458 Transactions. — Chemistry and Physics 



Propionic 



CH 3 • CHo • COoH 



18 



Butyric. 



CH 8 • CH, • CH 2 • CO,H 

 20 



Methylethylaceti c . 



CH 3 • CH • C0 2 H 



CH 3 . CH 2 

 12 



On replacing the methyl in this compound by bromine the 

 rate of association remains practically the same : this was also 

 found in the case of bromacetic acid. 



Propionic. Bromacetic. 



CH 3 • CH, • C0 2 H Br • CH, ■ C0 2 H 



18 17 



Methylethvlacetic. Bromethylacetic. 



CH 3 • CH 2 • CH(CH 3 ) • CO,H CH 3 ■ CH 2 • CHBr • C0 2 H 

 12 11 



Whereas the phenyl group has an effect approximately equal 

 to an alkyl or a halogen in the monosubstituted acids, its 

 influence is strongly normalising in the diacetic acids. 



It is interesting to compare the rates of association of the 

 following acids : — 



Phenylacetic. Butyric. 



C 6 H 5 ■ CH 2 ■ C0 2 H C,H 5 ■ CH, * CO,H 



20 20 



Chlorphenylacetic. Chlorethylacetic. 



C 6 H 5 • CHC1 • C0 2 H C 2 H 3 • CHC1 • C0 2 H 



_ 4 (12) 



The number in brackets is calculated by analogy from the 

 similarly constituted brombutyric acid. 



Diphenylacetic acid also probably associates very slowly, 

 as its oxy-derivative shows the low value of 5'5. Thus we see 

 that when one of the groups in a disubstituted acetic acid is 

 a phenyl the acids associate much less rapidly than the com- 

 pounds with alkyl or halogen groups. The explanation of this 

 perhaps lies in the stereo-chemistry of the phenyl in the mole- 

 cule. At any rate, it renders the a hydrogen which causes 

 the association in these acids practically inactive. 



It is interesting to note that all the acids of this type have 

 a high value for the initial molecular depression. 



2. (c.) Trisubstituted Acids. — As would be expected, tri- 

 chloracetic acid possesses a rate of association of only 2-5 per 

 cent. The onlv other acid of this class in the list is benzilic 

 acid, (C 6 H 5 ), ■ COH ■ CO,H, whose association (7 per cent.) is 

 no doubt increased owing to the action of the hydroxyl group 

 (see under 4). 



3. The Dicarboxylic Acids. — These acids, as well as the 

 tricarboxylic acids, with their halogen and oxy derivatives, are 



