INSECTICIDES AND REPELLENTS 3 



CHEMISTRY OF INSECTICIDES AND REPELLENTS 



DDT 



History and Preparation 



DDT was first synthesized in Germany in 1874, but it was not 

 until 1939 that the Swiss firm of J. R. Geigy, S. A., reported the 

 compound to be effective as an insecticide against clothes moths, 

 flies in stables, and certain agricultural insect pests. 



Samples for the initial research were furnished by the Geigy Co., 

 Inc., New York City, a subsidiary of J. R. Geigy, S. A. The first 

 samples were received at Orlando in October 1942. 



The chemical name of DDT is l,l,l-trichloro-2,2-bis(p-chloro- 

 phenyl)ethane. The term U DDT" is derived from the generic name 

 dichloro-diphenyl-trichloroethane. The compound is prepared from 

 2 molecules of monochlorobenzene and 1 molecule of chloral or chloral 

 hydrate in the presence of a suitable condensing agent, such as sulfuric 

 acid. The equation is as follows: 



I H2SO4 \ r\\ 



+ o=c— CCI3 ► C— CCI3 



! CI 



Monochlorobenzene Chloral p.p'-DDT 



The chlorine atoms may occur in different positions on the benzene 

 ring to form different isomers. The DDT crystallizes from the 

 reaction mass, and the sulfuric acid is removed by washing. The 

 technical product contains from 70 to 77 percent of the p,p f isomer 

 and from 15 to 25 percent of the o,p' isomer, which is much less 

 toxic. Pure ^,2/ -DDT may be obtained by several recrystallizations 

 of the technical material from ethyl alcohol. 



Properties 



Pure p,p'-DT)T is a white crystalline substance melting at 108.5°- 

 109° C. The technical product melts at a lower temperature, and the 

 Federal specification calls for a minimum setting point of 89°. The 

 aerosol grade has a melting point of not less than 103°. The specific 

 gravity of the compound is about 1.6. It is stable under ordinary 

 conditions. 



DDT is nearly insoluble in water, but is moderately soluble in 

 petroleum and vegetable oils, and readily soluble in many other 

 organic solvents. Table 1 shows the approximate solubilities at 

 27°-30° C. (81°-86° F.) for once-recrystallized DDT (m. p. 107.5°- 

 108°. C.) or technical DDT in some of the common solvents. 



