Disproportionation/recombination reactions

In contrast to several other rearrangements of the penicillin sulfoxide, the reaction with 155 was also successful with the free acid (4) wherein 157 was isolated with no reported decarboxylation. Treatment of the acid (157) with sodium bicarbonate afforded a disproportionation-recombination reaction resulting in the azetidinone disulfide U58) and the benzthiazoyl disulfide (159). 

It is seen that the rate constant ks is lower for compounds with electron-accepting substituents than with electron-donating substituents, which implies a dependence of the rate of Ar20 and R02 recombination on the electron density at the para- and ort/zo-positions of the benzene ring of the phenoxyl radical. The activation energies of this reaction vary from -33 to 10 kJ mol-1 however, the concurrent variation in the pre-exponential factor from 103 to 1010 L mol-1 s-1 causes a strong compensatory effect. It can also be seen that phenoxyl radicals readily react with peroxyl radicals k= 10s—109 L mol-1 s-1), whereas the disproportionation of peroxyl radicals is sufficiently slower (see Chapter 2). Hence, during the oxidation of hydrocarbons in the presence of phenols when k7[ArOH] > /c2[RH], the recombination reaction of ArO with R02 is always faster than the reaction of disproportionation of peroxyl radicals. 

The following reactions may occur combination, recombination, reaction with radical acceptors, disproportionation and transfer reactions. 

It is known that polystyrene irradiated at 45"C, for example, has G(x) = 0.04 and G(s) — 0.02 (20,29,30) which are similar values to those found in this study. Increasing the temperature during irradiation, favored the disproportionation reaction of chain scission radicals relative to recombination reactions and thereby increased main chain scission yields (20) (Figure 7). 

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. 

Termination steps are generally recombination reactions, though disproportionation can occur. Recombination reactions occur when two radicals combine to give one molecule, while disproportionation occurs when two radicals react to form two molecules, rather than recombining to form one ... 

Phenoxyls (and their mesomeric forms) participate in autorecombination (coupling) processes, recombination with alkylperoxyls and disproportionation reactions. The transformation of individual nuclei in polynuclear phenols proceeds independently. This accounts for a rather complicated mixture of products. Recombination reactions are influenced by steric effects of substituents. Fully hindered phenols (containing tertiary alkyls in both ortho positions to the phenolic hydroxy group, such as in 11,12,14 and 16) are... 

A rather indirect mode of reaction accounts for 60% of the product trans-stilbene (reaction sequence 1, 2, 4, 5, and 6 in Table VII). The importance of this path may be traced to both the low disproportionation/recombination ratio for resonance stabilized radicals, and the high rate constant for 3-bond scission of the intermediate radical recombination product, 1,2,3,4-tetraphenylbutane. Analogous decomposition routes involving formation of an adduct, followed by irreversible destruction of the adduct, may be important reaction paths in many other coal related reaction systems. 

The question of cage recombination 22) merits special consideration in these systems. The most sensitive way to check for it was to test if meso/DL equilibrations occurred in the course of the thermolysis reaction of a pure diastereomer I8, 20). Additional evidence for the unimportance of cage dimerizations are the high disproportionation-recombination ratios found for most of the radicals involved 9- 23) and the high fluid-... 

In the important reactions discussed above we have three free radicals HCO, CjHs, and CH3CH2CO. We have already considered all radical-molecule reactions. Now we must consider all radical-radical reactions. Such reactions are of two kinds, recombination and disproportionation. The reactions of radical recombination are ... 

The constancy of product ratio relative to changes of light intensity and reactant pressure, indicate pripaary photochemical production of four radicals, CH3, CHO, H, and CH3CO, with, therefore, ten recombination reactions and a large number of disproportionation reactions. 

It has been proposed that the decarbonylation of aldehydes by the Wilkinson catalyst [RhCl(PPh3)3] involves a radical pair disproportionation or recombination reaction. A radical pair intermediate in solution is equivalent to a cage reaction (Scheme 6). Table 15 shows the results obtained from the decarbonylation of a series of chiral cyclopropyl aldehydes ... 

