Global Change 2797.3) produced industrially as a racemic mixture of two enantiomers (Fig. 7.29 &
Box 7.3). a-HCH is a pesticide that has been widely applied throughout the USA,
primarily to control cotton pests. At the time of application both a-HCH enan-
tiomers were present in equal proportions.
a-HCH, is a SVOC, and thus will evaporate following its application as a
pesticide, to be transported in the atmosphere and then redeposited by conden-
sation (Section 7.4.1). Interestingly, one of the a-HCH enantiomers is selectively
degraded. Thus, as time passes one of the enanitomers decreases in concentra-
tion while the other maintains its concentration. This changes the concentration
ratio of the enantiomers from an initial ratio of 1 to a progressively lower value
with time. The ratio of a-HCH enantiomers in air above the Great Lakes is now
0.85, identifying its source as ‘old’ a-HCH that has been degraded during
storage, but that is now escaping from the lake bed sediments, through the water
and re-equilibrating with the atmosphere. Moreover, this re-emission has a sea-
sonal signal, being greatest in the warm summer months when volatilization is
encouraged by higher vapour pressures. By measuring not only the concentra-
tion of a-HCH above the Great Lakes, but also the concentration ratio of the
enantiomers, it is clear that the Great Lakes now behave as a source of a-HCH
and not a sink. This case study highlights the complexity of mitigating the effects
of pollution by exotic chemicals. Removal of a contaminant from the environ-
ment involves far more than just stopping manufacture or use.
ClClClClCl
ClClClClClCl
ClClClClCl
ClClClClClCl
ClClMirrorFig. 7.29The enantiomers of a-hexachlorocyclohexane (a-HCH). Bold wedge shaped
bonds represent bonds rising from the plane of the page toward the viewer, and bonds
represented by dashed lines are receding from the plane of the page away from the viewer.
See also Box 7.3.