Ozone reaction mechanism

Ozonation ofAlkenes. The most common ozone reaction involves the cleavage of olefinic carbon—carbon double bonds. Electrophilic attack by ozone on carbon—carbon double bonds is concerted and stereospecific (54). The modified three-step Criegee mechanism involves a 1,3-dipolar cycloaddition of ozone to an olefinic double bond via a transitory TT-complex (3) to form an initial unstable ozonide, a 1,2,3-trioxolane or molozonide (4), where R is hydrogen or alkyl. The molozonide rearranges via a 1,3-cycloreversion to a carbonyl fragment (5) and a peroxidic dipolar ion or zwitterion (6). 

It is thus anticipated that compressive stress inhibits while tensile stress promotes chemical processes which necessitate a rehybridization of the carbon atom from the sp3 to the sp2 state, regardless of the reaction mechanism. This tendency has been verified for model ring-compounds during the hydrogen abstraction reactions by ozone and methyl radicals the abstraction rate increases from cyclopropane (c3) to cyclononane (c9), then decreases afterwards in the order anticipated from Es [79]. The following relationship was derived for this type of reactions ... 

One strategy in limiting the formation of ozone and other photochemical oxidants has been the use (in the past) of low reactivity fuels in internal combustion engines. More recently, alternate fuels (methanol, for instance) have been proposed for regions that suffer from elevated levels of photochemical air pollution. The effect of switching to such a low-reactivity fuel may be seen in Equation E2 for methanol, which has a simple atmospheric reaction mechanism. 

This simple oxygen-only mechanism consistently overestimates the O3 concentration in the stratosphere as compared to measured values. This implies that there must be a mechanism for ozone destruction that the Chapman model does not account for. A series of catalytic ozone-destroying reactions causes the discrepancy. Shown below is an ozone-destroying mechanism with NO/NO2 serving as a catalyst ... 

The most effective antiozonants are the substituted PPDs. Their mechanism of protection against ozone is based on the scavenger-protective film mechanism [68-70]. The reaction of ozone with the antiozonant is much faster than the reaction with the C=C bond of the rubber on the rubber surface [56]. The rubber is protected from the ozone attack tUl the surface antiozonant is depleted. As the antiozonant is continuously consumed through its reaction with ozone at the mbber surface, diffusion of the antiozonant from the inner parts to the surface replenishes the surface concentration to provide the continuous protection against ozone. A thin flexible film developed from the antiozonant/ozone reaction products on the mbber surface also offers protection. 

Atkinson, R., Carter, W.P.L. (1984) Kinetics and mechanisms of gas-phase ozone reaction with organic compounds under atmospheric conditions. Chem. Rev. 84, 437 470. 

The second term characterizes the chain decomposition of ozone. The mechanism of chain reaction was found to include the following reactions ... 

Ozone rate constants, 27 771 Ozone reactions activation of, 27 779 kinetics and mechanism of, 27 778-779 Ozone resistance, of ethylene-propylene polymers, 10 704, 717 Ozone synthesis, energy requirements for, 27 798... 

The reaction of alkenes with ozone constitutes an important method of cleaving carbon-carbon double bonds.138 Application of low-temperature spectroscopic techniques has provided information about the rather unstable species that are intermediates in the ozonolysis process. These studies, along with isotope labeling results, have provided an understanding of the reaction mechanism.139 The two key intermediates in ozonolysis are the 1,2,3-trioxolane, or initial ozonide, and the 1,2,4-trioxolane, or ozonide. The first step of the reaction is a cycloaddition to give the 1,2,3-trioxolane. This is followed by a fragmentation and recombination to give the isomeric 1,2,4-trioxolane. The first step is a... 

The reaction mechanism shown for ozone depletion includes chorine. Chlorine in this reaction acts as a catalyst. A principal source of this chlorine is from the ultraviolet breakdown of CFC (chlorofluorocar-... 

This is a major chain propagation step in the overall reaction mechanism for ozone formation in photochemical air pollution. Because H02 is intimately tied to OH through reaction (17) and cycles such as that in Fig. 1.4, when NO is present the sources and sinks of H02 are, in effect, sources or sinks of the OH radical. 

