CHEMISTRY OF THE IIEXOSES 659 



Acetaldchyde in the presence of water forms a liydrato (coiiiparal)lc to cliloral 

 hydrate). 



H H 



I 

 CH3-C=0 + HOHfiCH,— C—OH 



OH 



This hydrate possesses ionizable hydro(?en in its OH groups, and in the presence 

 of a metallic hydroxide, MOH, can accordingly form a salt (comi)arahle to an 

 H 



alcoholate) ; thus : CH3 - C - OH, 



OM 



and this salt being highly dissociable falls apart into MOH and a "methylene 

 enol " (in this case hydroxyethylidene) : 



H 



I I 



CHs-C-OH^CHa-C-OH + MOH 



OM 



(Herein lies the point of departure of Nef's view from the foregoing.) This 

 methylene enol then rearranges to form the "olefine dienol" CH2 = CHOH (in this 

 case vinyl alcohol). Ketones, on the other hand, form the olefine dienol directly 

 without forming the methylene enol. In the case of KOH and acetone (dimethyl 

 ketone) there is the same formation of the hydrate followed by salt formation and 

 the loss of KOH, but the latter does not all come from one C atom as it does 

 when split out of aldehydes, thus : 



OH OK 



I I 



CHs-C-CHa + KOH^CHj-C-CH,, + HjO^ CH3-C-CH2 

 I 

 OH OH OH 



(hydrate of acetone) 



In a manner entirely analogous to what occurs in the simple aldehydes and 

 ketones, the two aldohexoses, glucose and mannose, and the ketohexose levulose, 

 can form one and the same enol molecule. And vice versa this enol molecule may 

 open its double bond in two ways as shown below ( (a), (b) and (c) ) and the 

 dissociated molecules. 



Enol molecule 

 (a, 1, 2 d-glucose olefine dienol) 



(a) and (b) will be in dynamic equilibrium with the enol (c). Now if H and 

 OH are again taken on by (a) and (b) this assumption of the elements of water 



