30 



PLANT GROWTH SUBSTANCES 



CHCOOH 

 II 



\ 



•CH, 



XXXI 



HOOCCH 

 II 





COOH 



yym 



not, and there is evidence from the ultraviolet absorption spectra that 

 the active form of both has the m-configuration (2). The surface activity, 

 determined by the polarographic procedure, is the same for the two 

 isomers of XXXI. A detailed criticism of Veldstra's theory would be 

 out of place here, but at least in the case of XXXI the argument is open 

 to doubt. For, according to principles of organic chemistry, derived from 

 study of tetramethylethylene and other simple cases, when a carbon 

 atom forms a double bond the other two bonds He in the same plane. 

 Hence in both forms of XXXI the carboxyl would be expected to lie in 

 the plane of the ring. Because the right-hand ring is saturated, however, 

 it can be somewhat buckled, raising carbon atom number 2 above the 

 plane of the ring and hence making it possible for the COOH to he 

 slightly below the plane, and thus to avoid interference with the hydro- 

 gen atom on the aromatic ring. Judging from models, the effect is very 

 small. 



The same theory has been appUed to the nitrophenoxyacetic acids. 

 Of these only the ;;7-isomer shows appreciable activity. It is pointed out 

 by Veldstra that the 0- and /7-isomers are capable of resonance with a 

 quinoid structure, in which the oxygen atom would be held in the plane 

 of the ring. This restricts freedom of rotation and favors positions in 

 which the acid dipole is at an angle to the plane of the ring. But careful 

 examination of scale models shows that this restriction would in fact 

 hold the carboxyl at least as much out of the plane of the ring as in it, 

 that is, the o- and /^-compounds should have at least as high activity. 



Another more general consideration opposes this view also. If the 

 maintenance of the carboxyl out of the plane of the ring were the princi- 



