262 PHOTOSYNTHESIS 



COOH, yield aldehydes and formic acid and he thought that the same 

 reaction might he applied to carbonic acid. The carboxyl group (-COOH) 

 would be reduced to formic acid (H-COOH) and the remaining two free 

 hydroxy! groups united to form hydrogen peroxide. Finally the formic 

 might undergo a similar reaction with the formation of formaldehyde and 

 another molecule of hydrogen peroxide. These reactions may be written 

 as follows: 



OH 



/ 



= C +H.OH ^HCOOH + HO.OH 



\ 



OH I 



H2O + O 



H H 



/ 



= C +H.OH >0=:C +HO.OH 



\ 

 OH HI 



H2O + O 



According to Erlenmeyer's view, formaldehyde is obtained from car- 

 bonic acid by way of formic acid. This is the first proposal which assumes 

 the formation of a peroxide. The scheme also implies that it is not 

 carbon dioxide but carbonic acid, H2CO3, which is reduced and it is essen- 

 tially a formaldehyde theory. 



There have been a number of variations to the ideas of Liebig and of 

 Erlenmeyer ; most of these have had very little experimental work as a 

 basis.^" 



Thus Ballo," in a measure, united the views of Liebig and of Erlen- 

 meyer. He assumes that formic acid is the first reduction product, this is 

 oxidized to oxalic acid which in turn yields tartronic -^ tartaric -» saccharic 



acids. 



Bach ^^ has proposed a mechanism which supposes a reaction analogous 

 to the oxidation of sulfurous acid to sulfuric acid and sulfur; thus car- 

 bonic acid would yield percarbonic acid : 



3H2CO3 = 2H2CO4 + HsO-f C 



formaldehyde 



2H2CO4 = 2CO2 + 2H2O2 = 2CO2 + 2H2O + O2 

 3H2CO3 = 2H2CO4 + H,CO = 2COo + 2H2O + Oo 



"Brunner and Brandenburg, Ber. chem. Ges., 9, 984 (1876). Reinke, Ber. chem. 

 Ges 14, 2149 (1881). Brunner and Chuard, ibid., 19, 595 (1886). Konigs, ibid., 

 25, 800 (1892). 



"Ballo, Ber. chem. Ges., 17, 10 (1884). 



"Bach, Compt. rend., 116, 1145 (1893). 



