DEHYDRASE 139 



according to which the activation of hydrogen is the all-im- 

 portant preliminary condition for oxidation. It is well known 

 that the oxidation of a substance may be effected either by the 

 addition of oxygen or the removal of hydrogen ; as an example 

 of the latter type may be taken the conversion of hydroquinone 

 into quinone — 



OH O 



H 2 0+ ( 



o 



or the conversion of ethyl alcohol into acetic aldehyde — 

 CH 3 CH 2 OH + O -> CH3CHO + H 2 0. 



But even the conversion of acetic aldehyde into acetic acid, 

 CH3CHO + o = CH3COOH 



which might be regarded as a direct addition of oxygen, must, 

 according to Wieland, be regarded as a dehydrogenation, since 

 he has shown that the reaction 



CH 3 CHO + Ag 2 = CH s COOH + aAg 



cannot take place in the entire absence of water ; this he 

 regards as evidence for the preliminary formation of a hydrate 

 which may then be oxidised by dehydrogenation according 

 to the following stages : — 



/OH 



ch3cho + h,0 = ch 3 c^-oh 



\h 



/OH /OH 



CH 3 CAoiH : + = CH s C< +H 2 0. 



Nh J ^° 



This conversion of aldehyde into acid may be effected in 

 the entire absence of oxygen, provided some other acceptor 

 for the hydrogen is substituted ; such a substance is methy- 

 lene blue, which, provided a suitable catalyst be present, will 

 remove the hydrogen, becoming itself reduced to the colourless 

 leuco-compound :— 



/OH /OH 



CHjC^-qH: + MB = MBH, + CH 3 C^ 



Nh" 



