The Carbon Quandary: Essential and Detrimental - Sustainable Design: The Science of Sustainability and Green Engineering - page 260

The term aromatic comes from the observation that many compounds derived
from benzene are highly fragrant, such as vanilla, wintergreen oil, and sassafras.
Aromatic compounds thus contain one or more benzene rings. The rings are
planar ; that is, they remain in the same geometric plane as a single unit. However,
in compounds with more than one ring, such as the highly toxic polychlorinated
biphenyls (PCBs), each ring is planar, but the structure of the rings bound together
may or may not be planar. This is actually a very important property for toxic
compounds. It has been shown that some planar aromatic compounds are more
toxic than their nonplanar counterparts, possibly because living cells may be more
likely to allow planar compounds to bind to them and to produce nucleopeptides
that lead to biochemical reactions associated with cellular dysfunctions such as
cancer or endocrine disruption.
Aliphatic and aromatic compounds can both undergo substitutions of the
hydrogen atoms. These engender new properties to the compounds, including
changes in solubility, vapor pressure, and toxicity. For example, halogenation (sub-
stitution of a hydrogen atom with a halogen) often makes an organic compound
much more toxic: Trichoroethane is a highly carcinogenic liquid that has been
found in drinking water supplies, whereas nonsubstituted ethane is a gas with
relatively low long-term toxicity. This is also why one of the means of treating
wastes contaminated with chlorinated hydrocarbons and aromatic compounds in-
volves dehalogenation techniques. The important functional groups that are part
of many organic compounds are shown in Table 7.1. Green design success or fail-
ure can hinge on the type of organic compounds resulting from reactions. A step
that leads to a chlorinated compound, for example, can be made greener if that
step is removed or if it generates a less toxic, nonhalogenated compound instead.
Thus, the designer needs at least a rudimentary understanding of these structures.
Different structures of organic compounds can induce very different physical
and chemical characteristics, as well as change the bioaccumulation and toxicity
of these compounds. For example, the differences between an estradiol and a
testosterone molecule may seem small, but they cause significant differences in
the growth and reproduction of animals. The very subtle differences between
an estrogen and an androgen, female and male hormones, respectively, can be
seen in these structures. Incremental changes to a simple compound such as
ethane can make for large differences (see Table 7.2). Replacing two or three
hydrogens with chlorine atoms makes for differences in toxicities between the
nonhalogenated and the chlorinated forms. The same is true for the simplest
aromatic, benzene. Substituting a methyl group for one of the hydrogen atoms
forms toluene. Replacing a hydrogen with a hydroxyl group is equally significant.
For example, dry cleaning operations have progressively switched solvents, from
very toxic, chlorinated compounds to safer ones. In fact, many dry cleaners' now
use chlorinefree processes, especially those that take advantage of CO 2 when
it becomes supercritical. At sufficiently high pressure, CO 2 can dissolve most