PROCEEDINGS OP SECTION B. 115 



12.— THE DYNAMICS OF CERTAIN ACID CATALYSES. 

 By A. 0. D. Rivett, B.A., B.Sc. {Oxon.), D.Sc. (Melh.). 



(Abstract.) 



Broadly speaking, there would appear to be three types of acid 

 catalyses, represented respectively iu the following cases : — 



(1) The hydration of acetic, and probably other, anhydrides. 



(2) The hydrolysis of esters and acid amides. 



(3) The intramolecular rearrangement of acetchloranilide to 



p-chloracetanilide. 



In (1) it was found by Rivett and Sidgwick (Journ. Chem. Soc, 

 1910, 97, 733) that for acetic anhydride in aqueous solution the 

 velocity of hydration decreases as initial concentration of anhydride 

 increases, and this does not suggest catalytic activity of hydrogen ion. 

 But Orton and Jones (Journ. Chem. Soc, 1912, 101, 1708) proved that 

 presence of much hydrogen ion (as from mineral acids) the velocity 

 was increased, though but slightly. In anhydrous media, the acids 

 are, on the other hand, powerful catalysts. In media ranging from 

 pure acetic acid to pure water, the curious result is found that in 90 per 

 cen,t. acetic and above, molecular quantities of acids are equally effective, 

 but in 50 per cent, and below, equivalent quantities have equal effect. 

 The available figures are not extensive, and hardly suffice to decide what 

 the catalyst actually is. So far as they go, they suggest that hydrogen ion 

 may be the main active agent in aqueous solution, as Orton and Jones 

 suggest, but that in anhydrous media the undissoeiated molecules are 

 the principal catalysts. Possibly both ions and molecules are active, 

 and the activity varies with the solvent. 



As regards (2), it has been known for a long time that the rate of 

 hydrolysis of esteis is only roughly proportional to the concentration 

 of hydrogen ion. Following a suggestion of Arrhenius (Zeit. phys. 

 Chem., 1899, 28, 329), regarding the inversion of cane sugar, Lunden 

 (Zeit. phys. Chem., 1904, 49, 189) has shown that in the hydrolysis of 

 ethyl acetate the relations are best expressed by A; = am + hm^, 

 where Jc is velocity constant and m is hydrogen ion concentration, a and 

 h are constants. This second term gives the effect of the ions upon 

 the catalytic activity of the hydrogen ion. When salts of the acid 

 catalyst are present, another term has to be added for the effect of the 

 metallic ion on the activity. 



Concerning (3), detailed results were communicated upon the 

 dynamics of the intramolecular rearrangement of acetchloranilide to 

 p-chloracetanilide (see tvro papers in Zeit. phys. Chem., 1913). It was 

 shown that with hydrochloric acid as catalyst, the connexion between 

 acid Concentration and velocity constant was best expressed by 

 k = {I -a) C {A + BaC), where A and B are constants and a is 



