Global persistent organic polllutant equilibrium

The manufacture and use of many exotic organic compounds (see Section 1.4) has now been discontinued because of their persistence, potential health effects and global mobility. For example, PCBs were first manufactured and used in the 1930s and their usage increased until the early 1970s. Thereafter PCB usage was banned in many instances or subject to restrictions. However, PCBs did not immediately disappear from the environment. Even today PCBs are present in all of Earth's environmental reservoirs and they are distributed globally. What has changed, following discontinuation of widespread PCB use, is the equilibrium between PCB concentrations in air, water and soil. In the 1970s, when PCB concentrations were at their peak, PCBs in the atmosphere were at much higher concentrations than today. These high atmospheric concentrations caused a net flux of PCBs from the atmosphere to the Earth's surface waters and soils to establish equilibrium between these reservoirs (Fig. 7.28). Today, PCB concentrations in the atmosphere are much lower. This has disturbed the equilibrium between soils and water and the overlying air such that the net flux of PCBs is now out of the soil and water into the atmosphere to re-establish equilibrium (Fig. 7.28). This situation is termed re-emission or secondary emission.

Unequivocal evidence for the re-emission of POPs has been demonstrated in an elegant study of the Great Lakes that border the USA and Canada. This study focused on the POP a-hexachlorocyclohexane (a-HCH), a chiral compound (Box

Fig. 7.28 Environmental equilibration of polychlorinated biphenyls (PCBs) between soil and air.

7.3) 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 enantiomers 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 condensation (Section 7.4.1). Interestingly, one of the a-HCH enantiomers is selectively degraded. Thus, as time passes one of the enanitomers decreases in concentration 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 seasonal signal, being greatest in the warm summer months when volatilization is encouraged by higher vapour pressures. By measuring not only the concentration 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 environment involves far more than just stopping manufacture or use.

Fig. 7.29 The 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.

Box 7.3 Chiral compounds

Molecules that cannot be superimposed on their mirror images are said to be chiral (from Greek kheir—hand). A pair of molecules that fulfil this condition are called enantiomers (from the Greek enantio — opposite). Consider a molecule comprising a central carbon atom to which the following groups are attached in a tetrahedral array: -CH3, -H, -Br, -COOH (Fig. 1.). In this figure the molecule is represented as a tetrahedron. Bonds represented by lines are in the plane of the page; bonds represented by wedged lines are rising from the plane of the page towards the viewer, and bonds represented by dashed lines are receding from the plane of the page away from the viewer. The mirror image of this molecule cannot be superimposed upon the original molecule. Try rotating the mirrored-molecule around the vertical bond; you will find that as the groups rotate round they never arrive in a position that would allow them to be superimposed on the original molecule. Thus, this molecule is chiral and the carbon atom to which the groups are attached is termed the chiral centre. By contrast, a molecule comprising a central carbon atom to which the following groups are attached in a tetrahedral array,

-CH3, -CH3, -Br, -COOH (Fig. 1.), has a mirror image that can be superimposed upon the original molecule. This molecule is achiral and the molecule and its mirror image are not enantiomers.

It is possible to describe enantiomers without drawing the structures by assigning a particular enantiomer the letter 'R' or 'S'. This system ascribes priority to the groups attached to the chiral centre in accordance with their atomic weight. The higher the atomic weight the higher the priority assigned. In a simple case where, for example, the central carbon atom is attached to -Cl, -Br, -I and -H, then priority is given in the order I > Br > Cl > H on account of the atomic weights being 127, 80, 35 and 1, respectively. Where for example a -CH3 group and a -COOH group are attached to the central carbon atom, i.e. the central carbon atom is attached to another carbon atom in both cases, then the groups attached to these carbon atoms need to be considered. In this example priority is given to -COOH as 'O' has a greater atomic weight than 'H'. Having assigned priority to the groups, the group of lowest priority is projected away from the viewer. The viewer then establishes if the

COOH

3C H

Rotation axis

COOH C

H CH

Mirror

COOH

3C CH

Rotation axis

COOH

H3C CH3

Mirror

Cannot overlap upon rotation CHIRAL

Fig. 1 Contrasting examples of chiral and achiral molecules.

Can overlap upon rotation ACHIRAL

(continued)

route from the highest priority group to the second highest priority group lies in a clockwise or anticlockwise direction. Clockwise is assigned 'R' (from the Latin rectus, right) while anticlockwise is assigned 'S' (from the Latin sinister, left). In the chiral molecule we began with (Fig. 1), prioritizing the groups yields the order -Br > -COOH > -CH3 > -H. Figure 2 shows the assignment of 'R' and 'S' to the enantiomers. In this figure, as in Fig. 1, the wedge bonds rise from the plane of the page toward the viewer; and the dashed bond recedes from the plane of the page away from the viewer.

A mixture containing equal proportions of two enantiomers is termed a racemic mixture and is indicated by the prefix 'RS-'. During organic synthesis it is usual for a chiral product to be produced as an RS mixture (unless the reaction has been tailored to produce a specific chiral product). Chirality is important because it affects a molecule's activity. For example, the compound thalidomide has one enantiomer that is a

Anticlockwise

Clockwise

Anticlockwise

Clockwise

Mirror

Fig. 2 Assignment of 'R' and 'S' to the enantiomers of a molecule comprising a central carbon atom (central circle) to which -CH3, -H, -Br, -COOH groups are attached.

Mirror

Fig. 2 Assignment of 'R' and 'S' to the enantiomers of a molecule comprising a central carbon atom (central circle) to which -CH3, -H, -Br, -COOH groups are attached.

valuable therapeutic drug (a sedative) while the other enantiomer is highly toxic. Between 1957 and 1960 thalidomide was prescribed in at least 46 countries to alleviate morning sickness associated with pregnancy. As a consequence of the two enantiomers being present thousands of babies were born with birth defects.

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