Ozonations aromatic systems, ozone

When a stream of ozonized oxygen or air, usually under 6% ozone, is passed through a solution of an olefin, such as 2,4,4-trimethyl-2-pentene, absorption occurs as fast as the ozone is introduced and no ozone escapes through the solution until all the olefin has been converted to ozonide. If an aromatic hydrocarbon such as benzene is ozonized rather than an aliphatic olefin, absorption of ozone is not complete and several times the theoretical amount of ozone must be used to effect complete ozoniza-tion. When a molecule has both an aromatic system and an aliphatic double bond, the aliphatic bond may react selectively, with little or no reaction with the aromatic system. Anethole will absorb a mole of ozone and produce, on hydrolysis of the ozonide, a very good yield of anise aldehyde. Complete saturation of the molecule requires almost 10 moles of ozone, however. 

The various characterization experiments were used to derive the chamber characterization parameters and evaluate the ehamber eharaeterization model as discussed above. The single organic - NOx experiments were earried out to demonstrate the utility of the ehamber to test the mechanisms for these eompounds, for whieh data are available in other ehambers, and to obtain well-charaeterized meehanism evaluation data at lower NOx levels than previously available. The formaldehyde -i- CO - NOx experiments were carried out beeause they provided the most ehemieally simple system that model calculations indicated was insensitive to chamber effeets, to provide a test for both the basic mechanism and the light eharaeterization assigmnents. The aromatie - CO - NOx experiments were carried out because aromatic - NOx experiments were predieted to be very sensitive to the addition of CO, because it enhances the effects of radicals formed in the aromatic system on ozone formation. The ambient surrogate - NOx experiments were earried out to test the ability of the mechanism to simulate ozone formation under simulated ambient eonditions at various ROG and NOx levels. 

The simultaneous over prediction of O3 concentrations and under prediction of the NO oxidation rate and oxidising capacity of the aromatic systems poses a problem for meehanism development a reduction in ozone concentration can be achieved by a reduction in peroxy radical concentration, which limits the NO to NO2 conversion, but this will also lead to a reduction in OH production and in the oxidative capacity of the system. A number of possible mechanistic fixes have been investigated and a number of strategies for resolving some of these issues have been proposed (Bloss et al., 2005b) ... 

Better yields are often obtained when ozone is used for oxidative cleavage of olefins to carboxylic acids or of cycloalkenes to dicarboxylic acids. Olefinic double bonds are very much more easily attacked by ozone than are aromatic systems, so that arylethylene derivatives can be successfully treated with ozone without appreciable effect on the ring. If the ozonide which is formed initially is decomposed with water, the aldehyde is obtained together with hydrogen peroxide and other products ... 

The ozonization of aromatic compounds is more interesting and complex in that each bond in the aromatic system has a certain degree of double bond character and this is reflected in the amounts of ozonization products since this reaction is specific for do >le bonds. Benzene, for example, has six bonds of equal double bond character. This can be represented by its two Kekul resonance structures which we must realize do not denote independent, existing forms but they do approximate the relative amount of double bond character for each bond. 

The ozone induced reaction of polychloro benzenes and some related halogeno compounds with nitrogen dioxide is a novel non-acid methodology for the selective mono nitration of moderately deactivated aromatic systems [100]. In the presence of ozone and preferably with methanesul-fonic acid as a catalyst, the polychloro benzenes undergo mononitration... 

Chlorinated rubber is soluble in aromatic solvents, and paints made from it dry by solvent evaporation alone. In contrast to the vinyls, there is less difficulty in formulating systems that are suitable for brush application. It has excellent resistance to a wide range of chemicals and to water, but as it is extremely brittle it needs to be plasticised. To preserve chemical resistance it is necessary to use inert plasticisers such as chlorinated paraffin wax. Due to the presence of ozone depleting solvents, chlorinated rubber coatings are being phased out and largely replaced by vinyl acrylic coatings which have very similar performance and can be formulated from lower aromatic or aliphatic solvents. 

Solid styrene was exposed at — 196°C to ozone, in an attempt to discern whether the behavior of the system is similar to that of olefinic compounds, yielding an ozonide, or to that of aromatic compounds, yielding a 7r-complex. On heating to about — 100°C, an adduct is formed that is stable until about —55 °C, when benzaldehyde and a peroxidic polymer are slowly obtained. The structure of the adduct is probably that of a POZ, based on the similarity of the IR spectrum with the ozone adduct of vinyl chloride described in the preceding paragraph . 

As a third example we will discuss the ozonization29 of aromatic molecules mentioned previously, in particular that of naphthalene. Kooyman and Ketelaar postulated that as a first step the ozone molecule reacts with a pair of iz electrons in a bond. If now one asks whether there will be a difference between the 1—2 and the 2—3 bond, then the energy of the other 7u electrons after the bond localization of the one pair must be considered. It is clear that this is much lower with bond localization at 1—2 since the remaining bonds form a system like that in styrene, thus with the very stable aromatic ... 

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