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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

Structure and numbering of selected PAHs

TABLE 1

Physical-chemical characteristics of some PAHs *
(From Neff, 1979; CCREM, 1987; NRCC, 1983; USPHS, 1990)

PAH

Mol.Wt.
(g)

Solubility
at 25 °C
(µg/L)

Vap. Pressure
at 25 °C
(mm Hg)

Log Kow
(Log Koc)

Carcino-
genicity

Benzene (and total)
rings

Naphthalene

128.2

12500 to 34000

1.8x 10-2

3.37

NC

2

Acenaphthylene

152.2

3420

10 -3 - 10-4

4.07 (3.40)

NC

2

Acenaphthene

154.2

   

3.98 (3.66)

NC

2

Fluorene

166.2

800

 

4.18 (3.86)

NC

2 (3)

             

Anthracene

178.2

59

2.4x 10-4

4.5 (4.15)

NC

3

Phenanthrene

178.2

435

6.8x 10-4

4.46 (4.15)

NC

3

Acridine

179.2

   

(4.48)

NC

3

2-Methylanthracene

192.3

21.3

 

4.77

NC

3

9-Methylphenanthrene

192.3

261

 

4.77

NC

3

1-Methylphenanthrene

192.3

269

 

4.77

NC

3

Fluoranthene

202.3

260

 

4.90 (4.58)

NC

3 (4)

9,10-Dimethylanthracene

206.3

56

 

5.13

NC

3

Benzo[a]fluorene

216.3

45

 

5.34

NC

3 (4)

Benzo[b]fluorene

216.3

29.6

 

5.34

NC

3 (4)

             

Pyrene

202.1

133

6.9x 10-7

4.88 (4.58)

NC

4

Benz[a]anthracene

228.3

11.0

1.1x 10-7

5.63 (5.30)

C

4

Naphthacene

228.3

1.0

 

5.65

NC

4

Chrysene

228.3

1.9

 

5.63 (5.30)

WC

4

Triphenylene

228.3

43

 

5.63

 

4

Benzo[b]fluoranthene

252.3

2.4

 

6.04 (5.74)

C

4 (5)

Benzo[j]fluoranthene

252.3

2.4

 

6.21

C

4 (5)

Cholanthrene

254.3

2.0

 

6.28

C

4 (5)

7,12-Dimethylbenz[a]anthracene

256.3

1.5

 

6.36

SC

4

Dibenzo[a,h]fluorene

266.3

0.8

 

6.57

WC

4 (5)

Dibenzo[a,g]fluorene

266.3

0.8

 

6.57

C

4 (5)

Dibenzo[a,c]fluorene

266.3

0.8

 

6.57

WC

4 (5)

3-Methylcholanthrene

267.3

0.7

 

6.64

SC

4 (5)

Benzo[ghi]fluoranthene

214.2

0.5

 

6.78

NC

4 (5)

             

Benzo[a]pyrene

252.3

3.8

5.5x 10-9

6.06 (5.74)

SC

5

Benzo[e]pyrene

252.3

2.4

5.5x 10-9

6.21

NC

5

Perylene

252.3

2.4

 

6.21

NC

5

Indeno(1,2,3-cd)pyrene

276.3

-

 

6.58 (6.20)

C

5(6)

Dibenz[a,h]anthracene

278.3

0.4

 

6.86 (6.52)

C

5

             

Benzo[ghi]perylene

276.4

0.3

1.0x 10-10

6.78 (6.20)

NC

6

             

Coronene

300.3

0.14

1.5x 10-11

7.36

NC

7

* 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

Selected rules to numbering PAHS and organic molecules

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.

TABLE 2

Production, uses, and producers of PAHs

PAH

Commercial name(s)

Uses

Producers*

Acenaphthene

1,2-Dihydronaphthalene,
1,8-Dihydronaphthalene,
1,8-Ethylenenaphthalene,
Periethylene naphthalene,

Chemical intermediary in pharmaceutical & photographic industries; to a limited extent in the production of soaps, pigments and dyes, insecticides, fungicides, plastics, and processing of certain foods

4 and 16

Acridine

2,3,5,6-Dibenzoquinoline,
9-Azanthracene,
Benzo(b)quinoline

Laboratory chemical (as a dye) & to a limited extent in pharmaceuticals

4, 16, and 18

Anthracene

Paranaphthalene,
Tetra olive N2G, Green oil

As a dye or chemical intermediary for dyes, diluent for wood preservatives

4 and 16

Fluorene

o-Biphenylenemethane, Diphenylenemethane

Unknown

4 and 11

Naphthalene

 

In the production of phthalic anhydride, carbaryl insecticide, beta-naphthol, tanning agents, moth repellent, and surfactants

1-3, 5-10, 17, 19, & 20

Pyrene

Benzo[d,e,f]phenanthrene,
Beta-pyrene

 

4 and 12

Quinoline

1-Azanaphthalene,
1-Benzazine, Chinoline,
Benzopyridine

In the preparation of hydroxyquinoline sulfate, niacin, some dyes; as a solvent for resins & terpines; decarboxylation agent

1, 3, 13, 14, and 15

 

* 1. A & C. American Chem. Ltd., Montreal, Que
2. A & K PetroChem Ind. Ltd., Concord, Ont
3. ACP Chemicals Inc., Saint-Leonard, Que
4. Aldrich Chem. Co., Inc., Milwaukee, WI
5. Allied Chem. Canada, Mississauga, Ont
6. Allied Signal Inc., Morristown, NJ
7. Anachemia Canada Ltd., Montreal, Que
8. Aristech Chem. Co. Inc., Pittsburgh, PA
9. Ashland Chem. Co. Inc., Columbus, OH
10. Carbochem Inc., Mississauga, Ont
11. Chemical Dynamics Co., Inc., Milwaukee, WI
12. Chemsyn Science Laboratories, Lenexa, KS
13. Crowley Chem. Co., New York, NY
14. General Intermediates of Canada, Edmonton, Alta
15 Howard Hall Int'l, Cos Cob, CT

16. Jonas Chem. Corp., Brooklyn, NY
17. Mary & Baker CDA Inc., Mississauga, Ont
18. Polyscience, Inc., Warrington, PA
19. Recochem Inc., Montreal, Que
20. Texaco Chemical Co., Houston, TX

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