HYDROGEN EXCHANGE BETWEEN INTERMEDIATES 159 



SO that the over-all reaction becomes: 



(7.8) 4 {CO2I + 2 H2O "-^ {CH2O) + 3 {CO2} + H2O + O2 



catalysts 



The reaction mechanism (7.8) is represented graphically in scheme 7. III. 



4C02 SHjO 



3CO2 +{CH20J -t-HjO 4HX Og 



Scheme 7.III. — Photosynthesis, with reduction of carbon dioxide (in the form of a 

 compound {CO2}) by an intermediate catalyst as the primary photochemical process 

 (second four quanta theory). 



If {CO2I is a carboxyhc acid (c/. Chapter 8), reaction (7.8c) is analo- 

 gous to the "Cannizzaro reaction" (4.22b). As an analogy to (7.8b), 

 we may mention the dismutation of semiquinones into quinones and 

 hydroquinones. For example, the reduction of thionine by ferrous ions 

 in light — a reaction whose first step was described on page 000 as similar 

 to the primary process in photosynthesis — runs to completion by the 

 dismutation of the primary reduction product (semithionine) into 

 thionine and leucothionine: 



(7.9a) 2 Thionine + 2 Fe+++ > 2 semithionine + 2 Fe++ 



(7.9b) 2 Semithionine > leucothionine + thionine 



2hi' 



(7.9) Thionine + 2 Fe+++ > leucothionine + 2 Fe++ 



In van Niel's mechanism (7.6), four water molecules participate in the 

 primary reaction, and three of them are recovered; in mechanism (7.8), 

 four carbon dioxide molecules participate in the primary reaction, and 

 three of them are restored at the end. 



The dismutation of the radicals, {HCO2}, can take place either 

 directly, as assumed in (7.8b), or through the intermediate formation of 

 "biradicals," {HC02J2, analogous to the peroxides {OHI2, postulated 

 in (7.4a,b). 



4. Hydrogen Exchange Between Intermediates as the Primary Process 



Third Four Quanta Theory 



In the absence of decisive arguments in favor of a direct association 

 of the primary photochemical process with either carbon dioxide or 



