Hydroxyl radicals reactions with hydrocarbons

Neeb, P. (2000), Structure-reactivity based estimation of the rate constants for hydroxyl radical reactions with hydrocarbons, J. Atmos. Chem., 35, 295-315. 

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

NMHC. A large number of hydrocarbons are present in petroleum deposits, and their release during refining or use of fuels and solvents, or during the combustion of fuels, results in the presence of more than a hundred different hydrocarbons in polluted air (43,44). These unnatural hydrocarbons join the natural terpenes such as isoprene and the pinenes in their reactions with tropospheric hydroxyl radical. In saturated hydrocarbons (containing all single carbon-carbon bonds) abstraction of a hydrogen (e,g, R4) is the sole tropospheric reaction, but in unsaturated hydrocarbons HO-addition to a carbon-carbon double bond is usually the dominant reaction pathway. 

For unreactive hydrocarbons such as benzene and the pentanes, the reaction between hydroxyl radicals and the hydrocarbon is so slow that most of the hydroxyl radicals react with CO rather than with the hydrocarbon. In these systems the rise in the NO oxidation rate is the same as that observed in the absence of the hydrocarbon. 

The alkyl radical lies at the centre of the oxidation scheme shown in Fig. 2.1 and the first two sections discuss its method of generation both in the initiation and chain propagation processes. The former has already been discussed in some detail in Chapter 1 and is not amenable to study by direct techniques. It is included here to produce a more complete picture of the overall methodology needed to quantify a reaction mechanism and to emphasize that direct techniques cannot provide all of the answers. The hydroxyl radical (OH) is the main chain carrier and the bulk of Section 2.3, is devoted to the measurement of rate constants for its hydrogen atom abstraction reactions with hydrocarbons. Much of the data and the methodology described in this section derive from studies of tropospheric chemistry, where the oxidative chain, at least in its early stages, shows a close relationship to the higher temperature processes which are central to this book. Indeed, it is fair to say that many of the developments in... 

The hydroxyl radical, in turn, is integral to many atmospheric reactions including those that activate the various lower hydrocarbons to more reactive radical groups. For example, the hydroxyl radical combines with methane to produce the methyl radical (CH3), as shown in Equation (16.63) ... 

Hydrocarbon reactivity is best based upon the interaction of hydrocarbons with hydroxyl radical. Methane, the least-reactive common gas-phase hydrocarbon with an atmospheric half-life exceeding 10 days, is assigned a reactivity of 1.0. (Despite its low reactivity, methane is so abundant in the atmosphere that it accounts for a significant fraction of total hydroxyl radical reactions.) In contrast, P-pinene produced by conifer trees and other vegetation, is almost 9000 times as reactive as methane, and i7-limonene, (from orange rind) is almost 19000 times as reactive. 

Nitrations are highly exothermic, ie, ca 126 kj/mol (30 kcal/mol). However, the heat of reaction varies with the hydrocarbon that is nitrated. The mechanism of a nitration depends on the reactants and the operating conditions. The reactions usually are either ionic or free-radical. Ionic nitrations are commonly used for aromatics many heterocycHcs hydroxyl compounds, eg, simple alcohols, glycols, glycerol, and cellulose and amines. Nitration of paraffins, cycloparaffins, and olefins frequentiy involves a free-radical reaction. Aromatic compounds and other hydrocarbons sometimes can be nitrated by free-radical reactions, but generally such reactions are less successful. 

The reaction of volatile chlorinated hydrocarbons with hydroxyl radicals is temperature dependent and thus varies with the seasons, although such variation in the atmospheric concentration of trichloroethylene may be minimal because of its brief residence time (EPA 1985c). The degradation products of this reaction include phosgene, dichloroacetyl chloride, and formyl chloride (Atkinson 1985 Gay et al. 1976 Kirchner et al. 1990). Reaction of trichloroethylene with ozone in the atmosphere is too slow to be an effective agent in trichloroethylene removal (Atkinson and Carter 1984). 

The transformation of arenes in the troposphere has been discussed in detail (Arey 1998). Their destruction can be mediated by reaction with hydroxyl radicals, and from naphthalene a wide range of compounds is produced, including 1- and 2-naphthols, 2-formylcinnamaldehyde, phthalic anhydride, and with less certainty 1,4-naphthoquinone and 2,3-epoxynaphthoquinone. Both 1- and 2-nitronaphthalene were formed through the intervention of NO2 (Bunce et al. 1997). Attention has also been directed to the composition of secondary organic aerosols from the photooxidation of monocyclic aromatic hydrocarbons in the presence of NO (Eorstner et al. 1997) the main products from a range of alkylated aromatics were 2,5-furandione and the 3-methyl and 3-ethyl congeners. 

Damall, K.R., Lloyd, A.C., Winer, A.M., Pitts, J.N. (1976) Reactivity scale for atmospheric hydrocarbons based on reaction with hydroxyl radicals. Environ. Sci. Technol. 10, 692-696. 

Ohta, T., Ohyama, T. (1985) A set of rate constants for the reactions of hydroxyl radicals with aromatic hydrocarbons. Bull. Chem. Soc. Jpn. 58, 3029-3030. 

Perry, R.A., Atkinson, R., Pitts, J.N. (1977) Kinetics and mechanisms of the gas phase reaction of the hydroxyl radicals with aromatic hydrocarbons over temperature range 296 473 K. J. Phys. Chem. 81, 296-304. 

Reaction of peroxyl radical with secondary hydroperoxide produces very active hydroxyl radical. The latter attacks immediately the hydrocarbon molecule. 

Another important effect observed when reactions take place in the liquid phase is associated with the solvation of the reactants. Theoretical comparison showed that the collision frequencies of the species in the gas and liquid phases are different, which is due to the difference between the free volumes. In the gas phase, the free volume is virtually equal to the volume occupied by the gas species (FfwT), while in the liquid phase, it is much smaller than the volume of the liquid species (V < V). Since the motion and collision of the species occur in the free volume, the collision frequency in the liquid is higher than in the gas by the amount (V/Vf)U3 [32,33]. The activation energies for the reactions of radicals and atoms with hydrocarbon C—H bonds in the gas and the liquid phases are virtually identical, and that in the liquid is independent of the solvent polarity. This also applies to the parameter bre, which can be seen from the following examples referring to the interaction of the hydroxyl radical with hydrocarbons [30] ... 

As in the Mallard-Le Chatelier approach, an ignition temperature arises in this development, but it is used only as a mathematical convenience for computation. Because the chemical reaction rate is an exponential function of temperature according to the Arrhenius equation, Semenov assumed that the ignition temperature, above which nearly all reaction occurs, is very near the flame temperature. With this assumption, the ignition temperature can be eliminated in the mathematical development. Since the energy equation is the one to be solved in this approach, the assumption is physically correct. As described in the previous section for hydrocarbon flames, most of the energy release is due to CO oxidation, which takes place very late in the flame where many hydroxyl radicals are available. 

Chemical radicals—such as hydroxyl, peroxyhydroxyl, and various alkyl and aryl species—have either been observed in laboratory studies or have been postulated as photochemical reaction intermediates. Atmospheric photochemical reactions also result in the formation of finely divided suspended particles (secondary aerosols), which create atmospheric haze. Their chemical content is enriched with sulfates (from sulfur dioxide), nitrates (from nitrogen dioxide, nitric oxide, and peroxyacylnitrates), ammonium (from ammonia), chloride (from sea salt), water, and oxygenated, sulfiirated, and nitrated organic compounds (from chemical combination of ozone and oxygen with hydrocarbon, sulfur oxide, and nitrogen oxide fragments). ... 

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