Hydrocarbon reactivity scales

To measure the ozone-forming capability of individual organics, a number of hydrocarbon reactivity scales have been developed over the years (Carter, 1994). A useful definition of reactivity is that of incremental reactivity (IR), defined as the amount of O, formed per unit of VOC added or subtracted from the VOC mixture in a given airmass (Carter and Atkinson, 1987, 1989 Bowman and Seinfeld, 1994a, b). 

J. N. Pitts, Jr., A. C. Lloyd, A. M. Winer, K. R. Darnall, and G. J. Doyle, "Development and Application of a Hydrocarbon Reactivity Scale Based on Reaction with the Hydroxyl Radical," 69th Annual Air Pollution Control Association Meeting, Portland, Ore., June 27-July 1, 1976, Paper No. 76-31.1. 

More precisely, the rate of ozone formation depends closely on the chemical nature of the hydrocarbons present in the atmosphere. A reactivity scale has been proposed by Lowi and Carter (1990) and is largely utilized today in ozone prediction models. Thus the values indicated in Table 5.26 express the potential ozone formation as O3 formed per gram of organic material initially present. The most reactive compounds are light olefins, cycloparaffins, substituted aromatic hydrocarbons notably the xylenes, formaldehyde and acetaldehyde. Inversely, normal or substituted paraffins. 

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. 

The concept of reactivity scales is based on ranking organics in terms of their potential for ozone production. A number of different parameters have been used to rank organics by their reactivity, including observed rates of reaction, product yields, and effects observed from irradiated VOC-NO. mixtures. For example, the rates of 03 or N02 formation or the hydrocarbon loss have been used to develop reactivity scales, as have the yields or dosages of products such as 03 and PAN in... 

The importance of OH radicals in atmospheric chemistry is the basis of another reactivity scale for organics that do not photolyze in actinic radiation (Darnall et al., 1976 Wu et al., 1976). This scale is based on the fact that, for most hydrocarbons, attack by OH is responsible for the majority of the hydrocarbon consumption, and this process leads to the free radicals (H02, R02) that oxidize NO to N02, which then leads to 03 formation. Even for alkenes, which react with 03 at significant rates, consumption by OH still predominates in the early portion of the irradiation before 03 has formed. It has therefore been suggested that the rate constant for reaction between OH and the... 

Table 16.5 shows the results of a historical grouping of hydrocarbons based on their rate constants for reaction with OH (Darnall et al., 1976), and Table 16.6 is a summary of reactivity using a variety of different assessment parameters, including the yields of ozone/oxidant, PAN, HCHO, or aerosol particles, as well as eye irritation and plant damage (Altshuller, 1966). In general, regardless of the reactivity scale... 

Figure 10. Average reactivity history for Commerce, 1969 hydrocarbon consumption scale (from Altshuller)...

The characteristic magnitudes of detonation cells for various fuel-air mixtures (Table 3.2) show that these restrictive boundary conditions for detonation play only a minor role in full-scale vapor cloud explosion incidents. Only pure methane-air may be an exception in this regard, because its characteristic cell size is so large (approximately 0.3 m) that the restrictive conditions, summarized above, may become significant. In practice, however, methane is often mixed with higher hydrocarbons which substantially augment the reactivity of the mixture and reduce its characteristic-cell size. 

In vitro studies of DNA interactions with the reactive ben-zo[a]pyrene epoxide BPDE indicate that physical binding of BPDE occurs rapidly on a millisecond time scale forming a complex that then reacts much more slowly on a time scale of minutes (17). Several reactive events follow formation of the physical complex. The most favorable reaction is the DNA catalyzed hydrolysis of BPDE to the tetrol, BPT (3,5,6,8,17). At 25°C and pH=7.0, the hydrolysis of BPDE to BPT in DNA is as much as 80 times faster than hydrolysis without DNA (8). Other reactions which follow formation of physical complexes include those involving the nucleotide bases and possibly the phosphodiester backbone. These can lead to DNA strand scission (9 34, 54-56) and to the formation of stable BPDE-DNA adducts. Adduct formation occurs at the exocyclic amino groups on the nucleotide bases and at other sites (1,2,9,17,20, 28,33,34,57,58). The pathway which leads to hydrocarbon adducts covalently bound to the 2-amino group of guanine has been the most widely studied. 

While a majority of laboratory-scale dehydrocyclization studies involve carefully chosen feedstocks, often a single alkane, commercial operators use a naphtha fraction consisting of a complex mixture of hydrocarbons. At least some of these will be incapable of easily undergoing direct dehydrocyclization and need to be isomerized into reactive structures if aromatics are to be formed. The work of Davis suggests that the acidity of dual function catalysts is an important added factor in these isomerizations, one which likely complements the different set of isomerizations that may be catalyzed by the platinum function. 

The collected papers of a symposium at Dallas, April 1956, cover all aspects of the handling, use and hazards of lithium, sodium, potassium, their alloys, oxides and hydrides, in 19 chapters [1], Interaction of all 5 alkali metals with water under various circumstances has been discussed comparatively [2], In a monograph covering properties, preparation, handling and applications of the enhanced reactivity of metals dispersed finely in hydrocarbon diluents, the hazardous nature of potassium dispersions, and especially of rubidium and caesium dispersions is stressed [3], Alkaline-earth metal dispersions are of relatively low hazard. Safety practices for small-scale storage, handling, heating and reactions of lithium potassium and sodium with water are reviewed [4],... 

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