding available data, estimated (instead of measured) values should be used with caution because considerable deviations from experimentally determined values can occur. For an enhanced understanding of the behavior of single PACs in comparison with each other, some of the properties mentioned above are plotted vs. the number of rings or the degree of alkylation. Also, some physico-chemical data are correlated with different functional groups as substituents of the PAHs. This article reveals that rather little is known about the less common PACs, e.g., higher molecular weight compounds, alkylated or otherwise substituted aromatics, for instance, keto-, oxo-, amino-, nitro-, cyano-PAHs, or some heterocyclic aromatic compounds, including their derivatives. It mirrors the limited state of knowledge about the variety of PACs that do not belong to the 16 EPA PAHs.

Keywords: environmental occurrence; polycyclic aromatic compounds; polycyclic aromatic hydrocarbons; properties of polycyclic aromatic compounds; toxicity.

Figure 1. Comparison of experimental (EXP) and estimated (EST) data using the EPI suite (9) for naphthalene, pyrene, benzo[a]pyrene, coronene, and 2-aminonaphthalene; estimated values should be used with care.

Figure 2. Boiling points of PAHs (including alkylated PAHs) vs. number of rings.

Figure 3. Vapor pressures of PAHs (including alkylated PAHs) vs. number of rings.

Figure 4. Water solubilities of PAHs (including alkylated PAHs) vs. number of rings.

Figure 5. Log K_OW of PAHs (including alkylated PAHs) vs. number of rings.

Figure 6. Physico-chemical data for naphthalenes with increasing degree of alkylation: boiling point and log K_OW (top) and vapor pressure, water solubility, and Henry's Law constant (bottom); naphthalene (Nap), 1-methylnaphthalene (1-MNap), 1,3-dimethylnaphthalene (1,3-DMNap), 1,2,6-trimethylnaphthalene (1,2,6-TrMNap), 2,3,6,7-tetramethylnaphthalene (2,3,6,7-TeMNap).

Figure 7. Log K_OW data for naphthalenes with different functional groups, and quinolones.