290 LIBERATION OF OXYGEN CHAP. 11 



With this compound as hydrogen donor, the primary photochemical 

 process (7.2) becomes: 



(11.15b) 4 HR'— R"OH + 4 X ^ 4 HX + 4 HR'R"0 



with the arrow indicating a free valency. X is used in equation (11.15b) 

 as the designation for the oxidant (instead of Z, as in Eq. 7.2), because 

 the position of the catalyst Z-HZ in Scheme 7.1 is occupied in Franck 

 and Herzfeld's scheme, by the system RO-ROH. 



The peroxide formation (7.4a) is now described by equation: 



t 

 (11.15c) 4 HR'R"0 > 2 HR'R"0— OR"R'H 



and the peroxide decomposition (7.4b), by equation: 



(11.15d) 2 (HR'R"0)2 > O2 + 2 H2O + 4 R'=R" 



The reaction cycle is completed by regeneration of the water-acceptor 

 complex by reaction (11.15a). 



Comparison of equations (11.15a-d) with the Franck-Herzfeld 

 reaction mechanism (7.12) shows that, in the latter, the abbreviation 

 ROH is used for HRiR20H, and the order of reactions {ll.ldd) and 

 (11.15a) is reversed, that is, the organic peroxide is assumed ^irsf to react 

 with water to form an organic hydroperoxide: 



r 4R0 >2R00R 



(ll.ibaj I2 ROOR + 2 H2O > 2 ROH + 2 ROOH (c/. 7.12f) 



and then to liberate oxygen: 



a "catalatic" 

 enzyme 



(11.16b) 2 ROOH > 2 ROH + O2 (c/. 7.12g) 



The primary photochemical reaction (11.15b) is, in the formulation of 

 Franck and Herzfeld: 



(11.16c) 4ROH + 4X >4RO + 4HX (c/. 7.12a) 



and water is thus removed one step away from the primary photochemical 

 process. 



The reaction of an organic peroxide group RO — OR with water, 

 assumed by Franck and Herzfeld, is thermochemically possible (cf. be- 

 low), but the same is also true of the addition of water to C=C double 

 bonds, as assumed in (11.15a) (of. Table 9. III). Thus, both the order 

 of reactions suggested in (11.15), and that assumed in (11.16) are not 

 implausible. 



To sum up, the formation of organic peroxides as precursors of free 

 oxygen can easily be fitted into the picture of photosynthesis as a hydro- 

 gen transfer from water to carbon dioxide. However, if we inquire into 

 the proofs of this hypothesis, all we find are inhibition experiments with 

 hydroxylamine (and certain other poisons) which indicate that one of 



