Products in the Absence of Oxygen

Pyrimidines. Detailed studies concerning the products that are formed upon OH-attack (radiolysis of N20-saturated solutions) are available for Ura, Thy, Cyt and l,3Me2Ura (Tables 10.9-10.12). 

In addition, the radiolysis of Ura in deoxygenated solutions (in the absence of N20) has also found attention (Infante et al. 1974 Shragge et al. 1974). Under such conditions, however, not only the OH-induced reactions play a role, but also the electron-adduct radical with all the ensuing mechanistic complications contributes to the products. 

In competition, the C(6)-yl and C(5)-yl radicals may disproportionate, possibly via an adduct [reactions (80) and (81)]. This yields the hydrate via an enol [reaction (83)]. The other product is the glycol [reaction (82)]. In the original paper (Al-Sheikhly and von Sonntag 1983), it has been proposed that it maybe formed in an ET reaction. Due the considerable rearrangement energies involved in ET reactions as compared to radical recombination reactions, it is now considered that this ET reaction might occur via an addition/elimination process [reactions (80) and (81)] such as has also been found for other systems. 

In this context, it should be mentioned that the glycols derived from Thd (Tg) are now also very well characterized (Jolibois et al. 1996). 

As can be seen from Table 10.12, there are practically no changes in the product yields when neutral and basic solutions are compared. However, the yield of the dimers is drastically reduced in acid solutions, while that of the glycol is enhanced. Altogether, the G value of consumption is also reduced which points to an increase of the importance of disproportionation reactions. This has been explained by an acid-catalyzed transformation of the reducing C(6)-yl radical into the oxidizing C(5)-yl radical [reactions (85) and (86) Al-Sheikhly and von Sonntag 1983]. 

In fact, the dominant radical seen by EPR when uracil is y-irradiated in low-temperature sulfuric-acid glasses, is the 5-yl one [34], which supports this contention under the assumption that the nucleobase is not protonated under these conditions. 

In the presence of a thiol, the OH-adduct can be reduced, the reducing C(5)-OH-C(6)-yl radical, paradoxically, apparently more readily than the oxidizing C(6)-OH-C(5)-yl one. Nevertheless, the rate of this reaction must be very slow, as in the radiolysis of N20-saturated solution of 5 -thymidylic acid in the presence of 5% cysteamine, thioethers Le. the recombination products of a cysteamine-derived thiyl radical with a 5 -thymidylic-acid-derived OH-adduct radical, are formed with a G value as high as 1.5 X 10 mol J [35] in a competitive reaction [36]. 

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The detectable reaction products, in the absence of oxygen, were N2, N20, CO and C02, formed according to the following overall reactions ... 

Reduction of the simple AT-methylpyridinium ion, 103, is believed initially to give the expected AT,AT -dimethyltetrahydro-4,4 -bipyridine, but the end product (in the absence of oxygen) is the A/, A/ -dimethyIbipyridine radical cation formed by a formal loss of two hydride ions and one-electron reduction of A/. A/ -dimethylbipyridinium [297,298]. The isolated product, 27,27 -dimethylbipyridinium dication, results from air-oxidation of the radical cation [298] ... 

Conversion of quartz into tridymite under the effect of various mineralizers is dealt with in numerous works. The results of Holmquist (1961) imply that alkali oxides bring about tridymitization with cristobalite as a possible intermediate transition product. In the absence of oxygen, alkali chlorides always brought about the formation of cristobalite but not tridymite. The conversion to tridymite therefore obviously requires the formation of a solid solution. The problem of the formation of tridymite in the presence of mineralizers is significant, in particular in the field of refractories where it is dealt with in more detail. 

The heat of decomposition, AH, of the products in the absence of oxygen. The reaction is considered very hazardons, with potential for deflagration if AH > 0.2 to 0.3 MJ/kg (AlChE, 1995), bnt the absolnte valne varies. King and CHETAH (King, 1990) cite AH > 1.25 MJ/kg as being hazardons with AH, > 2.9 MJ/kg being likely to explode when snbjected to mild heat or shock. 

Thermally, dithiins may react as dienophiles in [2 -f 4] cycloadditions <86SR123>. Photochemically, they afford [2 -I- 2] cyclodimerization products in the absence of oxygen in its presence, they are degraded to carbon dioxide and hydrogen sulfide among other photooxidation products <82TL2651 >. 

Equilibrium (3) is the main (but not the only) equilibrium which determines whether thiyl radical generation implies oxidant or reductant production. In the absence of oxygen (see below. Section 3.2), the fraction of thiyl in the unconjugated, oxidizing form,/os is easily calculated as ... 

In the absence of oxygen the photodecomposition of adenosylcobalamin leads to the formation of Co +-cobalamin (22) and a 5 -deoxyadenosyl that cy-clizes to 8,5-cyclic-adenosine (23). In the presence of oxygen, aquocobalamin and adenosine-5 -carboxaldehyde are formed (24). Photolysis of methylcobala-min occurs very rapidly in aqueous solution with formation of formaldehyde and aquocobalamin as the major products. In the absence of oxygen the reaction is rather slow and gives rise to the formation of Co -cobalamin and methane (25,26). Remarkably, photolysis of methylcobalamin in the presence of homocysteine yields methionine, a methylation reaction that under aerobic, intracellular conditions occurs only in an enzyme-catalyzed reaction with reductive activity... 