An equilibrium between 0- and N-centered radical is suggested. Complication in this simple mechanism is caused by the participation of the respective radicals in coupling, disproportionation, oxidation and recombination reactions. It is connected with the formation of products different from those in the original mixture and not involved in the regeneration cycle. A stepwise depletion of antioxidant active species depedent on both the amine and phenol structures should therefore be considered. 

It is very clear that if the initiator has hydroxyl groups, and if the termination takes place exclusively by recombination then a polymeric diol is obtained [2, 3], which is ideal for polyurethane. If the termination takes place by disproportionation, only monofunctional compounds are obtained, which cannot be used in PU. The vinylic and dienic monomers used in practice have various termination mechanisms. Some monomers give only recombination reactions, such as styrene, acrylates (methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethyl hexyl acrylate), acrylonitrile and butadiene. Other monomers give both mechanisms of termination, around 65-75% disproportionation and 25-35% recombination, such as methacrylates (methyl methacrylate, ethyl methacrylate, butyl methacrylate etc.), substituted styrenes and other monomers [2, 3, 4]. 

Under higher-pressure conditions, particularly at low temperatures, collisional deactivation of excited association products producing ClOOCl and CIOCIO may be important kinetically. In fact for the most stable intermediate, ClOOCl, its formation is the dominant process under the stratospheric condition [107, 108, 1 12,1 14, 116]. On the other hand, the disproportionation processes producing ClOO and OCIO by (12b) and (12c), respectively, are endothermic by 3 - 4 kcal/mol [119] accordingly, they cannot compete effectively with the recombination reaction in the stratosphere (with low temperature and medium pressure). These endothermic reactions may, however, become dominant processes in the combustion of the AP propellant. 

One of the big surprises of the recent past has been the observation that radical-radical termination processes via recombination or disproportionation have no activation energies and very high A-factors. A-factors for recombination reactions of alkyl recombinations vary from lO10-4 1/mole-sec for CH3 radicals (28) to a low of about 109 5 for tert-... 

If one now jumps to some higher pressure at which steady-state reaction conditions can again prevail, similar semi-quantitative arguments can be used to explain the phenomenon known as the second explosion limit. At these somewhat higher pressures the large majority of the events by which chains are terminated will occnr in the gas phase. The higher pressure hinders the diffusion of radicals to the vessel surfaces and provides a number density of gas-phase radicals that is sufficient for radical disproportionation and recombination reactions to become mnch more significant than surface termination processes. 

The same authors later studied the evolution of the radicals formed after rupture of a single knotted alkane molecule using first-principles molecular dynamics calculation [284]. In knotted chains, recombination of the radicals is totally bypassed in favor of ultrafast (about several hundred femtoseconds) phenomena such as diradicals which generate cyclic alkanes, and disproportionation to form carbon-carbon double bonds. Saitta and Klein suggested that the trefoil knot imposes topological constraints to the velocity distribution of the recoiling radicals at rupture, leading to deviations from the canonical recombination reaction. 

Buschow (1994) analyzed these effects thermodynamically using the van t Hoff relation. The (Nd,Zr)2FeuB and/or Co- and Zr-containing regions have the higher equilibrium hydrogen partial pressure (Ph2)> which means that in the phases with additives the completion of the disproportionation process becomes difficult, and also the recombination process should be accelerated under the same conditions. The recombination reaction depends on thermally activated bulk diffusion which increases with temperature (McGuiness et al. 1990). 

