140 CHEMICAL PROPERTIES AND COMPOSITION 



Since in general the limit of esterification varies little 

 with composition amongst the acids, the esters of the 

 diortho-substituted acids also show a very small velocity. 



Since the general lines of these striking phenomena have 

 thus been indicated, Goldschmidt l followed out further the 

 effect of ortho- substitution by measuring velocities of re- 

 action. When sufficiently diluted with ethyl alcohol the 

 acids suffer under the influence of hydrochloric acid a re- 

 action whose velocity is proportional to the mass of the 

 acid itself, and to that of the hydrochloric acid, thus giving 

 a simple monomolecular equation for the reaction, since 

 the quantity of hydrochloric acid does not change during 

 the experiment. 



In this way the influence of the ortho-substituting group 

 appears very clearly, even when only one of the hydrogen 

 atoms in that position is replaced : 



Benzoic acid 0*0428 Phenyl-acetic acid 3-33 



ortho meta para 



Methylbenzoic acid . . . o-oni 0-047 0-024 



Oxybenzoic acid .... 0-0021 



Nitrobenzoic acid .... 0-0028 0-0296 0-0261 



Bromobenzoic acid . . . 0-0203 0-0553 0-045 



The very great velocity, 3-33, for phenylacetic acid, 

 C 6 H 5 CILCOOH, in which two hydrogen atoms adhere 

 to the carbon which carries the carboxyl group, falls to 

 i per cent, of its value, or 0-0428 for benzoic acid, in 

 which these are wanting. If in the latter the nearest 

 hydrogen atoms, those in the ortho position, are replaced 

 the velocity sinks still further, but without any very 

 remarkable influence due to the mass of the substituting 

 group. 



Another series of phenomena that are most probably 

 due to spatial effects, is associated with Pasteur's decom- 

 position of optical antipodes by organisms described in 

 Part II, p. 115. As Emil Fischer found, this decomposition 

 may be produced outside the organism by enzymes, since, 



1 Berl. Ber. 20. 3218. 



