PHOTOCHEMISTRY 29 



mechanism of the reaction for such a case has not been estabhshed with 

 reasonable certainty. 



The photochemical oxidation of formaldehyde (Style and Summers, 

 1946) is a good example of this type of reaction. It has been studied in 

 the temperature range of 100° to about 275°C and at a variety of pressures 

 and compositions. Its principal products are CO, CO2, HCOOH, Ho, 

 and H2O. Their several yields vary from values of less than 1 to 30 or 40, 

 depending in a coniplex way on the conditions. It is well established that 

 H, CHO, and HO2 are reaction intermediates. The (chief) primary 

 process is 



HCHO + hp^U + CHO. 



Although the detailed mechanism is not known, the experimental evidence 

 is compatible with the postulate that the following reactions serve as 

 (some of) the secondary steps of the reaction: 



H + HCHO ^ H. + CHO, 

 CHO + HCHO -^ Ho + CHO + HCHO, 

 2CH0 ^ CO + HCHO, 

 M + H + O2 ^ H62 + CO, 

 CHO + 02^ H62 + CO, 

 HO2 + HCHO ^^ [CO + CO2 + HCOOH + H], 

 2HO2 -^-^ H2O + 3.2O2. 



Organic peroxides are the principal products of some reactions of this 

 general type. Peroxides may also serve as photochemical sensitizers. 

 For example, the chief product of the photochemical oxidation of cyclo- 

 hexene is the corresponding peroxide. As the concentration of the perox- 

 ide builds up in an illuminated solution containing oxygen and cyclo- 

 hexene, the peroxide absorbs an increasing amount of the incident light, 

 and the reaction is accelerated (Bateman and Gee, 1948). These 

 observations are consistent with the postulate that the primary process 

 in the absence of the peroxide is 



RH (cyclohexene) + /( j^ -^ R + H, 



and in the presence of the peroxide is predominantly 



ROOH + hv^ ROO + H. 



At temperatures at which the thermal reaction can be neglected, the over- 

 all process is a short chain reaction. It seems very probable that R, ROO, 

 H, and HO2 are important intermediates in this process. Since the 

 quantum yield of the reaction is inversely proportional to the square root 

 of the intensity of the absorbed light, the chain-breaking step must be a 

 bimolecular reaction between chain carriers (i.e., intermediates) leading 

 to the production of stable molecules. 



