Ozone reactions with aromatic hydrocarbons

Butkovic, V., Klasinc, L., Orhanovic, Turk, J., Glisten, H. (1983) Reaction rates of polynuclear aromatic hydrocarbons with ozone in water. Environ. Sci. Technol. 17, 546-548. 

As the reaction temperature is increased, chemiluminescence is observed in the reactions of ozone with aromatic hydrocarbons and even alkanes. Variation of temperature has been used to control the selectivity in a gas chromatography (GC) detector [35], At room temperature, only olefins are detected at a temperature of 150°C, aromatic compounds begin to exhibit a chemiluminescent response and at 250°C alkanes respond, giving the detector a nearly universal response similar to a flame ionization detector (FID). The mechanisms of these reactions are complex and unknown. However, it seems likely that oxygen atoms produced in the thermal decomposition of ozone may play a significant role, as may surface reactions with 03 and O atoms. 

Ozone vigorously reacts with the double bonds of alkenes and quite rapidly with the iT-bonds of aromatic rings. Its reaction with the C—H bonds of hydrocarbons occurs relatively slowly and is accompanied by the formation of free R and HCV radicals followed by the decomposition of the unstable radical H03 (see Chapter 3). 

FIGURE 3 12 Possible initial steps for ozone, atomic oxygen, nitrogen dioxide, and hydroxyl-radical reaction with aromatic hydrocarbons. 

The reaction does not have preparative significance. However, it has been surmised that polycyclic aromatic hydrocarbons adsorbed on atmospheric particulate matter can become activated as a result of atmospheric oxidation. Samples of such materials could display a degree of mutagenicity not associated with inactivated hydrocarbons. Although in many cases the precise nature of the oxidant is not clear, Pitts et al.60 have been able to show that ozone can convert benzo[a] pyrene, adsorbed on a glass filter, to its 4,5-oxide (28). 

On the other hand, the indirect type of ozonation is due to the reactions of free radical species, especially the hydroxyl radical, with the organic matter present in water. These free radicals come from reaction mechanisms of ozone decomposition in water that can be initiated by the hydroxyl ion or, to be more precise, by the hydroperoxide ion as shown in reactions (4) and (5). Ozone reacts very selectively through direct reactions with compounds with specific functional groups in their molecules. Examples are unsaturated and aromatic hydrocarbons with substituents such as hydroxyl, methyl, amine groups, etc. [45,46],... 

The largest sink for alkanes in the atmosphere is reaction with OH and NO3 radicals. The formation of photochemical smog is described in detail in (Chapter 9.11, Sillman). Mono-aromatic hydrocarbons react only slowly with O3 and NO3 radicals in the troposphere. The only important atmospheric processes for mono-aromatic hydrocarbons, and naphthalene and dinaphthalenes are reactions with OH radicals (Atkinson, 1990). The products of these reactions include aldehydes, cresols, and, in the presence of NO, benzylnitrates. Methane can be an important contributor to ozone formation, especially in the remote troposphere, as described in (Chapter 9.11, Sillman). 

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. 

Gasoline hydrocarbons volatilized to the atmosphere quickly undergo photochemical oxidation. The hydrocarbons are oxidized by reaction with molecular oxygen (which attacks the ring structure of aromatics), ozone (which reacts rapidly with alkenes but slowly with aromatics), and hydroxyl and nitrate radicals (which initiate side-chain oxidation reactions) (Stephens 1973). Alkanes, isoalkanes, and cycloalkanes have half-lives on the order of 1-10 days, whereas alkenes, cycloalkenes, and substituted benzenes have half- lives of less than 1 day (EPA 1979a). Photochemical oxidation products include aldehydes, hydroxy compounds, nitro compounds, and peroxyacyl nitrates (Cupitt 1980 EPA 1979a Stephens 1973). 

The last general category—namely, the reaction of ozone with aromatic hydrocarbons, has received an enormous amount of attention by ozone chemists. Most of this attention has concerned rate and reactivity studies in an attempt to correlate these experimental quantities with some known parameters of the hydrocarbons. Several reactivity correlations have been proposed, including those with bond localization energy, atom localization energies, and oxidation-reduction potentials. This category is also represented by a paper in this section, in which a possible correlation between ozone reactivity and carcinogenicity of some polycyclic aromatic compounds is explored. 

Polycyclic hydrocarbons are extremely reactive with ozone. For example, naphthalene reacted with ozone at a rate about 1500 times faster than benzene (Hoigne and Bader, 1983b), and higher polycyclic hydrocarbons such as phenanthrene, pyrene, and benzo[a]pyrene were also extremely reactive (Butkovic et al., 1983). The experiments of Hoigne and Bader also indicate that the rate constants for reaction of ozone with aromatic hydrocarbons in water were about 100 times greater than in nonpolar solvents such as CCI4. However, aliphatic compounds did not show such a profound solvent effect. 

Perraudin E, Budzinski H, Villenave E (2007) Kinetic study of the reactions of ozone with polycyclic aromatic hydrocarbons adsorbed on atmospheric model particles. J Atmos Chem... 

With polycyclic aromatic hydrocarbons, the site of ozone attack may be dependent upon substrate stmcture and reaction solvent (eq 21). ... 

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