Peroxy radicals elimination reactions

Here, the peroxy radical eliminates superoxide to form a carbocation which then undergoes hydrolysis to form a product leading to chain scission. Reactions (23)-(24) show an alternative route for the same peroxy radicals. In this case, there is a bimolecular reaction with another peroxy radical ... 

The biperoxy radical produced by the ceric ion oxidation of 2,5-di-methylhexane-2,5-dihydroperoxide decays rapidly with first-order kinetics [k = ioio.e exp( -11,500 1000)/RT sec.1 = 180 sec."1 at 30°C. (30)]. After the first-order decay has run to completion, there is a residual radical concentration (—4% of the initial hydroperoxide concentration) which decays much more slowly by a second-order process. The residual second-order reaction cannot be eliminated or changed even by repeated recrystallization of the dihydroperoxide. This suggests that a small fraction of the biperoxy radicals react intermolecularly rather than by an intramolecular process and thus produce monoperoxy radicals. The bimolecular decay constant for this residual species of peroxy radical is similar to that found for the structurally similar radical from 1,1,3,3-tetra-methylbutyl hydroperoxide. Photolysis of the dihydroperoxide gave radicals with second-order decay kinetics which are presumed to be 2,5-hydroperoxyhexyl-5-peroxy radicals. 

If oxygen is present, organic peroxy radicals ROO° can be formed. These can further react, eliminating 02°7H02° and so enter again into the chain reaction. 

The yield of superoxide radicals given in Table 1 amounts to some 60% of the hydroxyl-radical yield and most likely arises from the elimination of superoxide from peroxy radicals, the latter being formed in Reaction (15). From the careful design of these experiments, the yield of superoxide also represented the maximum yield of those hyaluronic-acid peroxy free radicals... 

The rate of this reaction is furthermore independent of acid or base concentration and would therefore be a significant process even at neutral pH values. Peroxy radicals formed on the same carbon atom as hydroxy groups are also expected to eliminate superoxide radicals relatively rapidly [98]. For such species, the rate of elimination is accelerated in both acid and base. A typical reaction is... 

Some chemicals retard or suppress free-radical polymerization by reacting with primary radicals or macroradicals to yield radicals that are very stable to further reaction or yield nonradical products. These materials could be retarders or inhibitors. Retarders slow down the formation of polymer but inhibitors completely eliminate it. Oxygen is one of the most commonly known inhibitors for vinyl polymerization and good practice requires the removal of air from the reactor vessels before the reaction is started. It combines with active radicals to form unreactive peroxy radicals. 

According to Reaction sequence (4.35), one diphenylamine molecule eliminates four peroxy radicals and, by definition, the stoichiometric factor is 4. For a steri-cally hindered monophenol, this factor is 2. Hence diphenylamines perform better as peroxy radical scavengers than do monophenols at temperatures <120°C. Under... 

Phosphites with substituted phenoxy groups also behave as peroxy and alkoxy radical scavengers forming relatively stable phenoxy radicals, which again eliminate peroxy radicals. Reaction (4.55) ... 

Under thermal conditions, hydroxylamine ethers can reversibly decompose (Reaction 15). The radicals formed disproportionate to eliminate olefins and yield hydroxylamine (Reaction 16). In the presence of sufficiently effective acceptors of alkyl radicals (e.g., oxygen), the reaction rate of peroxy radical formation is much higher than that of hydroxylamine formation. Thus, in the process of polymer photooxidation, nitroxyl radicals regenerate and can break multiple oxidative chains. 

Neither of the peroxy radicals derived from HCFC-123 and HFC-125 contain a-hydrogen atoms. This has two implications. First, the molecular channel, reaction (30) is not feasible thus kjk = 1. Second, reaction (31) between the alkoxy radical and O2 is not possible, eliminating the interference due to secondary removal of the peroxy radicals by HO2. However, both alkoxy radicals provide other sources of secondary complications due to their dissociation. 

