64 RESPIRATION 



processes of plants, it is desirable to examine the mechanism 

 of oxidation somewhat closely. 



THE MECHANISM OF OXIDATION. 



Oxidation may be effected in one of two ways ; either by 

 the addition of oxygen to, or by the removal of hydrogen 

 from, a given compound. Similarly reduction is regarded as 

 the addition of hydrogen to, or the removal of oxygen from, a 

 given compound. 



The change from hydroquinone (I.) to quinone (II.) by the 

 removal of two atoms of hydrogen may be regarded as oxi- 

 dation. 



OH o 



/\ /\ 



| | + 0->H,0 + | I 



OH O 



(I.) (II.) 



Likewise the conversion of alcohol into aldehyde, which in- 

 volves the removal of two atoms of hydrogen, is an example 

 of oxidation 



CH 3 CH 2 OH + O = H 2 O + CH a CHO. 



This conversion of alcohol into aldehyde can be catalytically 

 effected by shaking the alcohol with palladium black in the 

 absence of oxygen.* This is due to the affinity of palladium 

 for hydrogen, resulting in the formation of palladium hydride, 

 PdH 2 . The reaction, however, soon comes to an end unless 

 the hydrogen is removed ; this may be effected by means of a 

 substance, such as quinone or methylene blue, which will act 

 as an hydrogen acceptor; if quinone is employed, this sub- 

 stance takes up the hydrogen, becoming reduced to hydro- 

 quinone, a colourless substance ; by the use of methylene blue, 

 however, the change is made visible since the reduction of the 

 quinonoid methylene blue involves a loss of colour with the 

 formation of a colourless, or leuco, compound. A more appro- 

 priate example, in that it recalls a once not uncommon labora- 

 tory experiment, is found in the changes brought about in an 

 aqueous solution of methylene blue by the living plant : a 



* Wieland : " Ber. deut. chem. Gesells.," 1912, 45, 488, 2606. 



