OH, reactivity

The concept of OH reactivity has been applied to give a first-cut assessment of the contribution of various individual organics and sources to photochemical oxidant formation in a number of situations. For example, Chameides et al. (1992) scaled the contribution of various VOC concentrations in a variety of atmospheres from remote to polluted urban areas using OH reactivity. They concluded that while NOx concentrations decreased from polluted urban areas to rural to remote regions, the total VOC reactivity assessed in this manner was comparable at all continental areas from remote to polluted. 

In short, while the OH reactivity scale has a number of caveats associated with its use, it has proven useful in providing at least an initial assessment of relative contributions of organics to photochemical smog formation. 

Chameides and co-workers (1992) examined the observed concentrations of ozone and its precursors, NOx and VOC, in a variety of tropospheric locations, from remote marine areas to polluted urban regions. Figure 16.38 shows ranges of observed NOx and OH-reactivity adjusted VOC (expressed relative to propene) in four... 

FIGURE 16.38 Observed NO, and OH-reactivity-adjusted VOC (expressed as propene) in various regions of the troposphere. Isopleths shown are midday rates of Oj production (ppb h l) calculated using a box model (adapted from Chameides et ol., 1992). 

The amount of grafted metal is usually low, for several possible reasons low support OH reactivity, chlorination of the surface, and release of metal species during hydrolysis. Bond and Bruckman [30] mentioned that it is difficult to obtain a monolayer of vanadium species on Ti02 because of the chlorination of the support surface by the HC1 evolved during the anchoring of VOCI3 at 313 K. 

There appears to be a gradual increase in OH reactivity with uracil as the pH is varied from 5 to 10.5 (Figure 8). There is no abrupt change in reactivity at the pKa indicating that the changes in charge and tautomeric form which occur at the pKa have no effect on the OH reaction. The lower values for the -OH reactivity obtained at pH 2 and 13.5 are both considerably in doubt. The value at pH 2 is doubtful because of the possibility that H- may be entering into the reaction in some way, and the pH 13.5 value because this is above the pKa for the dissociation... 

Rates with Other Nucleic Acid Derivatives. The -OH reaction rates with uridine, UMP, and adenine are compared in Table VI with those for uracil obtained using the same CNS" competition method. Uridine and 2, 3 -UMP (mixed) at neutral pH show lower corrected OH reaction rates of 2.9 X 109M"1 sec. 1 and 3.5 X 109M 1 sec."1 respectively, indicating that the ribose sugar and phosphate groups are not highly reactive sites for -OH attack. Adenine, the complementary purine base to uracil in RNA has a much lower -OH reactivity than uracil of 1.9 X... 

With strong reservation one may suggest the following order of OH reactivities in glycosides 6-OH > open-chain OH (e.g., 5-OH in furanosides) > 2-OH > 3-OH = 4-OH and equatorial OH > axial OH. These rules are, however, by no means reliable. 

To account for the combined effect of OH reactivity and concentration, one may adopt an OH-reactivity-based method (National Research Council, 1991). In this method, one defines a propene-equivalent concentration, Prop-Equivfy), for each VOC species j. This equivalent concentration is given by... 

Conc(7) is the concentration of species j expressed in ppbC, oH(y> is the rate constant for the reaction between species j and OH, and oh(C,H6) is the rate constant for the reaction between OH and propene. Prop-Equiv(7) is a measure of the concentration of species j on an OH-reactivity-based scale normalized to the reactivity of propene and is literally the concentration (in parts per billion carbon) required of propene to yield a carbon oxidation rate equal to that of VOC species j. Thus if a VOC species has an atmospheric abundance of 10 ppbC and is twice as reactive as propene, its Prop-Equiv is 20 ppbC if the species is half as reactive as propene, its Prop-Equiv is 5 ppbC. The use of propene s reactivity as the normalization factor in the above formulation is completely arbitrary, and equivalent results would be obtained if the reactivity of any other species is chosen. 

Examples of NO, sinks are the formation of organic nitrates and PANs (which are also sinks for radicals). The generation or loss of radical species can lead to a net formation (y > 1) or net loss (7 < 1) of OH radicals. This in turn leads to an enhancement or suppression of radical concentrations in the air parcel and to an enhancement or suppression of the overall reactivity of all VOCs in that air parcel by affecting the rate of formation of per-oxy radicals. Although the concentrations and OH reactivities of VOCs can vary by orders of magnitude from one species to another, the mechanistic reactivities of the VOC species generally found in the atmosphere are fairly uniform, varying only by factors of 2 or 3 from one species to another (Table 5.6). 

Figure 8. Hypothetical mechanisms for C and HCOj -enhanced radiolytic killing of a bacterium by extracellular pathways. Cl radical formation is suppressed by OH-reactive compounds in solution, but COj radical formation is not. In this case, protection by free radical scavengers must arise by direct competition for the CO3 radical. The reaction scheme also includes an intracellular pathway, although target sites are not specified.

Reactions were run at 295 K at 1 1 and 1 3 ratios of NCO OH reactive groups. Reaction product spectra were taken on a JR-75 device in the range of 400-4000 cm at 295 K with specimens in the form of KBr disks. 

The C-3 OH is always the least reactive regardlessofthe type of reaction. However, in some cases the preferred OH group depends on the reactants and on the D.S. Thus esterification with toysl chloride occurs at the primary alcohol but benzoyl chloride gives the C-2 and C-6 derivatives. For the reaction of alkali cellulose with chloroacetic acid the reaction occurs at the C-6 OH at low D.S. values but as the D.S. increases the C-2 OH reactivity increases. 

Table 1 Rate constants at room temperature and formal activation energies for the total OH reactivity of aromatics, qh = abstr + add obtained from measurements in 130 mbar Ar (slightly below the high-pressure limit) and within the indicated temperature range. Rate constants for the abstraction pathway are given at temperatures were the observed losses are not dominated by the unresolved loss of adduct not leading back to OH.

In a preliminary study also involving model compounds [81], the kinetics of urethane formation was followed by FTIR spectroscopy using an aliphatic and an aromatic monoisocyanate and their homologous diisocyanates. Both the model reactions and the polymer synthesis gave clear cut second-order behaviour, indicating that the hydroxyl groups borne by the suberin monomers displayed conventional aliphatic-OH reactivity. 

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