2.0 PAHs and THEIR CHARACTERISTICS

The discussion in this chapter is mainly based on Neff (1979) and Handbook of Chemistry and Physics (Weast, 1968). Other sources of information, if any, are also referenced at appropriate places.

2.1 Characteristics

Polycyclic aromatic hydrocarbons (also known as polynuclear aromatic hydrocarbons) are composed of two or more aromatic (benzene) rings which are fused together when a pair of carbon atoms is shared between them (Figure 1). The resulting structure is a molecule where all carbon and hydrogen atoms lie in one plane. Naphthalene (C10H8; MW = 128.16 g), formed from two benzene rings fused together, has the lowest molecular weight of all PAHs. The environmentally significant PAHs are those molecules which contain two (e.g., naphthalene) to seven benzene rings (e.g., coronene with a chemical formula C24H12; MW = 300.36 g). In this range, there is a large number of PAHs which differ in number of aromatic rings, position at which aromatic rings are fused to one another, and number, chemistry, and position of substituents on the basic ring system.

Physical and chemical characteristics of PAHs vary with molecular weight (Table 1). For instance, PAH resistance to oxidation, reduction, and vapourization increases with increasing molecular weight, whereas the aqueous solubility of these compounds decreases. As a result, PAHs differ in their behaviour, distribution in the environment, and their effects on biological systems. PAHs can be divided into two groups based on their physical, chemical, and biological characteristics. The lower molecular weight PAHs (e.g., 2 to 3 ring group of PAHs such as naphthalenes, fluorenes, phenanthrenes, and anthracenes) have significant acute toxicity to aquatic organisms, whereas the high molecular weight PAHs, 4 to 7 ring (from chrysenes to coronenes) do not. However, several members of the high molecular weight PAHs have been known to be carcinogenic (Table 1).

Figure 1

* NC= non-carcinogenic; WC=weakly carcinogenic; C=carcinogenic; SC=strongly carcinogenic; Kow=Octanol/water partition coefficient; Koc= partitioning coefficient for organic carbon
2.2 Nomenclature

Several systems of nomenclature have been used to describe PAH ring structures. The most important rules of the system adopted by the International Union of Pure and Applied Chemistry (IUPAC) are outlined below and briefly illustrated in Figures 1 and 2.

1. The structure diagram of PAH is written such that the greatest possible number of rings is in a horizontal row.
2. Horizontal and vertical axes are drawn through the centre of a horizontal row, while orienting the molecule in such a way that maximal number of rings (those which are not lined up horizontally) are placed in the upper right quadrant and the minimal number of rings in the lower left quadrant.
3. Carbon atoms are numbered in a clockwise direction starting with the carbon atom that is not a part of another ring and is in the most counterclockwise position of the uppermost ring or, if there is a choice, of the uppermost ring farthest to the right. Carbon atoms common to two or more rings are not numbered.
4. Ring faces, which are not common to two rings, are lettered in alphabetical order with the side between carbon atoms 1 and 2 designated "a". Alphabetical order is continued clockwise around the molecule.
5. Compounds (or isomers) formed by the addition of a component are named with numbers and letters enclosed in brackets. These are placed immediately after the name of the added component to describe where a substituent group is attached or where a ring is fused to the face of the molecule. Appropriate letters are used where a ring is fused to more than one face of the molecule.
6. The structural formulas used show aromatic rings as plain hexagons and a methylene group as CH2.

A few PAHs (e.g., phenanthrene and anthracene) depart from these rules of nomenclature as seen in Figure 2. Further details on rules of PAH (and other organic molecules) nomenclature, and exceptions to them, can be found in the Handbook of Physics and Chemistry (Weast, 1968).

2.3 Production and uses

Among a large number of compounds in the category of polycyclic aromatic

Figure 2

hydrocarbons, only a few are manufactured in North America (Table 2). These PAHs are mostly used as intermediaries in pharmaceutical, photographic, and chemical industries. Naphthalenes are also used in the production of fungicides, insecticides, moth repellent, and surfactants. PAH-specific uses are also shown in Table 2.