Radical disproportionation/combination

The resulting radical is stabilized by electron delocalization and eventually reacts with either another inhibitor radical by combination (dimerization) or disproportionation or with an initiator or other radical. 

Chains terminate by either of two mechanisms combination or disproportionation. Two chain radicals may combine to form a single bond between... 

Even though the rate of radical-radical reaction is determined by diffusion, this docs not mean there is no selectivity in the termination step. As with small radicals (Section 2.5), self-reaction may occur by combination or disproportionation. In some cases, there are multiple pathways for combination and disproportionation. Combination involves the coupling of two radicals (Scheme 5.1). The resulting polymer chain has a molecular weight equal to the sum of the molecular weights of the reactant species. If all chains are formed from initiator-derived radicals, then the combination product will have two initiator-derived ends. Disproportionation involves the transfer of a P-hydrogen from one propagating radical to the other. This results in the formation of two polymer molecules. Both chains have one initiator-derived end. One chain has an unsaturated end, the other has a saturated end (Scheme 5.1). 

Bizilj et at reported that disproportionation is more important for oligomeric radicals. While combination products were unequivocally identified, analytical difficulties prevented a precise determination of the disproportionation products. Accordingly, they were only able to state a maximum value of ktself reaction of 11 and <1.50 for reaction between 8 and 11. 

As for any chain reaction, radical-addition polymerization consists of three main types of steps initiation, propagation, and termination. Initiation may be achieved by various methods from the monomer thermally or photochemically, or by use of a free-radical initiator, a relatively unstable compound, such as a peroxide, that decomposes thermally to give free radicals (Example 7-4 below). The rate of initiation (rinit) can be determined experimentally by labeling the initiator radioactively or by use of a scavenger to react with the radicals produced by the initiator the rate is then the rate of consumption of the initiator. Propagation differs from previous consideration of linear chains in that there is no recycling of a chain carrier polymers may grow by addition of monomer units in successive steps. Like initiation, termination may occur in various ways combination of polymer radicals, disproportionation of polymer radicals, or radical transfer from polymer to monomer. 

The analogy between electron-transfer via addition/elimination (Eq. 2b,c) or abstraction/elimination (Eq. 2a, c) and classical solvolysis involving closed-shell molecules (nonradicals) is seen by comparing Scheme 1 with Scheme 3, in which XY, the precursor of the ions X and Y , is formally derived from the two radicals X and Y". Analogous to Scheme 1, on the way to the ionic products that result from the interaction between X and Y there are two possibilities if XY denotes a transition state, the reaction (Eq. 3a, a ) is a case of outer-sphere electron transfer. If, however, a covalent bond is formed between X and Y, the path (Eq. 3b, b ) is an example of inner- sphere electron transfer. Obviously, part b of the scheme describes the classical area of S l solvolysis reactions (assuming either X or Y to be equal to C) [9, 10]. If a second reaction partner for C (other than the solvent) is allowed for (the (partial) ions then represent transition states), then Eq. 3b also covers Sn2 reactions. If looked upon from the point of view of radical-radical reactivity, Eqs. 3a and b show well-known reactions radical disproportionation in Eq. 3a,a and combination in Eq. 3b. 

Reaction between CF2and radicals has not been demonstrated in the gas phase but recently Mastrangelo65 has shown that -CF3 and CF2 trapped upon a liquid nitrogen cold finger, may interact on warming up to yield perfluoropropane and perfluorobutanc. Perfluoropropane has also been detected as a by-product in the photolysis of hexafluoroacetone. It is possible that all these reactions take place upon the walls of reaction vessels. 1,4-Dichlorooctafluorobutane was also detected in the high conversion photolyses but it was present only in trace quantities. From estimates of the concentration of these products, an approximate value for the disproportionation/combination ratio for -CFaCl radicals may be calculated. A value of 0.04 is obtained which is remarkably constant and independent of concentration and light intensity. 

Gunning66 has shown that disproportionation takes place when CF2C1 radicals are generated by the mercury-photosensitized decomposition of CF2CI2 and in recent studies confirming his results47 it was found that the disproportionation/combination ratio was again 0.04. 

The process of addition of monomer units to the growing chain can be interrupted in different ways. One is chain termination by combination or disproportionation of radicals. Explicitly, two growing-chain radicals can combine to form a carbon-carbon bond, or disproportionation can occur with a hydrogen atom being transferred from one chain to the other ... 

Falconer and Cvetanovic (40) attempted to obtain a more quantitative value for the fraction of nonterminal addition in the case of propylene. They produced hydrogen atoms by mercury photosensitized decomposition of H2, using at least 100 times as much H2 as C3H6 and total pressures of 40 and of 250 mm. Under these conditions the reactions of importance were the combination and disproportionation of the iso- and n-propyl radicals and their cross reactions, the combination of the two radicals with H atoms (assumed to be equally probable), and a very small amount of decomposition of hot n-propyl radicals. Disproportionation to combination ratios were taken as 1.64 for two iso-propyl, 1.14 for two w-propyl, and hence 1.39 was taken as the mean of the two values for one iso- and one n-propyl radical. Using these values and the analysis of the products, the nonterminal addition of H atoms to C3H6 and C3D6 was found to amount to 6 1%. 

