The Organochlorine Insecticides - Organic Pollutants: An Ecotoxicological Perspective - page 102

The cyclodiene insecticides aldrin, dieldrin, endrin, heptachlor, endosulfan, and
others were introduced in the early 1950s. They were used to control a variety of
pests, parasites, and, in developing countries, certain vectors of disease such as the
tsetse fly. However, some of them (e.g., dieldrin) combined high toxicity to verte-
brates with marked persistence and were soon found to have serious side effects in
the field, notably in Western European countries where they were extensively used.
During the 1960s, severe restrictions were placed on cyclodienes so that few uses
remained by the 1980s.
HCH, sometimes misleadingly termed benzene hexachloride (BHC), exists in a
number of different isomeric forms of which the gamma isomer has valuable insec-
ticidal properties. These were discovered during the 1940s, and HCH came to be
widely used as an insecticide to control crop pests and certain ectoparasites of farm
animals after the Second World War. Crude technical BHC, a mixture of isomers,
was the first form of HCH to be marketed. In time, it was largely replaced by a refined
product called lindane , containing 99% or more of the insecticidal gamma isomer.
Those OCs that came to be widely marketed were stable solids that act as neuro-
toxins. Some OCs, or their stable metabolites, proved to have very long biological
half-lives and marked persistence in the living environment. Where persistence was
combined with high toxicity, as in the case of dieldrin and heptachlor epoxide (stable
metabolite of heptachlor), there were sometimes serious environmental side effects.
Because of these undesirable properties, no fewer than eight out of twelve chemi-
cals or chemical groups identified by the United Nations Environment Programme
(UNEP) as persistent organic pollutants (POPs or, more informally, “the dirty
dozen”) are OCs. These are aldrin, chlordane, DDT, dieldrin, endrin, heptachlor,
mirex, and toxaphene. The intention is that high priority should be given by national
and international environmental regulatory bodies to the eventual removal of POPs
from the environment.

5.2 ddt [1,1,1,-trIcHLoro-2,2-BIS (P-cHLoroPHenyL) etHane]

5.2.1 c h e m i c a l p r o p e r T i e s
The principal insecticidal ingredient of technical DDT is p,p ′-DDT (Table 5.1
and Figure 5.1). The composition of a typical sample of technical DDT is given in
Table 5.2.
The composition of the technical insecticide varies somewhat between batches.
However, the pp ′ isomer usually accounts for 70% or more of the total weight. The
o,p ′ isomer is the other major constituent, accounting for some 20% of the technical
product. o,p ′-DDT is more readily degradable and less toxic to insects and verte-
brates than the p,p ′ isomer. The presence of small quantities of p,p ′-DDD deserves
mention. Technical DDD has been marketed as an insecticide on its own (rhothane)
and the p,p ′ isomer is a reductive metabolite of p,p ′-DDT.
p,p ′-DDT is a stable white crystalline solid with a melting point of 108°C. It has
very low solubility in water and is highly lipophilic (log K ow = 6.36); thus, there is a
high potential for bioconcentration and bioaccumulation. It has a low vapor pressure,