Direct spectroscopic measurements of HCFC-based alkoxy radicals and real-time determinations of their reaction rates are scarce. The laser-induced fluorescence (LIF) spectrum [124,12S] of CF3O has been measured and a UV absorption feature has tentatively been assigned [92] to CF3CFHO. Three-center elimination of HCl from CH3CHCIO and CHjClO has been observed by real-time measurement of the HCl production [10,87], but the alkoxy radicals have not been directly detected. In other cases, our understanding of alkoxy radical chemistry comes from product studies and theoretical considerations. Below we summarize the fate of the alkoxy radicals derived from the peroxy radicals listed in Tables 6 and 7. 

Elimination of HOO- from peroxy radicals having a-hydrogens is another common pathway for the formation of superoxide. For example, ethanol reacts with OH by hydrogen atom transfer to give a mixture of several radicals dominated by 5, which rapidly reacts with oxygen to form a peroxy radical that eliminates OOH (with the concomitant production of acetaldehyde). This reaction is not as efficient with the peroxy radical of t-butanol, which has no a-hydrogens (von Sonntag, 1987). 

Addition reactions of peroxy radicals with olefins (Equation 4.63) have often been described (Mayo, 1958 Hamberg and Gotthammar, 1973). Among the stable products are epoxides, possibly formed by elimination of alkoxy radicals. The structural constraints on epoxide formation are quite stringent and the overall rate constants for their formation can vary by three or more orders of magnitude. 

A chain branching process can result from addition reactions of an alkyl peroxy radical to olefins or acetylenes, where vinyl, or acetyl peroxides are formed. The initial radical formed by the addition of the peroxy radical would undergo an addition reaction with molecular oxygen, then undergo molecular elimination of HO2 to form an unsaturated peroxide. The unsaturated peroxide would then undergo rapid cleavage of the weak peroxide bonds. A mechanism that implements this chain branching results from ROO addition to olefins is illustrated by ... 

Attack of NO3 on isoprene apparently proceeds in much the same manner, but there is considerable controversy about the precise reaction pathway because of the variety of peroxy radicals that can be formed. The products, such as 4-nitroxy-2-methyl-l-butan-3-one and methacrolein, are consistent with the initial addition of NO3 to the terminal carbon atoms to form nitro-oxy-peroxy radicals in the presence of oxygen apparently the NO3 adds preferentially to position 1 (Fig. 12). 3-methyl-4-nitroxy-2-butenal was found as the main product in these experiments. The nitro-oxy-peroxy radicals can react with NO2, in the presence of O2, to yield thermally unstable nitroxy-peroxynitrate compounds. One particularly important feature of the addition of NO3 is the extent to which the initial adduct, which might eliminate NO2 to form an epoxide, is actually converted to the nitro-oxy-peroxy radicals in the atmosphere. 

While amines and some annular hydrocarbons are suitable chain terminators, hindered phenols such as di-l-butyl-p-cresol (alias butylated hydroxytoluene or BHT) are most popular because they avoid discolorization and they eliminate two free radicals per BHT molecule (Fig. 54.3). The resonance-stabilized aryloxy radical is protected by the bulky electron-releasing l-butyl groups in the 2 and 6 positions, so the hindered phenol can combine with a second peroxy radical but cannot combine readily with molecular oxygen or with another aryloxy radical nor abstract H atoms from the polymer to initiate a new firee-radical chain reaction. 

Dietz, et al., (7 ) which was subsequently expanded by Gibian and Ungermann (12). The course of the reaction is dependent upon both the alkyl halide structure and the reaction solvent. For a primary alkyl halide, the displacement of halide ion (eq. 4) is dominant over the elimination reaction (eq. 5) as determined by product analysis e.g., for the reaction of KO2 with 1-bromo-octane and 2-bromooctane in DMSO, olefins were isolated in yields of 1% and 34%, respectively ( ). It is apparent that secondary halides give significant olefin yields under these experimental conditions. The H02 formed in equation 5 may ionize to superoxide (eq. 6) or be reduced (eq. 7). The bulk reaction of the alkyl peroxy radical with 0 (eq. 8) is the analogue to equation... 

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