ISOMERIC CHANGE 229 



velocity is only attained by increasing the concentration of the 

 catalyst about a thousand-fold. 1 



Neutral Salts have no marked catalytic action on neutral or 

 feebly-acid compounds, such as the sugars, the simple nitro- 

 paraffins, and nitrocamphane. Stronger acids are, however, 

 capable of displacing the mineral acid from a salt, and when this 

 is the case a neutral salt may exert a catalytic action comparable 

 with that of the free bases. Quantitative experiments with nitro- 

 camphor have shown that sodium chloride is a powerful catalyst, 

 although about 800 times less efficient than sodium ethoxide. 

 This action of neutral salts is of the utmost importance in 

 attempting to check the progress of isomeric change by purify- 

 ing the materials, since the use of glass vessels inevitably 

 results in serious contamination, and satisfactory conditions can 

 only be obtained when contact with glass is avoided by using 

 apparatus made of silica. 2 



Acids exert a marked catalytic action on the isomeric changes 

 of the sugars and of nitrocamphor, but this is much less powerful 

 than when bases are used ; in aqueous solutions of glucose 

 hydrochloric acid was found to be about 100 times less efficient 

 than caustic soda ; in the case of nitrocamphor dissolved in 

 benzene trichloracetic acid was about 1,000 times less efficient 

 than piperidine. 



Water also acts as an efficient catalyst, 1 per cent, of water 

 added to an alcoholic solution of nitrocamphor producing the 

 same acceleration as sodium ethoxide at a concentration of 

 N/ 1 0,000. 



(d) Arrest of Isomeric Change. — In the case of compounds 

 which do not change rapidly and are not specially sensitive to 

 the action of catalysts, there is no difficulty in holding back 

 isomeric change even in presence of a solvent. This fact is 

 illustrated by practically the whole of the compounds discussed 

 in connection with the first group of isomeric changes ; it is 

 seen in a particularly striking form in the case of compounds 

 which only change in presence of an acid catalyst, and may be 

 preserved indefinitely in alkaline solutions. In the case of 

 compounds of the second group, which are specially sensitive 

 to alkaline catalysts, opposite conditions prevail ; the presence 



1 Forster, Trans. Che in. Soc. 1901, 79, 999. 



s Silica tubes and discs for polarimetric work can be obtained of excellent 

 quality and at a low price from the Silica Syndicate. 