Pasteur effect Yeast and other cells can break down sugar in the presence of oxygen (eventually to CO2 and H2O) or in its absence (to CO2 and ethanol). The decomposition of sugar is often greater in the absence of oxygen than in its presence, i.e. the Pasteur effect. With oxygen, less toxic products (alcohol) are produced and the breakdown is more efficient in terms of energy production. 

Coal can be converted to gas by several routes (2,6—11), but often a particular process is a combination of options chosen on the basis of the product desired, ie, low, medium, or high heat-value gas. In a very general sense, coal gas is the term appHed to the mixture of gaseous constituents that are produced during the thermal decomposition of coal at temperatures in excess of 500°C (>930°F), often in the absence of oxygen (air) (see Coal CONVERSION PROCESSES, gasification) (3). A soHd residue (coke, char), tars, and other Hquids are also produced in the process ... 

Pyrolysis. Heating in the absence of oxygen releases moisture at low temperatures, carbon dioxide at temperatures >200° C, and a variety of gaseous products at very high temperatures. Acid washing of the raw coal is used to remove extractable cations, followed by treatment with selected cations. Yields of CO2, CO, CH, H2, and H2O depend on the amounts of inorganic species in the coal (42). 

Temperature. The temperature for combustion processes must be balanced between the minimum temperature required to combust the original contaminants and any intermediate by-products completely and the maximum temperature at which the ash becomes molten. Typical operating temperatures for thermal processes are incineration (750—1650°C), catalytic incineration (315—550°C), pyrolysis (475—815°C), and wet air oxidation (150—260°C at 10,350 kPa) (15). Pyrolysis is thermal decomposition in the absence of oxygen or with less than the stoichiometric amount of oxygen required. Because exhaust gases from pyrolytic operations are somewhat "dirty" with particulate matter and organics, pyrolysis is not often used for hazardous wastes. 

Chlorine and bromine add to benzene in the absence of oxygen and presence of light to yield hexachloro- [27154-44-5] and hexabromocyclohexane [30105-41-0] CgHgBr. Technical benzene hexachloride is produced by either batch or continuous methods at 15—25°C in glass reactors. Five stereoisomers are produced in the reaction and these are separated by fractional crystallization. The gamma isomer (BHC), which composes 12—14% of the reaction product, was formerly used as an insecticide. Benzene hexachloride [608-73-17, C HgCl, is converted into hexachlorobenzene [118-74-17, C Clg, upon reaction with ferric chloride in chlorobenzene solution. 

A Japanese process developed by Taogosei Chemical Co. chlorinates ethylene directly in the absence of oxygen at 811 kPa (8 atm) and 100—130°C (32). The products ate tetrachlorethanes and pentachloroethane [76-01-7J, which ate then thermally cracked at 912 kPa (9 atm) and 429—451°C to produce a mixture of trichloroethylene, perchloroethylene [127-18-4] and hydrochloric acid. 

As mentioned above (Section 2.13.2.1.3), bipyrimidine photoproducts can arise, probably by reaction between two radicals. Thus, irradiation of an aqueous solution of 5-bromouracil (ill R=Br) in the absence of oxygen produces a variety of products including uracil, barbituric acid, 5-carboxyuracil (111 R = CO2H), several non-pyrimidine compounds and, as a stable end-product, the biuracil (114 R = H). A similar product (114 R = Me) is formed from 5-bromo-l,3-dimethyluracil (ilS). When two such related uracil derivatives are irradiated together, a mixed bipyrimidine product is formed, inter alia (B-76MI21302). 

Free-radical polymerisation techniques involving peroxides or azodi-isobutyronitrile at temperatures up to about 100°C are employed commercially. The presence of oxygen in the system will affect the rate of reaction and the nature of the products, owing to the formation of methacrylate peroxides in a side reaction. It is therefore common practice to polymerise in the absence of oxygen, either by bulk polymerisation in a full cell or chamber or by blanketing the monomer with an inert gas. 

Polyethylene displays good heat resistance in the absence of oxygen in vacuum or in an inert gas atmosphere, up to the temperature of 290°C. Higher temperature brings about the molecular-chain scission followed by a drop in the molecular-weight average. At temperatures in excess of 360°C the formation of volatile decomposition products can be observed. The main components are as follows ethane, propane, -butane, n-pentane, propylene, butenes and pentenes [7]. 

Chlorine has been identified as a reaction product, and furthermore it has been shown that cracking can occur in chlorine gas in the absence of oxygen. 

Determinations have been made of the solubility of lead linoleate prepared in the absence of oxygen and extracted with air-free water. Under these conditions, lead linoleate had a solubility of 0-002% at 25°C and the extract was corrosive when exposed to the air. When, however, the extraction was carried out in the presence of air, the resulting extract contained 0 07% solid material and was non-corrosive. It was concluded that in the presence of water and oxygen lead linoleate yielded soluble inhibitive degradation products. 

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