Isooctane photooxidation

Figure 2. Peracid formation in isooctane photooxidation against time of irradiation, for different rates of radical initiation I. ...

Proposed Model of Isooctane Photooxidation. Based on our experimental results, the photooxidation of isooctane is not likely to proceed by a classical chain reaction of propagating peroxy radicals according to equation (5) in Scheme I. The results rather indicate that the photooxidation proceeds by peracid radicals, which, by H-abstraction from the substrate, will lead to peracids. The question is, from where do these peracid radicals originate. 

Figure 4. Fit between kinetic model of isooctane photooxidation and experimental data. (A) Relative rate of peracid formation against -log I. (B) Relative rate of iodometrically...

Figure 5. Consumption of > -H and formation of >N0- in isooctane photooxidation I 5 x 10 M isooctyl radicals/h, corresponding (according to Scheme II) to a rate of peracid formation of...

Photooxidation of Isooctane. The overall rate of photooxidation of isooctane was investigated using carefully selected conditions and well-defined rates of radical initiation. Radicals were, at constant rate, initiated in isooctane by photolysis or tertiary butyl peroxide (tertiary BuOOBu) under oxygen flushing. The radical initiation rates applied, Io> were varied over a relatively wide range between the limits... 

According to the results shown in Figure 1, the kinetic chain length of the photooxidation of isooctane is very low. Even for the lowest rate of radical initiation applied, I 10 M/h, the kinetic chain length of the non-inhibited photooxidation did not exceed a value of 1. Radical termination, therefore, seems to dominate over a peroxy radical chain reaction according to equation (5) in Scheme I. 

Characterization of Photooxidation Products Formed in Isooctane. Among the lodometncally titratable species formed in the photooxidation of isooctane, a substance, developed in appreciable amounts, was observed which was easily distinguishable from ordinary alkyl-hydroperoxides. This substance is, at room temperature, readily destroyed by addition of olefins to the reaction medium. This is a reaction which is typical and specific for peracids (10) and can be employed for quantitative assessment of peracids in the presence of alkylhydroperoxides. In contrast to alkylhydroperoxides, peracids react with olefins forming the corresponding epoxide, according to the equation... 

Products reacting in this way in photooxidized isooctane have therefore been assigned to peracids. Their rate of formation has been determined in our photooxidation experiments by use of 7-tetradecene, which turned out to be a convenient olefin for peracid-analysis. The results obtained for different rates of radical initiation are shown in Figure 2. 

After removal of peracids by tetradecene addition, substantial amounts of iodometrically titratable oxidation products remained in photooxidized isooctane. Their relative rate of formation did not depend markedly on the rate of radical initiation and was found to be in the order of 40 to 45% of I. This is shown in Figure 3. Although the chemical composition of these products has not been identified, it is, however, assumed that these products were mainly hydroperoxides and probably dialklyperoxides other than those formed by self-recombination of tertiary peroxy radicals. 

Figure 3. Relative initial rate of formation of photooxidation products other than peracids in isooctane against -log Iq.

This reaction actually takes place in the photooxidation of isooctane. This is shown in Figure 5 for the case of the secondary TMP derivative used in our photooxidation experiments. 

Quantitative assessments turned out to be much more difficult in 2,4-dimethylpentane than in isooctane, due to formation of relatively unstable photooxidation products. 

The photooxidation of isooctane turned out to be more complicated than originally expected. Of special interest was the rather surprising result that, during tertiary BuOOBu photolysis, practically statistical radical attack of the different C-H sites of isooctane was observed. At first glance, this result was not expected from literature data (11, 12). For example, according to investigations carried out by Niki and Kamiya (12) tertiary BuO- attack on primary, secondary and tertiary C-H sites occurs with a selectivity of 1 7 20. This selectivity ratio derives from experiments on hydro-... 

Scheme II. Model for photooxidation of isooctane (dominating reactions).

At the present time, a discussion of the results of our model investigations in terms of possible consequences for polypropylene must be completely speculative. Apart from the differences expected between liquid phase and solid polymer photooxidation kinetics, differences in the chemical structure between our model substance, isooctane, and the structural unit of polypropylene have to be also considered. With respect to the number of CH,-groups per structural unit, isooctane and polypropylene differ by a ratio of 5 1. 

PROBABLE FATE photolysis-, probably occurs slowly, in an isooctane solvent, it hardly adsorbs any radiation above 300 nm, direct photolysis in the environment should not be significant oxidation resistant to autooxidation by peroxy radical in water oxidation by hydroxyl radicals occurs in atmosphere photooxidation half-life in air 6.4 days-63.7 days hydrolysis not important, first-order hydrolysis half-life >879 years volatilization generally rapid volatilization occurs, half-life <9 hr, volatilization from soil surfaces may be an important transport mechanism sorption significant amount of adsorption by organic materials expected in environment biological processes bioaccumulated more than chlorobenzene, volatilization is more important than biodegradation will wash out in rain water... 

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