CHEMORECEPTION 133 



the CH3 (in the above representation) is increased the stimulating 

 efficiency decreases. Thus, glycols are less effective than alcohols, and 

 diketones are as a rule less effective than ketones. Increase of the 

 number of substitutions of those polar groups to the left of the CH3 

 tends to increase stimulating effectiveness. Thus, dibromo compounds 

 are more effective than monobromo homologues. 



The effects of the positions of the functional groups are illustrated by 

 experiments with the glycols. The following rules are found to hold : 



(1) juxtaposition of two hydroxy! groups in a short molecule 

 (e.g., 1, 2-butanediol) makes for a high threshold; this effect is re- 

 duced as chains become longer; 



(2) in a chain with no terminal OH groups, i.e., subterminal but 

 not adjacent (e.g., 1, 3-butanediol) the thresholds are low; 



(3) if both OH groups are terminal thresholds are intermediate; 



(4) branching tends to raise the threshold, other factors being 

 equal. 



Taken all together, the foregoing results state essentially that the 

 length of the free alkyl group largely determines the stimulating 

 effectiveness and that its power is modified to varying degrees by the 

 nature of the attached polar groups. The length of the alkyl group is 



4' 



-i\\ 



M W. " 



-3 -2 



LOe SOLUBILITY GRAM MOLES PER UTRE 



Fig. 78. Correlation between concentration required for stimulation and 

 the solubility of compounds at 25-27 degrees C. in water. Each of the 

 forty-six points represents a different aliphatic compound. (From 

 Dethier and Chadwick, 1950.) 



