292 



Freed, Reithel, and Remmert 



reactions. The suggestion of Veldstra (30) that the compounds may be 

 acting at a protein-lipoid surface leads to speculation that the action 

 of these compounds may be physical in nature. If the action of these 

 compounds is physical, then there should be some measurement 

 which ^^•ould show obvious differences in the physical properties of 

 the biologically active and the biologically inactive members of the 

 series. The work of \\'right (35) in correlating the biological activity 

 of insect repellents to their infrared absorption spectra suggested that 

 the infrared spectra of the chlorophenoxyacetic acids may reveal some 

 relation to activity. 



INFRARED AND ULTRAVIOLET SPECTRA OF 

 CHLOROPHENOXYACETIC ACIDS 



A series of chlorinated phenoxyacetic acids was examined quali- 

 tatively for infrared absorption, using 1 mg. of chemical to 300 mg. 

 of potassium bromide in a pellet form. The spectra ^vere run from 

 about 2 fx out to 14 /x (Figure 1). The most obvious difference noted 

 between an active (2,4-D) and a relatively inactive compound (2,6-D) 

 was to be found in the absorption in the region of 12 ju, (825 cm.-^). 

 The major difference noted in this region is that the biologically active 

 compound had an intense absorption band at 794 cm.-^, whereas the 

 inactive compound had only a moderately strong absorption in this 

 region, characterized by a missing Q branch. This fact is illustrated by 



1 r 



2,4-Dichlorophenoxyocetic ocid 



2,6-Dichlorophenoxyocetic ocid 



J L 



14 12 10 8 6 4 2 



MICRONS 

 Fig. 1. Infrared absorption spectra of 2,4- and '^.G-dicliloioplicnDwacclic aciil 



