Radical nonselective

Steam Cracking. Steam cracking is a nonselective process that produces many products from a variety of feedstocks by free-radical reactions. An excellent treatise on the fundamentals of manufacturing ethylene has been given (44). Eeedstocks range from ethane on the light end to heavy vacuum gas oil on the heavy end. All produce the same product slate but in different amounts depending on the feedstock. 

Compared with the sensors for atoms and radicals, the calibration of EEP sensors is also somewhat specific. To calibrate detectors of atomic particles, it will be generally enough to determine (on the basis of sensor measurements) one of the literature-known constants, say, tiie energy of parent gas dissociation on a hot Hlament. For the detection of EEPs when nonselective excitation of gas is taking place, in order to calibrate a sensor use should be made of some other selective methods detecting EEPs. The calibration method may be optical spectroscopy, chemical and optic titration, emission measurements, etc. 

If the initiator is a peroxide compound capable of generating very active but nonselective alkoxyl radicals, the concentration of the inhibitor must be sufficiently high to provide the above conditions. For instance, the ratio krJk2i=50 (408 K) for the reaction of the tert-butoxyl radical with ionol and cyclohexane [46]. In this case, alkoxyl radicals will react with InH more readily than with RH, provided [InH] > 0.2 mol L-1. 

Selective combination of the secondary geminate radical pairs occurs in the micelle, compared to nonselective free-radical combination reactions in solution. This results from the micelle host effectively constraining the separation of the geminate radical pair. 

Paraquat (1,1 dimethyl, 4,4 bipyridyl) is a nonselective contact herbicide. It is used almost exclusively as a dichloride salt and usually is formulated to contain surfactants. Both its herbicidal and toxicological properties are dependent on the ability of the parent cation to undergo a single electron addition, to form a free radical that reacts with molecular oxygen to reform the cation and concomitantly produce a superoxide anion. This oxygen radical may directly or indirectly cause cell death. Diquat, l,T-ethylene-2,2 -dipyridylium, is a charged quaternary ammonium compound often found as the dibromide salt. The structure of diquat dibromide and that of the closely related herbicide paraquat can be seen in Fig. 4.5. 

Because advanced oxidation processes are based on hydroxyl free-radical chemistry, chemical interactions are highly nonselective. Rates of destruction vary with such factors as the nature of the contaminant mixture, pH, concentration of contaminants, presence of scavengers, and inorganic nature. 

Piroxicam, an oxicam (Figure 36-1), is a nonselective COX inhibitor that at high concentrations also inhibits polymorphonuclear leukocyte migration, decreases oxygen radical production, and inhibits lymphocyte function. Its long half-life (Table 36-1) permits once-daily dosing. 

In gas-phase hydrobromination, where a radical mechanism is operative, the bromine atom always adds to the central carbon atom of the allenic system. As a result, vinylic bromides are formed through the stable allylic radical. In the solution phase under ionic addition conditions, either the vinylic or the allylic cation may be the intermediate, resulting in nonselective hydrobromination. Allylic rearrangement or free-radical processes may also affect product distributions. 

As a result, free-radical chlorination of alkanes is a nonselective process. Except when only one type of replaceable hydrogen is present (methane, ethane, neopentane, unsubstituted cycloalkanes), all possible monochlorinated isomers are usually formed. Although alkyl chlorides are somewhat less reactive than alkanes, di- and polychlorinations always occur. The presence of a chlorine atom on a carbon atom tends to hinder further substitution at that carbon. The one exception is ethane that yields more 1,1-dichloroethane than 1,2-dichloroethane. The reason for this is that chlorination of an alkyl chloride occurs extremely slowly on the carbon atom adjacent to the one bearing the chlorine atom (vicinal effect).115... 

Since free-radical chlorination is a nonselective process, overchlorination may be a problem in the manufacture of ethyl chloride. Temperature-induced pyrolysis to yield ethylene and hydrogen chloride may occur, too. A fluidized-bed thermal chlorination reactor may be used to overcome these problems. The best selectivity achieved in the temperature range of 400-450°C is 95.5% with a chlorine to ethane ratio of 1 5. 

R (CH3 OH, ho2 ) + CH3CH2COCH3 RH(CH4, h2o, h2o2) + CH3CHCOCH3 The hydroxyl radical is typically nonselective, and will give... 

The indirect reaction pathway involves radicals. The first step is the decay of ozone, accelerated by initiators, e. g. OH , to form secondary oxidants such as hydroxyl radicals (OH°). They react nonselectively and immediately (k = 108 - I010 IVT1 s-1) with solutes (Hoigne and Bader, 1983 a, b). The radical pathway is very complex and is influenced by many substances. The major reactions and reaction products of the radical pathway based on the two most important models are discussed below (Staehelin and Hoigne, 1983 a, b Tomiyasu et al., 1985). 

The decay of ozone, which can be initiated by the hydroxide ion, leads to a chain reaction and produces fast-reacting and thus nonselective OH-radicals. This also means that OH-radicals have a very short half-life, e.g. less than 10 ps at an initial concentration of I O 4 M. 

Advanced oxidation processes (AOPs) have been defined by Glaze et al. (1987) as processes which involve the generation of hydroxyl radicals in sufficient quantity to effect water purification . The most common processes are 03 /H202, 03/UV and H202/U V. The development of the AOPs is an attempt to produce the nonselective and rapid OH0 to oxidize pollutants. Each of these processes involves chemistry similar to that discussed above. 

Organic micropollutants are found in surface and ground waters, always in conjunction with more or less NOM, but at relatively low concentrations in the range of 0.1 pg L I to 100 pg L-1 (in water sources of sufficient quality for a water supply). Their degradation by ozone to oxidized metabolites or even to mineral products is a complex process, due to the influences of various water quality parameters (pH, inorganic and organic carbon etc.) on the two known major reaction pathways direct electrophilic ozone reaction and the oxidation via the nonselective, fast reacting OH-radicals. 

Rate constants for the reaction of hydroxyl radicals with different compounds were determined by Haag and Yao (1992) and Chramosta et al. (1993). In the study of Haag and Yao (1992) all hydroxyl radical rate constants were determined using competition kinetics. The measured rate constants demonstrate that OH0 is a relatively nonselective radical towards C-H bonds, but is least reactive with aliphatic polyhalogenated compounds. Olefins and aromatics react with nearly diffusion-controlled rates. Table 4-3 gives some examples comparing direct (kD) and indirect (kR) reaction rate constants of important micropollutants in drinking water. 

Write initiation, propagation, and termination steps for this radical-chain reaction. Estimate a AH0 for the overall reaction using the bond-dissociation energies of Table 4-6. Would you expect bromotrichloromethane to be a selective or nonselective brominating agent Explain. 

---