292 



LIBERATION OF OXYGEN 



CHAP. 11 



Reversible peroxide formation appears possible also in the case of 

 "double bond peroxides." According to the standard bond values, the 

 reaction : 



(11.22) —6=6— + O2 > — C— C— 



i- 



-0 



should liberate 9 kcal; but the resonance stabilization of the — C=C — 

 double bond may make the energy of oxygenation smaller or may even 

 change its sign. The same may be true for the peroxides of quinones, 

 and other conjugated double bond systems, as, for example: 



(11.23) 



O 



o 



o/^o 



o 



A/ 







+ 02^ 



Our knowledge regarding the actual existence of peroxides of the type 

 of (11.22) or (11.23) is very limited. So-called "moloxides," whose 

 intermediate formation has often been assumed in the oxidation of 

 ethylenic double bond compounds, probably belong to the type (11.22). 

 However, they exhibit the tendency to split into aldehydes or ketones: 



(11.24) 



R 



R" R"' R" R'" 



■^ R'— CO + OC— R' 



0—0 



or to polymerize (c/. Rieche 1931, 1936). The catalytic effect of oxygen 

 on the polymerization of double bond compounds, e. g., of drying oils, 

 is probably due to the primary formation of peroxides of this type (c/. 

 Milas 1932). 



One of the few experimentally known reversible organic peroxides is 

 the colorless peroxide of the red hydrocarbon rubrene (or "rubene") 

 discovered by Moureu, Dufraisse, and Dean (1926). 



Moureu, Dufraisse, and Girard (1928) found that ru- 

 brene peroxide has, at 16° C, a dissociation pressure of 

 about 0.5 mm. This corresponds to AF = 6.3 kcal; 

 direct calorimetric measurements by Dufraisse and 

 Enderlin (1930) gave for the heat of dissociation a value 

 of AH = 23 kcal (which seems to be much too high for 

 a reversible reaction). 



As described in chapter 20, carotene and its deriva- 

 tives, which contain long chains of conjugated double 

 bonds, absorb considerable amounts of oxygen; it is 

 Rubrene probable that, in this case too, double bond peroxides are 