Quantum yields for adduct 63 and total product (63-65) formation from the reaction of - -t with several tertiary amines are summarized in Table 12. Quantum yields measured at 1.0 M amine concentration are lower than the values extrapolated to infinite amine concentration due to incomplete quenching of It. Extrapolated total quantum yields range from 0.07 to 0.33, providing a lower limit for the efficiency of the proton transfer step, kh> in Fig. 11. The other reaction products, 1,2-diphenylethane (64) and 1,2,3,4-tetraphenylbenzene (65), are formed mainly via in-cage radical pair disproportionation and out of cage combination, respectively. The relative importance of radical pair combination, disproportionation, and cage escape is dependent... 

No evidence has been found that two methoxy radicals ever combine to give dimethyl peroxide, but they do disproportionate to give methanol and formaldehyde... 

Dithiophenylbutane 66 is obtained by a radical-radical combination reaction, while 3-isopropylthiophene 67 and 3-(propen-2-yl)thiophene 68 form by a radical-radical disproportionation process. The solid-state reaction was shown to be highly temperature dependent. While irradiation of powdered samples at 20°C led to no observable product after 2 days, samples exposed to the same UV source for 24 h at ca. 45°C gave 66 as the only product in ca. 5-10% yield. Photochemical reactions were carried out with nanocrystalline suspensions of 65 in a Pyrex tube acting both as a container and a light filter (X > 290 nm). 

The mechanism of radical-radical reactions involves either combination or disproportionation. In the case of a-hydroxyalkyl radicals the following reactions have been suggested from product analysis. In methanol solutions the radicals predominantly combine to form ethylene glycol (40) in the ethanol system both processes take place... 

Radicals being neutral species tend to react together. Indeed, the most common side reactions in free-radical processes involve the formation of adducts between two radicals, via combination or disproportionation. These unwanted termination steps usually occur much faster than the desired reactions between radicals and substrates. Thus, the key to control in both radical addition and polymerization procedures consists in lowering the concentration of transient radical species. This will minimize the side reactions between radical species, yet the kinetics of the useful reactions will also be affected. 

However, this is probably an oversimplification, as the disproportionation reaction is surface-catalyzed and the two reactions must have different energies of activation. This behavior of dimethylamino radicals is in marked contrast to the iso-electronic isopropyl radicals, the disproportionation-combination ratio of which is unaltered by temperature and siuface (2, 30). The surface-promoted radical decomposition is also in marked contrast to the absence of such effects in the isopropyl radical reactions (2) and must be attributed to the presence of the nitrogen atom. Amino radicals also undergo surface reactions (28). The occurrence of TMA suggests the participation of methyl radicals in the reactions, and a possible route is provided by Reactions 28 and 34. 

The radicals formed combine, disproportionate or abstract an hydrogen atom the ions abstract a hydride ion (equations 14-16) ... 

The yield of ethylene is exceptionally high for s-butylcyclopropane. Foldiak and coworkers explained it as due to its formation not only from a simultaneous cleavage of two bonds of the ring but also from an ethyl radical which is detached from the tertiary carbon atom in the side chain. A support to this suggestion is the observation of formation of higher yield of ethane. However, this correlation can hold only if these ethyl radicals are excited since otherwise the amount of ethylene produced in this way should equal that of the ethane and the small difference in ethane yield is not sufficient to explain the larger difference in the yields of ethylene. Moreover, since for ethyl radical the ratio of disproportionation/combination is about 1, the yield of ethylene from ethyl radicals should be equal to the yield of butane which is only 0.12. 

The disproportionation-combination ratio is temperature dependent. For sec-butyl radicals, this ratio is 2.3 at 375°K. (11) and 11 at 90°K. (10). Adopting the same values for pentyl radicals we calculate a ratio of 7.5 at 143°K. (melting point of n-pentane) at which recombination occurs. 

If dimers are formed only by combination of pentyl radicals after melting of the sample, then 4,5-dimethyloctane must be characterized by a much higher yield than all other dimers. Indeed, this dimer is formed by combination of 2-pentyl radicals that are found to be the most abundant species in the irradiated solid (16). Table I shows effectively that the highest yield is measured for dimer 2-2. High values are also obtained for 1-1 and 1-2 isomers. Table I allows calculation of the individual yields of pentyl radicals if it is assumed that the disproportionation-combination ratio is the same for all pentyl radicals formed in the solid during radiolysis. If k is a constant and if [Ri ], [R2], and [R3 ] are the concentrations of 1-, 2-, and 3-pentyl radicals respectively, then ... 

The alkyl radicals disappear by combination and disproportionation, but considerable confusion exists as to their basic yield. From experiments with scavengers results a value of about 2 for the G-value of primary formed hexyl radicals (22, 34) the disproportionation/combination ratio of hexyl radicals has been determined as 0.7 (I). These two values are, however, not compatible with the measured product distribution as a function of temperature (7). 

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