Substituted Benzoic Acids 



251 



COOH 

 /F 



/\: 



COOH 



COOH 



n 



COOH 

 .F 



COOH 



.F 



NH2' 



NO; 



IV 



V 



vn 



vni 



COOH 

 .F 



VI 



COOH 



.CI 



F 



IX 



at the 2 position in 2,5-dichlorobenzoic acid with fluorine. Ahhough 

 the 2,5-dichloro compound has the highest S/'^'> value yet calculated 

 for the series and is one of the more active compounds, the 2-fluoro-5- 

 chlorobenzoic acid (VIII) is completely inactive. 



Professor Fukui has kindly sent us his calculations of S/<^^ values 

 for the 2-chloro-4-fluoro- and 2-fluoro-6-chloro- compounds and these 

 are given in Figure 1. It is readily apparent that the effect of fluorine 

 is not a consequence of its size but of its contribution to the electron 

 density at the ortlio position. Fluorine in the para position reduces 

 the S/*^' value at the ortJw position of the chlorine atom to 0.6614, 

 below the value associated with activity in elongation, and the com- 

 pound is completely inactive. With fluorine at the ortho position the 

 S/'^' value for the other ortho position is increased to 0.7152. Thus 

 2-fluoro-6-chlorobenzoic acid has an auxin activity index of 0.1, which 

 is the same as the value for 2,6-dichlorobenzoic acid. 



The S/'N' calculations are of interest also in connection with an 

 apparent effect of the size of the iodine atom on activity. Although 



