MECHANISM OF DEHYDROGENATION 65 



healthy twig of Elodea^ placed in a test tube of cold well- 

 boiled water coloured blue with the dye, is placed in the dark ; 

 after a time the water is quite colourless but quickly assumes 

 its blue colour on exposure to sunlight, owing to the evolution 

 of oxygen by the plant. This may be explained on the as- 

 sumption that during the respiration of the plant hydrogen is 

 absorbed by the methylene blue, the hydrogen acceptor, which 

 is thereby converted into the leuco compound. The subse- 

 quent presence of oxygen, on exposure to light, brings about 

 the re-oxidation of the leuco compound into methylene blue 

 and water. 



This mechanism of dehydrogenation is, according to Wie- 

 land, the one underlying many, if not all, oxidations and 

 especially those associated with enzymes. 



Reverting once more to the dehydrogenation theory of 

 oxidation, it has been shown by Wieland that the oxidation 

 of aldehyde to acid may be explained on the same principle, 

 since palladium black has no action whatever upon the dry 

 aldehyde. This is explained by the fact that the dry aldehyde 

 has not in itself any hydrogen for activation but that in the 

 presence of moisture it forms the hydrate 



.0 OH 



+ HOH = C 



H H 



Palladium black can then activate two hydrogen atoms in 

 this latter compound and these are then removed by a suitable 

 hydrogen acceptor : 



/OH /OH 



CH 3 C^-0;H! -> CHjC/ + H 2 



^Hj ^Q 



This view is supported by the fact that whereas chloral, 

 CC1 3 CHO, is unacted upon by palladium and methylene blue, 

 that well defined substance chloral hydrate, CC1 3 CH(OH) 2 , is 

 at once oxidized. Further corroborative evidence concerning 

 the essential part played by water is provided in the fact that 

 even such a well marked aldehyde-oxidizing agent as silver 

 oxide has no action upon dry aldehyde but oxidizes it readily 

 when wet. 



//* / OH 



CH 3 C;f + HOH = CH 3 C^-OH + Ag 2 O = CH 3 COOH + aAg + H 2 O 



\H \H 



VOL. II. 5 