In the radiolysis of liquid cyclohexane the major products are H2, cyclohexane and the bicyclohexyl, with yields of 5.6, 3.3 and 1.8 (100 eV)" S respectively, which suggests an important contribution of cyclohexyl radicals. This has been substantiated by radical titration experiments, where a solute is added that reacts with the radicals R (e.g. I2) and the products (RI) can be measured Also, with radical scavengers present, the yields of cyclohexene and bicyclohexyl decrease with the ratio kjk for the cyclohexyl radical. When it is assumed that the dimer is formed only by the radical recombination reaction 37 [with a yield of 1.8 (100eV) ], it follows that c-C Hio is formed concomitantly with a yield of 1.1 x 1.8 = 2.0 (100 eV)" Since the total yield of c-C H q is 3.3 (100 eV)" it follows that a yield of c-C Hio equal to 1.3 (100 eV)" is found that is formed in another process than disproportionation of cyclohexyl radicals. It appears that this process is molecular H2 elimination of the Si state (that also fluoresces), with (/>h2 = 0.8 as obtained from photolysis at the lowest photon energies (equation 33). The... 

Rate data for the disproportionation of the )W-oxo-dimers [0 Mo 0(L)2 2] to [Mo OLg] and [Mo OzLz] and for the reverse recombination reaction are collected in Table 5. The reactions were studied by a concentration-jump, stopped-flow method. ... 

Free-Radical Polymerization. The highly reactive nature of radical species results in polymerization products with high levels of branching. The relative levels of recombination and disproportionation termination reactions determines the breadth of the molecular weight distribution. Chain-transfer agents also affect... 

Recombination of disproportionation termination reactions may yield other polymers having different combinations of terminals. 

The phenoxy radicals are relatively long-lived and can undergo self-disproportionation, recombination vith alkylperoxy radicals, or isomeric rearrangement followed by recombination. The resulting compounds may have some stabilization activity. Propionate-type hindered phenols constitute a special class in this category during reaction, the phenol is transformed into phenolic cinnamates, which are known to be efficient chain-breaking antioxidants [Eq. (85)]. 

Resonance in macroradicals increases their stability, opposes their tendency for disproportionation and favors recombination reactions. A typical example is natural rubber mastication, in which resonance causes the weak-... 

By suppressing secondary reactions, radical acceptors enable a calculation of the rate for chain scission. By comparing degradation rate constants for the presence and absence of radical acceptors, a study of reaction mechanisms is thus provided. Using this method, NRPRA researchers demonstrated the importance of the recombination reaction in natural rubber mastication [26] and the predominance of a disproportionation reaction as a termination mode during mastication of vinyl polymers [27-29]. 

Termination of hydrocarbon radicals is not the only recombination reaction. Other possibilities (depending on the structure of the radicals) are (i) the termination by disproportionation, where hydrogen is eliminated, which yields an olefin (R-CHj-CHj R-CH=CH2-I-H ), (ii) termination by transfer (e.g. in reaction with antioxidants), (ill) termination by recombination with a hydroxyl radical, which leads to an alcohol (R-CHj-CHj -I- HO R-CH -GHj-OH) and (iv) further oxidation of the terminal hydroperoxide (R-CHj-CHj -I- Oj -> R-CHj-CHj-O-O -> R-CHj-CHj-O-OH). In the terminal hydroperoxide, the link between oxygen atoms is then cleaved, which gives an alkanal (R-CHj-CHj-O-OH R-CH2-CH=0- -H20). Alternatively, the link between carbon atoms can be cleaved, which results in a shorter alkyl radical (R-CHj ). Some of these reactions are discussed in relation to the secondary reactions of hydroperoxides. 

There are studies that consider the intermolecular abstraction followed by a (1 cleavage reaction as the more important propagation reaction diuing the degradation of polyethylene [74]. This reaction produces a significant decrease in average molecular mass and elimination of volatile products. Finally, recombination reactions (2.11,2.12) and disproportionation (2.13, 2.14) are the most important final reactions of the thermal degradation. 

At a higher irradiation dose a number of free radical initiated reactions like chain scission, disproportionation, recombination of macroradicals take place (refer to Figure 8 - Scheme-3). With the increase of vinyl... 

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