PURDIE'S REACTION 195 



yielded tetramethyl gluconolactone. Bearing in mind the 

 tendency of gluconic and similar acids to form y-lactones, 

 it will be seen that the unmethylated carbinol group in tetra- 

 methyl gluconic acid and in tetramethyl a-methylglucoside 

 is that in the y-position. It is therefore the y-carbon atom 

 in the methylated and in the parent glucosides which is 

 united with the oxygen atom of the ring. These facts are of 

 great importance, since they constitute what is probably the 

 strongest experimental evidence in favour of Fischer's formula 

 for glucosides. 



It has been pointed out that, excluding a few doubtful 

 exceptions, the silver oxide reaction does not cause race- 

 misation of reacting substances ; the sugars furnish a striking 

 illustration of this point. Not only do the methylated 

 compounds mentioned above retain optical activity, but they 

 are chemically and optically more stable (if such a term may 

 be used) than the corresponding unmethylated substances. 

 It is beyond the scope of this paper to discuss, in a general 

 manner, the results which have been obtained by the study 

 of the optical activity of the methylated sugars and their 

 compounds. These results are of importance, and are likely 

 to prove of further value in any generalisations on the 

 activity of the sugar group, since in the methylated com- 

 pounds the constitutive influence of the hydroxyl groups is 

 much modified, if not entirely eliminated. 



Tetramethyl glucose, as was to be expected, is very 

 soluble in organic solvents. It is therefore possible to examine 

 the substance polarimetrically in solutions from which water 

 is absent and in which no ionisation can occur. As is well 

 known, the reducing sugars exhibit mutarotation in aqueous 

 and in alcoholic solutions, and this mutarotation is now 

 believed to be due to the interconversion, in such solutions, 

 of the a- and /8-forms of the dissolved sugar, these two forms 

 being dynamic isomerides, differing in the position in space 

 of the hydrogen and hydroxyl radicles attached to the terminal 



