RELATION OF ALDEHYDES TO ACIDS. Ill 



which, upon exposure to air, quickly changes by a direct absorp- 

 tion of oxygen into benzoic acid, thus : 



Benz-aldehyd Oxygen Benzoic acid 



C 7 H 6 + C 7 H 6 S 



Oxygen, then, which removes hydrogen from the alcohols, attaches 

 itself directly to the aldehydes, and thereby distinguishes the one 

 class of compounds from the other ; even when, as in the case of 

 ally 1-alcohol and propionic aldehyd, the two bodies have the same 

 ultimate composition, represented in their case by the empiric 

 formula C 3 H 6 O. 



(118.) Although, however, the characteristic property of the 

 aldehydes is to absorb oxygen with conversion into acids, they 

 may nevertheless be rehydrogenised into alcohols, as by the 

 method of Wurtz referred to in my last lecture, thus : 



Benz-aldehyd Hydrogen Benzyl- alcohol 



C 7 H 6 + H a C 7 H 8 



Or, still more curiously, one moiety of the reacting aldehyd may 

 be oxidised into the corresponding acid or its salt, and the other 

 moiety simultaneously hydrogenised into the alcohol, as in Can- 

 nizzaro's well-known process : 



Benz-aldehyd Potash Potas. benzoate Benzyl-alcohol 



2C 7 H 6 + KHO = C 7 H 5 K0 3 + C 7 H 8 



Correlated, then, with every alcohol and acid is an intermediate 

 aldehyd, several of which bodies, in addition to benz-aldehyd, are 

 familiarly known to us in the form of essential oils. Thus, the 

 essential oils of chamomile, cinnamon, spiraea, and rue, contain 

 the angelic, cinnamic, salicic, and methyl-rutic aldehydes, con- 

 vertible by oxidation into the angelic, cinnamic, salicic, and rutic 

 acids respectively. The prime characteristic then of the aldehydes 

 is their acidifiability by direct absorption of oxygen ; so that, con- 

 sidered as products of oxidation, they may be looked upon as in- 

 completely formed acids. When, therefore, we find that by treating 



