Nitric oxide indirect reactions

Reaction 2-6 is sufficiently fast to be important in the atmosphere. For a carbon monoxide concentration of 5 ppm, the average lifetime of a hydroxyl radical is about 0.01 s (see Reaction 2-6 other reactions may decrease the lifetime even further). Reaction 2-7 is a three-body recombination and is known to be fast at atmospheric pressures. The rate constant for Reaction 2-8 is not well established, although several experimental studies support its occurrence. On the basis of the most recently reported value for the rate constant of Reaction 2-8, which is an indirect determination, the average lifetime of a hydroperoxy radical is about 2 s for a nitric oxide concentration of 0.05 ppm. Reaction 2-8 is the pivotal reaction for this cycle, and it deserves more direct experimental study. 

Ozone is produced in substantial concentrations by industrial activity and, indirectly, from automobile exhausts. The most important sequence of reactions producing tropospheric ozone begins with hydrocarbon vapors, nitric oxide, and sunlight ... 

Several brief preliminary reports in the literature indicate the formation of products from nitric oxide containing nitrogen-nitrogen bonds. Lithium aluminum hydride plus nitric oxide is reported to give rise to hyponitrite ion (17), Grignard reagents (27) and aluminum triethyl (3), when reacted with NO, give rise to intermediates which upon hydrolysis produce nitrosated alkyl-substituted hydroxyl-amines. These materials are reported to be unstable and evidence for their existence is indirect. If these products are indeed formed, the reactions can be easily incorporated into the BNO scheme. 

In addition to chemical reactions brought about directly by light energy, there are other atmospheric chemical reactions that can occur in the dark, but that typically involve reactive chemical species previously produced photo-chemically (cf. indirect photolysis in surface waters). The rates of such thermal, or dark, reactions depend on the temperature and on the concentrations of the reactive chemicals involved. For example, consider the rate of the dark reaction between the photochemically produced compounds ozone (03) and nitric oxide (NO). The rate is proportional to the concentration of each reactant... 

However, these indirect effects of nitric oxide derived products are far more prevalent under pathological conditions such as inflammation, where the production of both NO and by the professional phagocytic cell NADPH oxidase enzyme, and induction of iNOS yields the potent cytotoxic species peroxynitrite. Whilst nitric oxide will react with metal centres (as discussed above) at a rate of 5x 10 M" s and the superoxide anion can be dismutated by SOD at a rate of 2.3x10 M s the combined reaction below (Eq. 9), proceeds at a rate faster than either of these individual reactions ... 

Nitrite Nitrite is an important indicator of fecal pollution in natural waters as well as a potential precursor of carcinogenic species. A rush of flow and sequential injection spectrophotometric method based on Griess-type reactions has been proposed, also coupled to online sorbent enrichment schemes. The catalytic effect of nitrite on the oxidation of various organic species constitutes the basis of fairly sensitive spectrophotometric methods. Fluorometric methods based on the formation of aromatic azoic acid salts, quenching of Rhodamine 6G fluorescence, and direct reaction with substituted tetramine or naphthalene species have been also reported. Indirect CL methods usually involve conversion into nitric oxide and gas-phase detection as mentioned in the foregoing section. The redox reaction between nitrite and iodide in acidic media is the fundamental of a plethora of flow injection methodologies with spectrophotometric, CL, or biamperometric detection. New electrochemical sensors with chemically modified carbon paste electrodes containing ruthenium sites, or platinum electrodes with cellulose or naphthalene films, have recently attracted special attention for amperometric detection. 

The presence of HMX as an impurity in RDX is not a problem when the product is used as an explosive. However, the need for an analytical sample of RDX makes other more indirect methods feasible. One such method involves the oxidation of 1,3,5-trinitroso-1,3,5-triazacyclohexane (109) ( R-salt ) with a mixture of hydrogen peroxide in nitric acid at subambient temperature and yields analytical pure RDX (74%) free from HMX." The same conversion has been reported in 32 % yield with three equivalents of a 25 % solution of dinitrogen pentoxide in absolute nitric acid. l,3,5-Trinitroso-l,3,5-triazacyclohexane (109) is conveniently prepared from the reaction of hexamine with nitrous acid at high acidity. ... 

The cerium (TV)-assisted process (Aurousseau, Roizard, Storck, Lapicque, 1996 Kreysa Jiittner, 1994 Nzikou, Aurousseau, Lapicque, 1995) is an indirect outercell process with Ce + as a redox mediator for the simultaneous oxidation of SO2 and NOx to sulfuric acid and nitric acid, respectively. The process scheme and the main reactions are depicted in Figme 14.3. 

Oxidations by peroxynitrite can take place either directly by ground-state peroxynitrous acid, ONOOH, or indirectly by ONOOH where ONOOH is an activated form of peroxynitrous acid (Goldstein et al. 1996). In the direct oxidation pathway the reaction is first order in peroxynitrite and first order in substrate, and the oxidation yield approaches 100%. In the indirect oxidation pathway the reaction is first order in peroxynitrite and zero order in substrate. In the presence of sufficient concentrations of a substrate that reacts by the indirect oxidation pathway, about 50-60% of the ONOOH directly isomerize to nitric acid, and about 40-50% of the ONOOH is converted to ONOOH. The involvement of hydroxyl radicals in indirect oxidations by peroxynitrite is ruled out on the basis of kinetics and oxidation yields. 

Oxidation of an Aldose to the Corresponding Aldonic Add, Bromine or nitric acid are the main oxidants, the latter under mild conditions. The best yields are obtained by the use of bromine in a slightly acid buffered solution (pH 5-6) (see p. 340). The products are generally isolated as the metallic salts by direct crystallization from the reaction solution or by precipitation into ethanol. Yields as high as 95% have been reported in the case of glucose. Commercially the indirect use of bromine as an oxidant is employed in the electrolytic oxidation process with calcium bromide as a catalyst the constant regeneration of free bromine in the solution allows a very economical operation. In the case of rhamnose, the oxidation product can be isolated directly as the lactone this is one of the few cases for which recourse to metallic salts is not necessary. 

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