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

chlorofluoro compounds are very stable and are used as refrigerants. Because these
molecules are quite small, they are fairly soluble in water and are therefore able to
penetrate the tissues of organisms quite readily. They are thus used as anaesthetics
and narcotic agents.
The chlorinated aromatics are a particularly interesting group of chemicals. The
chlorobenzenes are biologically active. 1,4 or paradichlorobenzene is widely used
as a deodorant and disinfectant. The polychlorinated biphenyls, or PCBs, and their
brominated cousins, the PBBs, are notorious environmental contaminants, as are
chlorinated terpenes such as toxaphene, which is a very potent and long-lived
insecticide. Many of the early pesticides, such as DDT, mirex, and chlordane, are
chlorinated hydrocarbons. They possess the desirable properties of stability and a
high tendency to partition out of air and water into the target organisms. Thus,
application of a pesticide results in protection for a prolonged time. As Rachel Carson
demonstrated in Silent Spring, the problem is that these chemicals persist long
enough to affect non-target organisms and to drift throughout the environment,
causing widespread contamination.
Fluorinated chemicals also possess considerable stability and, because the fluorine
atom is lighter than chlorine, they are generally more volatile. Polyfluorinated
substances are very stable in the environment as a result of the strong C-F bond.
Brominated chemicals are also stable, but with reduced volatility. A major use of
brominated substances is in fire retardants, specifically polybrominated diphenyl
ethers.
3.3.2.3 Oxygenated Compounds
The most common oxygenated organic compounds are the alcohols, ethanol
being among the most widely used. Others are octanol, which is a convenient
analytical surrogate for fat, and glycerol is of interest because it forms the backbone
of fat molecules by esterification with fatty acids to form glycerides.
The phenols consist of an aromatic molecule in which a hydrogen is replaced
by an OH group. They are acidic and tend to be biologically disruptive. Phenol, or
carbolic acid, was the first disinfectant. Substituting chlorines on phenol tends to
increase the toxic potency of the substance and its tendency to ionize, i.e., its pKa
is reduced. Pentachlorophenol (PCP) is a particularly toxic chemical and has been
widely used for wood preservation.
The ketones such as acetone, and aldehydes such as formaldehyde, are fairly
reactive in the environment and can be of concern as atmospheric contaminants in
regions close to sources of emission. Much of the smog problem is attributable to
aldehydes formed in combustion processes.
Organic acids such as acetic acid are also fairly reactive. They are not usually
regarded as an environmental problem, but trifluoroacetic acid, which is formed by
combustion of freons and from some pesticides, is very persistent. Some chlorinated
organic acids, e.g., 2,4-D, are potent herbicides. Longer-chain acids, such as stearic
acid, are mainly of interest because they esterify with glycerol to form fats. Humic
and fulvic acids are of considerable environmental importance. These are substances
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