80 PROCESSES OUTSIDE THE LIVING CELL CHAP. 4 



The first reaction in Wislicenus' scheme (4.18), is feasible, because all peroxides 

 have approximately the same energy content; but the decomposition of percarbonic 

 acid into formic acid and oxygen is as impossible as a spontaneous monomolecular 

 decomposition of hydrogen peroxide into hydrogen and oxygen. Peroxides decompose 

 spontaneously only by himolecular dismutation into oxide and oxygen {cf. Chapter 11). 

 If we replace (4.18b) by such a decomposition, the net result will be merely a carbonate- 

 catalyzed decomposition of hydrogen peroxide. 



Thus, none of the known chemical methods of reduction of carbon 

 dioxide appears significant from the point of view of artificial photo- 

 synthesis. However, it seems probable (c/. Chapter 8) that the immediate 

 substrate of reduction in nature is not free carbon dioxide at all, but 

 carbon dioxide incorporated, by enzymatic catalysis, into a large organic 

 molecule, probably with the formation of a carboxyl group: 



(4.20) RH + CO2 > RCOOH 



Once association (4.20) has taken place, the reduction of carbon dioxide 

 to carbohydrate can be replaced by the reduction of the carboxyl group, 

 RCOOH, to the carbinol group RCH2OH: 



(4.21a) CO2 + RH > RCOOH 



(4.21b) RCOOH + 4 H > RCH2OH + H2O 



(4.21c) RCH2OH > RH + { CH2O 1 



(4.21) CO2 + 4 H ^ {CH2O) + H2O 



Furthermore, the reduction of one molecule of acid to one molecule of 

 carbinol can be replaced by the reduction of two molecules of acid to 

 two molecules of aldehyde, and the dismutation of the latter compound 

 (Cannizzaro reaction) : 



(4.22a) 2 RCOOH + 4 H > 2 RCHO + 2 H2O 



(4.22b) 2 RCHO + H2O > RCOOH + RCH2OH 



(4.22) RCOOH + 4 H > RCH2OH + H2O 



Thus, the chemical problem of carbon dioxide reduction to a carbo- 

 hydrate, can be replaced by the problem of the reduction of a carboxylic 

 acid to an aldehyde. The methods by which this reduction is achieved 

 in organic chemistry are, however, as violent as those used for the 

 reduction of carbon dioxide, i. e., they involve either very strong reduc- 

 tants (sodium amalgam, or hydrogen and palladium under high pressure), 

 or high temperatures (dry distillation of calcium salts). Thermody- 

 namical constants show that there is not much difference between the 

 energies of reduction of carbon dioxide and carboxyl (cf. Table 9. IV); 

 but the substitution of a large molecule of a carboxylic acid for the small 

 molecule of carbon dioxide may decrease the activation energy, and thus 

 make the reduction easier. The free radicals, HCO2 and H3CO2, which 

 must arise as intermediates in the reduction of carbon dioxide if the 



