28 



PLANT GROWTH SUBSTANCES 



A heteroatom may take the place of carbon in the side chain, as in 

 naphthoxy-2-acetic acid (XXIII), with an oxygen atom, and 2-methyl- 

 4-chIorophenylthioacetic acid (XXIV), (11) with a sulfur atom, which 

 is highly active. (See Table i.) Phenoxyacetic acid and phenylthioacetic 

 acid, however, are inactive (10), while phenylpropionic acid has a little 

 activity, so that the oxygen and sulfur atoms are less effective than the 

 carbon. Phenylglycine with a nitrogen atom is also inactive. 



A striking effect is caused by introduction of a hydroxyl into the 

 side chain. The activities are as follows (compared to indoleacetic acid 

 as 100): 



Parent Compound 

 Phenylacetic 10 



a-phenylpropionic (XXV) 6 



Indeneacetic (III) 



20 



CH, 



I 



C — COOH 



H 



Hydroxy Derivative 

 Mandelic 



Atrolactic (XXVI) 

 Tropic (XXVII) 

 Indeneglycohc 

 ethyl ester (XXVIII) 



o 



CH, 



CHjOH 



C — COOH 



I 



OH 



•COOH 



xx? 



VXVI 



^ 



-C — CH.COOH 



II ' 



CH 



m 



XXVII 



-C — CHOHCOOC,H, 



II 



-CH 



XXVIII 



Recently Went has reported (17) that mandelic and tropic acids are 

 also virtually devoid of preparatory activity (see below). Thus the 

 hydroxyl in the side chain essentially destroys all activity. An original 

 explanation will be offered below for this clear-cut effect. It must be 

 noted, however, that auxin-a with its 3 hydroxy Is is again an exception. 



The Problem of Spatial Configuration. — To the cases of the cinnamic 

 acids and the chain-substituted phenylacetic acids many interesting 

 observations have been added. Unfortunately their meaning is not yet 

 clear. 



There are now three cases in which optical isomerism influences ac- 

 tivity. The optical isomers of indole-a-propionic acid (XXIX) were 

 tested by Kogl and Verkaaik (6) in the standard Avena test. The (+) 



