Scavenging Kinetics

The rate of aqueous ozonation reactions is affected by various factors such as the pH, temperature, and concentration of ozone, substrate, and radical scavengers. Kinetic measurements have been carried out in dilute aqueous solution on a large number of organic compounds from different classes (56,57). Some of the chemistry discussed in the foUowing sections occurs more readily at high ozone and high substrate concentrations. 

One method which seems to produce them is to bubble H2 gas, which has been partially dissociated by passage through an electrodeless discharge, into water. Czapski and Stein (19, 20) developed this technique and studied the reactive species produced by competitive scavenging kinetics. In this way Jortner and Rabani (54) demonstrated the very striking and complete transformation from H to e aq for solutions at pH 12 given by Reaction 2. 

The procedure can be extended to achieve selective a-bromination and iodination of carboxylic acids and the general mechanism of the a-halogenation is outlined in Chapter 5, p 170. The autocatalytic effects in the selective a-chlorination of propionic acid to the 2-chloro and 2,2-dichloro acids have been studied in a semibatch reaction at 90-130 °C. The reaction was performed in the presence of chlorosulfonic acid and dichloroacetic acid as catalysts and oxygen as the radical scavenger. Kinetic experiments indicated autocatalytic formation of both products and that the selectivity was independent of the chlorine concentration in the liquid phase. The results confirmed the validity of the proposed reaction scheme which involved formation of the reaction intermediate, propanoyl chloride from propionic acid and chlorosulfonic acid, the acid-catalysed enolization of the acid, and a hydroxyl-chlorine exchange reaction. The acid-catalysed enolization was the rate-determining step in the reaction sequence. ... 

To explain the discrepancy in the scavenging yield, it was hypothesised that the failure for the IRT algorithm to reproduce the scavenging kinetics arose because of... 

In an attempt to model the scavenging kinetics, another approach was formulated in which a killing term was introduced into the backward diffusion equation as... 

The inability for the IRT to reproduce the scavenging kinetics led to the conclusion that a correlation must exist between the recombination and scavenging processes. This possibility is explored in some detail in Sect. 7.2.6. 

Eigure 7.14a shows the scavenging kinetics to follow Smoluchowski s time dependent rate constant irrespective of when the pair recombine, which is to be expected for neutral species. Eigure 7.14b highlights the existence of a correlation between the scavenging and recombination times for ion recombination. Those species which tend to recombine faster are scavenged much more rapidly. Hence, scavenging is... 

One particle and n scavengers In the first set of simulations, the scavengers were distributed randomly on the spherical surface (which were made mobile), whilst the particle was fixed at the south pole (which was made stationary). Figure 9.16 shows that the exponential approximation is excellent in describing the scavenging kinetics across the parameter space investigated. The exponential approximation can... 

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