On form and function of plant growth substances 



molecular models reveal that in fact substituents in the 8-position of the 

 naphthalene ring interfere more with a flat structure than do 2-substituents. 



COOH COOH R COOH 



-CI 



y\ /^, 



R = CH3, CI, Br, I 



IX 



X 



XI 



We would expect therefore 8-substituted a-naphthoic acids to be active, as 

 actually was found for 8-methyl-, 8-chloro-, 8-bromo, and 8-iodo-a-naphthoic 

 acid (XI) {Figure 4). 



In these compounds the COOH group is really fixed in what we indicated 

 as the active spatial position (see Fioure 5), of which they thus provide one of 

 the best illustrations. 



Fioure 5. Molecular model of ^-methyl-y.-naphthoic acid, showing the fixed {'active') position of the 

 COOH group. 



The fact that for such a rotation of the carboxyl group out of the plane of 

 the ring-system (frequently enovigh to give the molecule in the mean an 

 active non-planar structure) the 'help' of a steric hindrance is apparently 

 needed, and since this is not effected to the same extent by the forces respon- 

 sible for the growth substance-receptor interaction it would appear that these 

 forces are weak ones. In our opinion, for this same reason, the participation 

 of the COOH group in a chemical bond (e.g. an amide bond, as proposed 

 throughout the discussions by Muir and Hansch) may be ruled out. 



VARIATIONS OF THE POLAR GROUP 



It has been shown before that activity is not restricted to carboxylic acids, 

 but that COOH may be replaced by a primary NOg group (Veldstra, 1 944) 

 or by SO3H (Veldstra et at., 1954). In the latter case, in a quantitative 



123 



