DDT: Structure, Uses and Environmental Persistence

DDT -- systematically p,p'-dichlorodiphenyltrichloroethane, and more fully 1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane -- was the first synthetic organochlorine insecticide to see widespread use. Its molecule consists of a central $\text{CH--CCl}_3$ unit flanked by two para-chlorophenyl rings, so the molecule contains two structurally distinct kinds of carbon-halogen bond: the three $\text{C--Cl}$ bonds of the aliphatic trichloromethyl ($\text{--CCl}_3$) group, and the two aromatic $\text{C--Cl}$ bonds on the para position of each phenyl ring (of the haloarene type discussed earlier in this chapter).

Historical use. DDT proved extremely effective and inexpensive as an agricultural pesticide and, notably, as a public-health tool against insect-borne disease -- large-scale spraying programmes using DDT played a major documented role in reducing malaria transmission (by killing the mosquito vector) and typhus (via lice) in several regions during and after the Second World War.

Why it persists. The very stability of DDT's multiple carbon-chlorine bonds, which makes it resistant to hydrolysis and to breakdown by soil microorganisms, is what makes it an environmentally persistent compound: once released, it degrades only extremely slowly, so residues remain active and present in soil and water for years. DDT is also highly lipophilic (fat-soluble) and poorly water-soluble, so once absorbed by an organism it is stored preferentially in fatty tissue rather than being excreted.

Bioaccumulation and biomagnification. Because DDT accumulates in fat and is broken down only slowly, an individual organism's body burden of DDT tends to increase over its lifetime (bioaccumulation), and because each predator in a food chain consumes many prey organisms' worth of stored DDT, the concentration found in tissue increases at each successive trophic level -- a phenomenon called biomagnification. This is why DDT residues have been measured at their highest concentrations in top predators, and why documented ecological harm from DDT (most famously, eggshell thinning that impaired the reproduction of birds of prey such as the peregrine falcon and the bald eagle, by interfering with calcium metabolism) appeared at the top of food chains even though the chemical itself had been applied much lower down. …