Page 176 - Soil and water contamination, 2nd edition

Organic pollutants 163

Cl
CH Cl CH
H C 2 CH HC CH HC CH
2
or or or
H C CH HC CH HC CH
2 Cl O
CH Cl CH
2 O
1,2-dichlorocyclohexane cyclohexene furan
or or or
N N
benzene
naphtalene quinoline
6642
CH
3

methyl benzene(toluene) biphenyl anthracene benzo[a]pyrene
Figure 9.Ib Examples of cyclic organic molecules.

Because of the symmetry of the planar ring, there are only three isomers for any
particular disubstituted benzene: those identified as 1,2; 1,3; and 1,4. However, another
commonly adopted system uses the prefixes ortho-, or o- for the 1,2-isomer; meta-, or m-
for the 1,3-isomer; and para-, or p- for the 1,4-isomer.
The prefixes cis- and trans- describe the relative position of two substituents relative to
a double (or triple) carbon bond or the ring plane. The term cis is used is used if the two
substituents are located on the same side of the double bond or plane and the term trans
is used if they are on opposite sides (see Figure 9.Ic).

H H H Cl
Cl Cl Cl H
cis-1,2-dichlorocyclohexane trans-1,2-dichlorocyclohexane 6642
Figure 9.Ic Cis and trans isomers of 1,2-dichlorocyclohexane.
In ring systems with more than two substituents, more isomers are possible. For
example, Figure 9.Id shows three possible isomers of 1,2,3,4,5,6-hexachlorocyclohexane
(HCH). The γ-isomer is known as the insecticide lindane. Note that, like lindane, many
organic compounds have another ‘trivial’ name, which is used more frequently than the
systematic name.

Cl Cl Cl Cl Cl Cl
Cl Cl Cl Cl Cl Cl
Cl H Cl H Cl H
Cl H Cl H Cl H
H H H H H H
H Cl H Cl H Cl
γ-HCH α-HCH β-HCH 6642
Figure 9.Id Isomers of 1,2,3,4,5,6-hexachlorocyclohexane.

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