THE CHEMISTRY OF PHOTOSYNTHESIS 



263 



The criteria upon which Bach bases his reactions have, however, been 

 found to be erroneous by Euler.^^ 



Another system involving the formation of organic acids as inter- 

 mediate products in the reduction of carbon dioxide to formaldehyde has 

 been formulated by Baur." He assumes the following steps : 



H2C2O4 + 2H2CO0 > CH20H.COOH + H,0 + 2C02 



CH2OH.COOH > CH0O + H2CO3 



2H2C2O4 ^ CH2O -f 3CO2 + H2O 



Bauer considers that oxalic acid is the first reduction product of carbon 

 dioxide and figures that the formation of this reduction product requires 

 ^ to % less energy than the direct reduction to formaldehyde ; in alkaline 

 solution, i.e. with the salts of the acids, even less energy is required. The 

 oxalic acid is supposedly reduced by means of the formic acid. The theory 

 has been critically examined by Euler. 



Tschelinzefif ^^ has also suggested that a condensation of carbonic acid 

 and the subsequent splitting out of hydrogen peroxide may be obtained 

 through the secondary valencies of the carbonyl group by the formation 

 of oxonium compounds. This theory he developed for the aldol con- 

 densation and has attempted to apply it to other compounds containing a 

 C = O group. Thus he considers the following steps as offering a means 

 of passing from carbonic acid to comix)unds containing less oxygen : 



HO — C — 



HO — C — 



The peroxide thus formed decomposes with the elimination of oxygen and 

 water and the formation of oxalic acid. 



"Euler, Bcr. cheni. Ges., 37, 3411 (1904). 



"Baur, Naturu'tsscnscliaften, 1, 474 (1913). Parnas, ibid., 1, 819 (1913). 

 Euler. Zcit. phxsiol. Chem., 59, 122 (1909). 



"Tschelinzeff, Bull soc. chim., 37, 181 (1925). 