Since then, there have been a number of studies of this reaction, which have been summarized by Hatakeyama and Akimoto (1994). The mechanism appears to involve formation of an adduct that can either decompose to S02 and an isomerization product of the Criegee intermediate or, alternatively, react with a second S02 molecule to generate other products. For the Criegee intermediate formed in the ethene-ozone reaction, for example, the proposed reaction sequence is the following ... 

Another area rarely dealt with in previous literature is the how-to of ozone experiments. This information is usually hard-earned by doctoral candidates and laboratory staff, and either not considered as appropriate information for a scientific treatise or considered as proprietary information that belongs to expertise. This lack of information has motivated us to write a book that contains not only fundamental information about the toxicology of ozone, its reaction mechanisms, and full-scale applications of ozonation (Part A Ozone in... 

Two of the strongest chemical oxidants are ozone and hydroxyl radicals. Ozone can react directly with a compound or it can produce hydroxyl radicals which then react with a compound. These two reaction mechanisms are considered in Section A 2.1. Hydroxyl radicals can also be produced in other ways. Advanced oxidation processes are alternative techniques for catalyzing the production of these radicals (Section A 2.2). 

As shown in Table 3-1, ozone can destroy other disinfectants. This should be avoided by dosing them not ahead of ozonation stages, but rather at the end of the total treatment before the distribution of water to the supply-net. A special case is the reaction of ozone with H202 (correctly with the species H02 ), which is used as an advanced oxidation process (AOP) for intensified formation of hydroxyl radicals and their oxidative attack on persistent organic target compounds (persistent against ozone in the direct reaction mechanism) (see Chapter A 2). 

Since reaction mechanisms and experimental observations are not independent of the system in which they are made, the experimental set-up and how the experiment is run affect the outcome. That means that it must be clear how equipment and procedures affect the outcome when they are chosen. It also means that experimental set-ups and procedures from drinking water treatment cannot be applied on waste water without appropriate evaluation and vice versa. In general, an experimental set-up consists of an ozone generator, reactor, flow meters and on-line analysis of at least the influent and effluent ozone gas concentrations and ambient air monitor (Figure 2-1). Each set-up will be tailored to the experimental goals and the resources available. 

The reaction mechanism of ozone with the solute should be known to establish a selective oxidation. Non-targeted compounds contained in the water phase should not have a higher reactivity to ozone. Otherwise ozone might already react to a large extent in the aqueous phase, consuming much of the ozone so that it is not available for the oxidation of the target solute in the solvent phase. The achievable selectivity depends much on the distribution of the solute between the gas, water and solvent phase, which should be checked by partition coefficients from the literature or experimentally determined. 

The mass transfer rate of the solute to the solvent phase has to be considered compared to its reaction rate in the solvent. The system is controlled by the chemical reaction, if the oxidation of the solute in the solvent is slower than the mass transfer rate water/solvent of ozone. This is reversed in the case of a diffusion controlled or mass transfer limited system. The reaction mechanism of the solute with ozone should be considered in order to utilize a... 

The absorption of ozone from the gas occurred simultaneously with the reaction of the PAH inside the oil droplets. In order to prove that the mass transfer rates of ozone were not limiting in this case, the mass transfer gas/water was optimized and the influence of the mass transfer water/oil was studied by ozonating various oil/water-emulsions with defined oil droplet size distributions. No influence of the mean droplet diameter (1.2 15 pm) on the reaction rate of PAH was observed, consequently the chemical reaction was not controlled by mass transfer at the water/oil interface or diffusion inside the oil droplets. Therefore, a microkinetic description was possible by a first order reaction with regard to the PAH concentration (Kornmuller et al., 1997 a). The effects of pH variation and addition of scavengers indicated a selective direct reaction mechanism of PAH inside the oil droplets... 

Approximately one-half of the total reaction of CIO and BrO results in the destruction of ozone. Other mechanisms exist, such as a catalytic cycles that are rate-limited by the reaction between CIO and O and between CIO and H02 (25), but the contribution from these reactions is small in the polar regions. 

---