McKay, Donald. "Front matter" Multimedia Environmental Models ...

3.3.2 Chemical Classes (see Fig. 3.1 for structures and Table 3.5 for
properties)
3.3.2.1 Hydrocarbons
Hydrocarbons are naturally occurring chemicals present in crude oil and natural
gas. Some are formed by biogenic processes in vegetation, but most contamination
comes from oil spills, effluents from petroleum and petrochemical refineries, and
the use of fuels for transportation purposes.
The alkanes can be separated into classes of normal, branched (or iso) species
and cyclic alkanes, which range in molar mass from methane or natural gas to waxes.
They are usually sparingly soluble in water. For example, hexane has a solubility
of approximately 10 g/m 3 . This solubility falls by a factor of about 3 or 4 for every
carbon added. The branched and cyclic alkanes tend to be more soluble in water,
apparently because they have smaller molecular areas and volumes.
Highly branched or cyclic alkanes such as terpenes are produced by vegetation.
They are often sweet smelling and tend to be very resistant to biodegradation.
The alkenes or olefins are not naturally occurring to any significant extent. They
are mainly used as petrochemical intermediates. The alkynes, of which ethyne or
acetylene is the first member, are also chemical intermediates that are rarely found
in the environment. These unsaturated hydrocarbons tend to be fairly reactive and
short-lived in the environment, whereas the alkanes are more stable and persistent.
Of particular environmental interest are the aromatics, the simplest of which is
benzene. The aromatics are relatively soluble in water, for example, benzene has a
solubility of 1780 g/m 3 . They are regarded as fairly toxic and often troublesome
compounds. A variety of substituted aromatics can be obtained by substituting
various alkyl groups. For example, methyl benzene is toluene.
When two benzene rings are fused, the result is naphthalene, which is also a
chemical of considerable environmental interest. Subsequent fusing of benzene rings
to naphthalene leads to a variety of chemicals referred to as the polycyclic aromatic
hydrocarbons or polynuclear aromatic hydrocarbons (PAHs). These compounds tend
to be formed when a fuel is burned with insufficient oxygen. They are thus present
in exhaust from engines and are of interest because many are carcinogenic.
Biphenyl is a hydrocarbon that is not of much importance as such, but it forms
an interesting series of chlorinated compounds, the PCBs or polychlorinated biphenyls,
which are discussed later.
3.3.2.2 Halogenated Hydrocarbons
If the hydrogen in a hydrocarbon is substituted by chlorine (or less frequently
by bromine, fluorine, or iodine), the resulting compound tends to be less flammable,
more stable, more hydrophobic, and more environmentally troublesome. Replacing
a hydrogen with a chlorine usually causes an increase in molar volume and area and
a corresponding decrease in solubility by a factor of about 3.
The stability of many of these compounds makes them invaluable as solvents,
examples being methylene chloride and tetrachloroethylene. The fluorinated and
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