228 SCIENCE PROGRESS 



(c) Influence of Catalysts. — The contrast between the two 

 groups of isomeric changes is seen in a very striking form 

 in the influence of catalytic agents. In the first group it is 

 usually necessary to employ somewhat drastic methods of treat- 

 ment in order to effect a change, and even powerful catalytic 

 agents, such as the mineral acids and the halogens, have often 

 to be applied at high temperatures and for prolonged periods 

 before the transformation can be brought about. In the second 

 group of compounds the problem is entirely opposite in character, 

 the changes taking place so easily that it is a matter of the 

 utmost difficulty to arrest them or to obtain proof that they 

 do not proceed spontaneously in the absence of any catalytic 

 agent whatever. Such evidence has, indeed, only been forth- 

 coming in a limited number of instances, but there is no reason 

 to doubt that similar methods of investigation would lead to 

 analogous results in other cases. 



The difficulty of investigation increases with the velocity of 

 isomeric change and its sensitiveness to the catalytic action 

 of minute traces of impurity. Relatively speaking, this velocity 

 is low in the case of the ketonic compounds, since many of these 

 can be recrystallised without undergoing isomeric change, 

 provided that care is taken to crystallise rapidly and at a low 

 temperature. The isodynamic sugars change more rapidly, 

 at least in aqueous solutions, and in the nitro-compounds the 

 velocity of change is still higher. These velocities are, in all 

 probability, related somewhat closely to the acidity of the 

 hydrogen atom, which is extremely slight in the ketones, but 

 very pronounced in certain of the nitro-compounds, such as 

 nitroform, CH(N0 2 ) 3 % C(N0 2 ) 2 : N0 2 H, benzoylnitromethane, 

 C H 5 . CO . CH 2 . N0 2 ■$- C 6 H 5 . CO . CH : N0 2 H, and nitro- 

 camphor. 



Catalytic Action of Bases. — All these compounds are peculiarly 

 sensitive to the catalytic action of bases, a fact that is perhaps to 

 be attributed to the greatly increased tendency to ionisation 

 which is observed when a weak acid is converted into a metallic 

 salt. A climax of sensitiveness is reached in the case of nitro- 

 camphor, which responds distinctly to the addition to its solution 

 in benzene of piperidine to a concentration of N/io,ooo,ooo, 

 i.e. roughly one part in one hundred million or one decigramme 

 per ton of solvent. The sugars are much less sensitive, and 

 in the case of a typical diketone, benzoylcamphor, an equal 



