Secondary reactions concentration profile

Figure 8.19 Concentration profiles for a concurrent reaction, e.g. of a secondary alkyl halide + OH- -> alcohol reaction (2) is twice as fast as reaction (1) in this example...

Hydroxyl radical, OH, is the principal atmospheric oxidant for a vast array of organic and inorganic compounds in the atmosphere. In addition to being the primary oxidant of non-methane hydrocarbons (representative examples of these secondary reactions are given in Table 6), OH radical controls the rate of CO and CH4 oxidation. Furthermore, the OH reaction with ozone also limits the destruction of O3 in the troposphere, it also determines the lifetime of CH3CI, CHsBr, and a wide range of HCFC s, and it controls the rate of NO to HNO3 conversion. Concentration profiles for hydroxyl radical in the atmosphere are shown in Fig. 2. 

Pore-water concentration profiles of redox-sensitive ions (nitrate, Mn, Fe, sulphate and sulphide) and nutrients (ammonium and phosphate) demonstrate the effects of degradation of OM. In freshwater sediments, the redox zones generally occur on a millimetre to centimetre scale due to the high input of reactive OM and the relatively low availability of external oxidators, especially nitrate and sulphate, compared to marine systems. A typical feature for organic-rich freshwater sediments deposited in aerobic surface waters, is the presence of anaerobic conditions close to the sediment-water interface (SWI). This is indicated by the absence of dissolved oxygen and the presence of reduced solutes (e.g. Mn, Fe and sulphides) in the pore water. Secondary redox reactions, like oxidation of reduced pore-water and solid-phase constituents, and other postdepositional processes, like precipitation-dissolution... 

Some of the identified products could be assigned unequivocally to secondary reactions. The time profile [34, 35] and the pressure dependence of the fluorescence [36] of the spin-forbidden by-product NH(A) observed during 121.6 nm photolysis [37] indicates its origin from the secondary reaction of HN3 with electronically excited N2. The concentration of NH(A) reaches 5% or less of the NH formed [36]. Other secondary products like NH2 and excited NH(A) were found spectroscopically at a quantum yield close to 2 when the used... 

For deactivated compounds this limitation does not exist, and nitration in sulphuric acid is an excellent method for comparing the reactivities of such compounds. For these, however, there remains the practical difficulty of following slow reactions and the possibility that with such reactions secondary processes might become important. With deactivated compounds, comparisons of reactivities can be made using nitration in concentrated sulphuric acid such comparisons are not accurate because of the behaviour of rate profiles at high acidities ( 2.3.2 figs. 2.1, 2.3). 

We looked briefly at reaction profiles in Section 8.2. Before we look at the reaction profile for the concurrent reactions of hydrolysing a secondary alkyl halide, we will look briefly at the simpler reaction of a primary alkyl halide, which proceeds via a single reaction path. And for additional simplicity, we also assume that the reaction goes to completion. We will look not only at the rate of change of the reactants concentration but also at the rate at which product forms. 

However, without knowledge of the source of the increased OH flux, extrapolation of the concentration-time profiles of both the primary and secondary pollutants observed in such smog chamber studies to real atmospheres becomes less certain. For example, the reactions leading to the unknown precursor(s) to OH may occur only in smog chambers. Extrapolation to ambient air would thus require subtracting out this radical source. On the other hand, the same reactions may occur in ambient air where surfaces are available in the form of particulate matter, buildings, the earth, and so on if this is true, then the rates would be expected to depend on the nature and types of surfaces available and may thus differ quantitatively from the smog chamber observations. 

The results showed that some of the expected reaction products were formed. Figure 2a shows the time profiles of NMP, NMS and succinimide, while Figure 2b shows the corresponding plots of product concentrations versus the concentration of NMP consumed. The latter plots clearly show that NMS was a primary reaction product, whereas succinimide was a secondary one. The carbon balance of NMS production is 55 1 %. This indicates that other primary reaction products have to be formed. Figure 2 shows that one unidentified compound was a primary product. Furthermore, the HPLC chromatograms contained a lot of unidentified and unresolved peaks (Figure 3). Therefore, an on-line mass spectrometer was designed in order to identify more reaction products. 

Dinitrogen trioxide reacts with the unshared pair of electrons on unprotonated secondary amine by a nucleophilic substitution reaction to form nitrosamines. The rate of nitrosation of secondary amines in a weakly acidic aqueous solution is proportional to the concentration of the amines and to the square of the nitrite concentration. The concentrations of these two precursors depend on the pH of the medium. While the concentration of unprotonated amines increases when pH increases, the concentration of nitrous acid increases when the pH decreases. Hence, the pH rate profile for the nitrosation of amines shows a maximum resulting from the interaction between these two opposite... 

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