454 J. van Ovcrbeek 



(5) Since the polar group of the auxin molecule has to become part 

 of an oscillating H-bond system, this requires it to be undissociated 

 and isolated electronically from the rest of the molecule. If a polar 

 group such as carboxyl is in resonance with the benzene ring, elec- 

 trons are withdrawn from the group and it becomes more highly 

 dissociated. The insulation of the polar group is achieved in the 

 phenyl and phenoxyacetic acids by the carbon atom between the 

 carboxyl group and the rest of the molecule. The insulation of the po- 

 lar group in the benzoic acids is achieved by forcing the carboxyl 

 group out of the plane of the ring by the two bulky ortJio substitu- 

 ents and thereby minimizing resonance interaction between the car- 

 boxyl group and the benzene ring. This simple reasoning also ex- 

 plains the interesting behavior of the naphthoic acids (1). 1-Naphthoic 

 acid is like an unsubstituted benzoic acid. Its carboxyl group is in 

 resonance with the ring; therefore, the acid is relatively strong and 

 poorly fit for H-bonding. 1-Naphthoic acid, therefore, is poorly ac- 

 tive as an auxin. Auxin activity is vastly increased by the simple 

 expediency of saturating the bond next to the carboxyl group (XIV, 

 XV). A tetrahedral structure is thereby achieved, whereby the car- 

 boxyl group is forced out of the plane of the ring, and thus removed 

 from resonance interaction with the double bonds in the ring. It 

 becomes a weaker acid and thus becomes more suitable for partici- 

 pation in H-bonding. In addition, of course, this lateral movement 

 of the carboxyl group places it in a more favorable position to par- 

 ticipate in the oscillating H-bond network (see number 3 above). 



Let us examine an auxin molecule and try to explain, with 

 the aid of Figure 2, why the molecule is active. Take 2,3,6-tri- 

 chlorobenzoic acid. It is active because: 



(1) Its chlorine atoms make it partition more into the fat of the 

 membrane of the cytoskeleton than the unsubstituted benzoic acid; 



(2) The chlorine atoms in the 2 and 6 positions force the carboxyl 

 group out of the plane of the ring. This insulates the carboxyl elec- 

 tronically from the ring system and, in addition, places the carboxyl 

 group in the lateral position needed for becoming a partner in the 

 H-bond network; 



(3) The clilorine atoms in the 2 and 6 positions prevent the ring 

 from sinking into the cytoskeleton too far; 



(4) The chlorine atom in the 3rd position helps in anchoring the 

 ring more firmly by van der Waals forces. This makes the 2,3,6- 

 substituted benzoic acid a more active auxin than the 2,6-substituted 

 benzoic acid. 



LITERATURE CITED 



I. Hcacock, R. A., Wain, R. L., and Wiglitman, V. Sliulics on plant growth- 

 regulating substances. XII. Polycyclic acids. Ann. Appl. Riol. 46: 352-365. 1958. 



