Secondary radical reactions

The main products of the photolysis are C2H6, CO, (CH3CO)2 and CH4 (refs. 95-97, 108, 122, 125, 126, 156). Biacetyl formation is significant only at temperatures below 100 C, while methane is produced in considerable amoimts only at higher temperatures. At not too low light intensities and at temperatures below 100 °C, the relationship 

Let us discuss first the questions related to the decomposition of the acetyl radical as well as to the self- and cross-combination reactions of the CH3 and CH3CO radicals. Most of the experimental results can be interpreted by taking into accoimt the following steps 

We prefer to explain the results of Herr and Noyes as well as those of Anderson and Rollefson on the pressure effect by the pressure dependence of the decomposition of the CH3CO radical. According to the Benson equation , based on the detailed theory of unimolecular reactions, the acetyl radical is in the fall-off region below a few atmospheres. O Neal and Benson demonstrated that the results, obtained in the absence of added foreign gases at low light intensities - , are in accordance with the assumption of a pressure-dependent decomposition and a homogeneous recombination of the acetyl radicals. 

The effect of added CO2 on the formation of CO was investigated by Calwell and Hoare - with about 10 quanta.l . sec absorbed intensities, and at temperatures of 50-200 °C. The value of / co increased with increasing presstire at all temperatures investigated however, above 120 °C the pressure effect was less pronounced and was more complex in character. It has been suggested that, in the temperature range 120-200 °C, the inert gas effect is due to the acceleration of the decomposition of electronically excited acetone molecules. However, below 100 °C, the increase of co with increasing inert gas pressure is mainly related to the pressure dependence of the homogeneous decomposition of the acetyl radicals. 

In their experiments at 3130 A and 25 °C, with 60-70 torr acetone pressure and 10 and 2x10 quanta.l . sec intensities, Howland and Noyes also observed the enhancement of CO formation as a result of addition of CO2. Like 

---

In the absence of cosolutes, the photodegradation rates depended on the orientation of the nitro group. Thus 1-nitropyrene decayed relatively fast by the nitro-nitrite primary intramolecular photorearrangement process, followed by secondary radical reactions. However, 2-nitropyrene and 2-nitrofluoranthene were stable toward photolysis, consistent with the N02 group being in the same plane as the aromatic rings. 

Greater concentrations of radicals can be produced at higher irradiation doses. However, at high doses, dose saturation effects may well become important. Normally, the radical concentration is proportional to the dose, but with dose saturation this proportionality becomes invalid as a result of secondary radical reactions occurring or a change in the physical nature of the polymer matrix. 

Fig. 7.21 Simplified scheme of interrelated photo-initiated oxidation and secondary radical reactions in the gas phase (atmospheric...

In flash photolytic experiments, second-order radical reactions play almost an exclusive role. It follows from the investigations of Khan et al and those of Wettermark that, at high light intensities, the main secondary radical reactions... 

At present, there are still insufficient experimental results at hand to postulate a mechanism for the secondary radical reactions those available are adequate only to explain some of the problems - - . 

Tladiation chemists have been aware for about 15 years that the presence of dilute solutes in liquid hydrocarbons can change the course of radiation chemical reactions by other than the normally expected secondary radical reactions. For example, Manion and Burton (40) in early work on the radiolysis of benzene-cyclohexane solutions, drew attention to the possibility of energy transfer from solvent to solute. Furthermore, it is known that in hydrocarbon solvents certain solutes are capable of capturing electrons, thus interfering with the normal ion-recombination process (14, 20, 65, 72). Though ionic products can be observed readily in hydrocarbon glasses [e.g., (19, 21)] demonstration of effects which can be specifically ascribed to electron capture in the liquid state has been elusive until recently. Reaction of positive ions prior to neutralization can play an important role as demonstrated recently by studies on... 

Radiation-induced effects in labeled molecules occur with compounds of high specific activity, labeled with weak P emitters or even a emitters. Since autoradiolysis lakes place mainly via secondary radical reactions of fragments formed from solvents or major components, radiolytic damage... 

---