Bobektson. — Molecular Complexity of Fatty Acids. 457 



The results obtained for the initial molecular depression, 

 on the other hand, do not show this wavy character, but 

 diminish regularly for some distance, apparently reaching a 

 minimum value in the neighbourhood of lauric acid, and then 

 increasing at about the same rate, or perhaps slightly faster. 

 The rate of association was shown to increase much more, 

 quickly than it diminished for the lower members. 



1. (b.) The Isomeric Fatty Acids. — When the chain 

 branches beyond the a carbon the rate of association in- 

 creases : this is shown in the case of isovaleric acid thus — 



Butyric. 



Isovaleric. 

 £g sX > CH • CH 2 • C0. 2 H 



COoH • CH., • CH, • CH 3 25 



8- 



20 



Normal Valeric. 



CH 3 ■ CHo ■ CH, • CH, • COoH 



16 



The numbers placed beneath the acids are their rates of 

 association. The influence of such a grouping probably 

 becomes less as it is further removed from the carboxyl. 



In the acids with one of the a hydrogens replaced by an 

 alkyl group, such as methylethylacetic acid, the rate of asso- 

 ciation is found to be less than that of the corresponding 

 normal acid. That this behaviour is general is confirmed by 

 the results obtained for other disubstituted acetic acids, which 

 are discussed under 2. 



Scanty as this evidence is, there is presented a simple 

 method for discriminating the isomers of higher members. 

 Compared with chemical methods it possesses two great ad- 

 vantages — namely, quickness of execution, and the small quan- 

 tity of material required for the experiment. 



2. (a.) Monosubstituted Acids. — In these acids the halo- 

 gens, methyl, ethyl, and phenyl have practically the same 

 influence, the numbers varyiog from 17 in the case of 

 bromacetic to 20 for phenylacetic acid. Thus the negative 

 groups behave like the positive ; if the substituents are of the 

 same nature the heavier group appears to have the greater 

 influence — e.g., in the case of chlor- and brom-acetic acids, the 

 values of which are 20 and 17. On the other hand, methyl- 

 acetic acid associates more slowly than ethylacetic. 



2. (b.) Disubstituted Acids. — Dichloracetic acid unfor- 

 tunately decomposes in the presence of phenol, giving a 

 solution with a beautiful fluorescence, transmitting light of 

 greenish hue and reflecting red. The only dialkyl acid ex- 

 amined was methylethylacetic, which associates at the rate 

 of 12 per cent. — i.e., slower than either of the acids from 

 which it is derived. 



