Fine Chemicals: Pesticides

James Richard Fromm


All other forms of animal life, including insects, compete with human beings for food directly or indirectly. Crop losses to rodents such as rats and mice are significant, especially when storage facilities are not secure, but the most significant competition is from insects. Accordingly, effective chemical warfare has been waged against insects and rodents. Effective chemical warfare has also been waged on behalf of our desired food plants by destroying other plants that compete with them. Those chemicals which are effective against undesired plants, insects, or other animals (pests) are collectively called pesticides. The more specific terms herbicides, insecticides, and rodenticides are used to describe the major groups of pesticides. Ecological effects, many of them unforeseen, have led to stringent controls on certain pesticides. This in turn has led to a continuing search by chemists for new and effective, but less damaging, compounds.

Insecticides

Insects are by far the greatest competitor with man for food, because they represent about 80% of the total animal mass of the world. It has been estimated that about one-third of the food crops of the world are consumed or destroyed by insects. In addition, insects are carriers of diseases such as malaria and typhus. Effective insecticides date from around 1900, at which time some natural materials along with the fungicide Bordeaux mixture (CuSO4.5H2O solution plus suspension of Ca(OH)2) were in use. The modern pesticide industry began in 1939 with the discovery of the insecticidal properties of 1,1,1-trichloro-2,2-di(p-chlorophenyl)-ethane, C14H9Cl5 or DDT, by Paul Mueller of the Swiss firm of Ciba-Geigy. Production of DDT begins with direct chlorination of benzene to chlorobenzene; DDT is produced from the chlorobenzene and trichloroethanal (trichloroacetaldehyde or chloral), which react in the presence of concentrated sulfuric acid. The reaction is:

CCl3CHO + 2C6H5Cl rarrow.gif (63 bytes) CCl3CH(C6H4Cl)2 + H2O.

The use of DDT, which is comparatively nontoxic to humans, toxic to insects, and persistent in the environment, has assisted greatly in the control of malaria and other insect-borne diseases as well as in the protection of food crops. However, since DDT is soluble in fat but not in water it accumulates up the food chain from prey to predators and has been responsible for severe damage to birds and other animals. For the twin reasons of persistence and accumulation, the use of DDT has been banned in some countries including the United States and Canada.

Modified forms of DDT, such as the water-soluble methoxychlor, are more biodegradable in the environment than is DDT while they retain the high toxicity to insects and comparatively low toxicity to humans which have made DDT such a useful insecticide. Other classes of organic compounds effective as pesticides are benzene hexachloride (in use since about 1940), and organic phosphate esters (became available 1937-1946). Over 90% of the pesticides in use are of these or related organic classes of compounds, many of them containing chlorine or phosphorus.

Herbicides

About 10% of North American food crops are lost each year due to competition from undesired plants (weeds) at an annual cost of tens of billions of dollars. Chemical herbicides are used to minimize this damage, and billions of dollars are spent on them each year.

Although salt has been used against weeds from ancient times, and mixtures of salt and lime and other inorganic compounds have been used since the 1850s, these compounds and mixtures are neither selective nor particularly effective. The modern herbicide industry began in 1945 with the introduction of 2,4-dichlorophenoxyacetic acid, or 2,4-D as it came to be known. This compound selectively kills broadleaf plants while causing little to no injury to narrow leaf plants. It is widely used to control weeds in major food crops such as rice, wheat, and corn as well as dandelions in household lawns and gardens.

In the late 1940s the related compound 2,4,5-trichlorophenoxyacetic acid, or 2,4,5-T, was first marketed; it has since been widely used for controlling brush along railroads and highways. During the Vietnam War, the United States used over twelve million gallons of a defoliant mixture called Agent Orange as a defoliant herbicide mixture sprayed from aircraft to destroy forests in Vietnam. Agent Orange, so-called from the orange color of its storage drums, is a 50:50 mixture of the butyl esters of 2,4-D and 2,4,5-T. Considerable controversy still exists over the long-term effects of this material on the area and on the combatants of both sides present there. It is probable that damage to humans would be due to the highly toxic impurity 2,3,7,8-tetrachlorodibenzo-p-dioxin, often simply called dioxin, present in Agent Orange.

A significant modern herbicide, patented by Monsanto Chemical and widely marketed under the trademark Round-up, is N-(phosphonomethyl)glycine, HOOCCH2NH2PO(OH)2. It was actually made first in 1950 by the Swiss chemist Henri Martin, who had once worked on DDT with Paul Mueller. However, its herbicidal activity was discovered only in 1970 by Dr. J. E. Franz at Monsanto.


Previous Topic: Fine Chemicals: Soaps and Detergents

Next Topic: Fine Chemicals: Pharmaceuticals

Return To Course Outline


Copyright 1997 James R. Fromm