Diels and Alder received the Nobel prize in chemistry in 1950 for their work on this type of diene synthesis. Chlordane is a nearly odorless, viscous, amber-colored liquid that can be distilled only under high vacuum and is soluble in the common organic solvents. Chlordane is also a good solvent for DDT. It decomposes in the presence of alkalies with a resulting loss of insecticidal toxicity; consequently it cannot be formulated with alkaline materials.

TWO OF THE NEWER SYNTHETIC chlorinated hydrocarbons, which were known during their experimental testing period as Julius Hyman and Company Compounds I18 and 497, have been given the common names, aldrin and dieldrin, honoring Alder and Diels. Aldrin has been defined as containing not less than 95 percent of 1,2,3,4, 10, 10-hexachloro- 1,4,4a,5,8,8a-hexahydro- 1,4,5,8-dimethanonaphthalene. Dieldrin (pronounced deel-drip) has been defined as containing not less than 85 percent of 1,2,3,4,10,10-hexachloro-6,7-epoxy- 1.4,4a.5,6,7,8,8a - octahydro - 1,4,5,8 - dimethanonaphthalene. Aldrin is a white solid with a melting point of 104 -104.5 C. It is practically odorless at room temperature, but it has a pine-like odor when warm. Dieldrin melts at 175 -176 C. and is odorless. Aldrin is soluble in the common organic solvents. Dieldrin is moderately soluble in the same solvents. Neither is soluble in water. Aldrin is stable in the presence of organic and inorganic alkalies and hydrated metallic chlorides and therefore is compatible with most agricultural chemicals.

Unlike DDT, the DDT analogs, toxaphene, and chlordane, dieldrin is unaffected by alkalies. Insecticidal effectiveness is not lost in the presence of alkaline and acid materials that would occur in formulation; thus it is compatible with most agricultural chemicals. It does react chemically with strong acids. Aldrin, with a vapor pressure approximating that of lindane, is about 20 times more volatile than dieldrin.

Because aldrin and dieldrin are highly toxic, technical products and insecticidal formulations containing them must be handled with extreme care.

Bis (p-chlorophenoxy) methane, formerly called K-1875, is a solid that melts at 68 -68.5 C. It is rather soluble in acetone, benzene, and ethyl ether. It is not appreciably soluble in ethanol and the aliphatic hydrocarbons. It is insoluble in water. It belongs to the class of compounds known as acetals and is stable to alkalies, but on boiling with dilute aqueous acids it is hydrolyzed. It is used to control mites in fruit orchards.

One of the new insecticides that came out of the Second World War the Germans called "Lauseto neu." It is chloromethyl p-chlorophenyl sulfone and is not a chlorinated hydrocarbon. It is a good insecticide but is less effective than DDT against certain strains of lice, flies, and mosquitoes.

AN INTERESTING ACCOMPLISHMENT in the preparation of synthetic organic insecticides was the synthesis of compounds that resemble the pyrethrins and cinerins, the toxic materials in pyrethrum flowers. In 1948, after 15 years of investigation of the structure of these naturally occurring insecticides, chemists of the Bureau of Entomology and Plant Quarantine prepared pyrethrin-type esters similar to cinerin I. One of these synthetic esters, the dl-2-allyl-4-hydroxy-3-methyl-2-cyclopenten-1-one ester of a mixture of cis and trans dl-chrysanthemum carboxylic acid, has been produced commercially and has been given the common name of allethrin by the Interdepartmental Committee on Pest Control. Allethrin is a light, yellow-colored oil and possesses solubilities similar to those of the natural products so that it may be used in the same manner in fly sprays and aerosols.

Diphenylamine, (C₆H₅),NH, is another type of organic compound that has been used successfully for screw-worm control. 1,1-Bis(p-chlorophenyl) ethanol, (CI₂C₆H₄),COHCH₃, (also called DMC from the generic name dichlorophenyl methylcarbinol) is used against mites. Pentachlorophenol, C₆C1₅OH, is used to control termites.

A NEW FIELD of organic phosphorus insecticides was opened up during the Second World War by Gerhard Schrader, a German chemist who was engaged primarily in the search for more powerful agents of chemical warfare. Schrader discovered a new series of highly toxic organic phosphorus compounds. From them, through extensive tests, came several effective insecticides. The list includes parathion, tetraethyl pyrophosphate ( including the so-called hexaethyl tetraphosphate), and octamethyl pyrophosphoramide.

Parathion, a remarkably effective insecticide, has been put to use in many countries to control many kinds of insects infesting various crops.

Tetraethyl pyrophosphate and octamethyl pyrophosphoramide are used chiefly against aphids and some mites.

Tetraethyl pyrophosphate kills insects rapidly, almost as soon as the material is applied, and then, having performed its task, the toxic insecticide soon decomposes by hydrolysis into nontoxic and water-soluble products. Thus, there is no spray-residue problem connected with its use.

Octamethyl pyrophosphoramide has been manufactured in the United States only on a relatively small scale.