PRINCIPLES OF STRUCTURE 



45 



result that only a limited number of water dipoles can be attached. 

 If very little water is present, all the hydroxyl groups of the alcohol 

 molecules accumulate round the few water dipoles available (Fig. 41b) ; 

 in 96% ethyl alcohol, for instance, 9-10 CH3CH0OH round each HgO 

 molecule. This water is bound so strongly, that it can only be separated 



op o 9 



00 0^0 



^•00.- 'QP^ 



Fig. 41. Solubility. Water molecules and OH-groups hatched, lipophilic 

 groups (-CH3, -CHj-, -O- bridges) black. O.x^'gen groups (-OH and -0-) 

 surrounded. fl)Ethanol CHa-CHg-OH and water HgO (unlimited miscibil- 

 ity). ^) bound water in 96 °o ethanol, c) ethanol and ethyl ether 

 CHg-CHa-O-CHg-CHg (unlimited miscibility), d) water in "moist" ethyl 

 ether (very limited miscibilit}-). 



from the hydroxyl groups by chemical means. As is well-known, ab- 

 solute ethyl alcohol cannot be obtained by distillation, but only by 

 chemical dehydration. 



A still more simple reasoning applies to the miscibility of alcohol and 

 ether (Fig. 41c). Notwithstanding the homopolar character of the 

 ether bridge, i.e., the -O-group, it still has a certain affinity for the 

 OH-group. Consequently, both the hydroxyl group and the alkyl group 

 of the alcohol can enter into some chemical relationship with the two 

 parts of the ether molecule. This is not so when we attempt to dissolve 

 water in ether. The -O-bridge has, admittedly, a certain affinity for 

 water, but this affinity is slight, so that only a limited number of water 

 molecules can be bound by a given number of ether molecules (Fig. 

 4 id). The circumstances are similar to those in 96% ethyl alcohol — 



