Eobertson. — Molecular Convexity of Fatty Acids. 463 



Connection between the Molecular Depression and the 



Bate of Association. 



Among the normal fatty acids both constants tend to in- 

 crease at the same time, though this is not absolute. The 

 disubstituted acids tend to have a slightly higher value for the 

 depression than the mono acids. Other regularities, if they 

 do exist, remain hidden. Even if the acids associate at the 

 same rate the molecular depressions may vary widely. Thus 

 nitroso valeric acid (rate of association 50 per cent.) has a 

 molecular depression of 64 ; ethylmalonic acid, on the other 

 hand (association 44 per cent.), gives the value of the con- 

 stant as 72. Again, chlorphenylacetic and trichloracetic acids 

 associate at the rates of 4 and 25, but their depression con- 

 stants are 71 - 5 and 68. Of the seven acids whose association 

 numbers vary between 18 and 24, all but one give molecular 

 depressions between 69 and 71. 



In spite of these minor relationships, it is clear that the 

 conditions that influence the molecular depression are in- 

 distinct ; in all probability it depends to a large extent on the 

 spatial arrangement of the molecule in solution. 



The Solubility op the Fatty Acids and their Deriv- 

 atives in Phenol. 



The following generalisations may be drawn up : — 



(1.) The fatty acids are readily soluble, but the solubility 

 becomes slight when stearic acid is reached. 



(2.) All the oxy, halogen, and phenyl derivatives are 

 readily soluble. 



(3.) The poly car boxylic acids are characterized by their 

 extreme insolubility : this applies also to their unsaturated 

 compounds, the halogen and oxy derivatives. Their solu- 

 bility, however, is much increased by the introduction of an 

 alkyl group, the effect being smaller the greater the number 

 of carboxyls. 



(4.) The oxy derivatives of the dibasic acids are less soluble 

 than the original acids. 



(5.) Of the unsaturated dibasic acids the cis compounds are 

 much more soluble than their isomers possessing the trans 

 configuration. 



(6.) The a amido acids are extremely insoluble ; their 

 substituted derivatives possess a slight solubility. 



In the following table the differences between the freezing- 

 points of phenol and its saturated solution are given for a 

 number of acids. This depression is roughly proportional to 

 the number of dissolved molecules in a saturated solution of 

 a constant weight : — 



