"On October 16, 1942, we contacted Dr. R. C. Roark at Washington, and handed. him a Swiss report giving the results of tests which had been made with 'Gesarol' on a great variety of insects, some of which, like the Colorado potato beetle, are familiar to us in the United States.

"Dr. R. Wiesmann, Assistant Chief of the Swiss Agricultural Station, was responsible for these reports, which, incidentally, seemed altogether too good to be true. These reports indicated that 'Gesarol' was not only a contact, but a stomach poison as well. Entomologists were at first reluctant to accept the statements which were made, and frankly, we felt the same skepticism, for, of over 3,000 products which had been tested by the United States Department of Agriculture, a rare one or two could claim both contact and stomach insecticidal properties. . .

"Up to that time nothing was known of the chemical constitution of the product, as little factual information could be obtained because of the difficulties of communication. Dr. H. L. Haller at Beltsville had extracted the active ingredient in 'Gesarol,' and analyzed and synthesized some of the active ingredient in his laboratory, simultaneous with which there arrived . . . the information of its chemical composition."

The symbol DDT is a contraction for dichloro-diphenyl-trichloroethane, the generic name of the active insecticidal principle. Theoretically, there are 45 possible dichloro-diphenyl-trichloroethanes, excluding stereoisomeric forms. However, the term DDT has been confined to the product obtained on condensation of chloral (or its alcoholate or hydrate) with chlorobenzene in the presence of sulfuric acid. The product thus obtained is termed technical DDT. It is a white to cream-colored powder, possessing a fruitlike odor. Its major constituent is 1-trichloro-2,2-bis(p-chlorophenyl) ethane, which has the formula CIC6H4)2 CHCC13 and is called p,p'-DDT.

This compound may be obtained pure by recrystallization of technical DDT from 95-percent ethanol. The pure compound melts at 108.50 to 109 C. It was first described in 1874 by Zeidler, who prepared it while working on his doctor's thesis at the University of Strasbourg.

The preparation of DDT is relatively simple. It requires no elaborate equipment, and all the starting materials ethanol, chlorine, benzene, and sulfuric acid are readily available. The basis of the commercial process is the original procedure of the first chemist to make the compound that is, the condensation of monochlorobenzene with chloral in the presence of sulfuric acid. A considerable amount of research has been carried out in this country, as well as in Great Britain and Canada, on the optimum conditions for the condensation, such as temperature and concentration of reactants, and for preparing the starting materials.

As its name implies, technical DDT is a commercial grade. Since it melts over a range of several degrees, the solidification point rather than the melting point is used to give an indication of its purify.

Intensive studies, involving fractional crystallization, chromatographic adsorption, distillation in high vacuum, and cryoscopic analysis showed the presence of 14 different compounds in the early samples of technical DDT. The p,p'-DDT varied in the samples of different manufacturers from 65 to 75 percent. The major impurity was found to be 1-trichloro-2- (o-chlorophenyl) -2- (p-chlorophenyl) ethane (which is called o,p'-DDT) which was present to the extent of 19 to 21 percent. This compound is considerably less toxic than p,p'-DDT to insects. The other impurities were isolated in amounts ranging from 0.007 to 4.0 percent. The impurity present to the extent of 4.0 percent was identified as 1,1 dichloro-2,2-bis (p-chlorophenyl) ethane (TDE). It is formed from dichloroacetaldehyde, an impurity in chloral, and chlorobenzene. TDE is also an effective insecticide but the other minor components are relatively nontoxic to insects.

DDT is practically insoluble in water, but is soluble in a wide variety of organic solvents, such as acetone, benzene, xylene, chloroform, carbon tetrachloride, vegetable oils, petroleum oils, and many others. Crude or unrefined kerosene can be used to prepare solutions containing 5 percent of DDT, but refined kerosenes require the addition of 10 to 20 percent of an auxiliary solvent. For this purpose xylene, cyclohexanone, and alkylated naphthalenes have been used.

When pure, p,p'-DDT is a reasonably stable compound. It can be heated at 115' to 120 C. for several hours without decomposition. Irradiation of the solid material, spread in a thin layer, for 35 hours with a mercury-vapor lamp caused only a slight depression in melting point and an alcoholic solution showed no change after exposure to sunlight for over a year. Under certain conditions (temperatures above 95 -100 C. or in some organic solvents) it has been shown that a number of materials, even in very small quantities, cause decomposition of DDT with the formation of products that are of little or no value as insecticides.