Alkyl radicals, disproportionation recombination

The recombination and the disproportionation of alkyl radicals play an important role in many other chain reactions, for example, pyrolysis, photolysis, and radiolysis of organic... 

The following explanation accounts for the facts. Alkyl radicals react with phenoxyl radicals by two parallel reactions, namely, recombination and disproportionation. 

If we assiime that there is no activation energy for the disproportionation or recombination, then fcj 109-5 liter/mole-sec. (see Table III). This is about a factor of 10 higher than the values to be expected of H-abstraction reactions of alkyl radicals. It is furthermore anomalous in having a negligible activation energy compared to the expected 8 3 kcal. Note that if we assign 1 kcal. of activation energy to the disproportionation then Ad 101 U liter/mole-sec. 

Rates of recombination and disproportionation of simple alkyl radicals are, as we have indicated earlier, very high, and special techniques are required to measure them. These methods measure total termination rate, recombination... 

Termination rates ordinarily quoted are 2k, where kt is the bimolecular rate constant for the sum of recombination and disproportionation. The factor of 2 appears because two radicals are removed from the system each time the reaction occurs. See K. U. Ingold, in Free Radicals, J. K. Kochi, Ed., Vol. I, p. 43, for references to particular alkyl radical results. Rotating sector measurements for allylic radicals have been reported by H. J. Hefter, C. S. Wu, and G. S. Hammond, J. Amer. Chem. Soc., 95, 851 (1973). 

The decay of free radicals taking part in oxidation of a polymer may occur as a recombination or disproportionation of alkyl radicals, alkyl and peroxyl radicals, or peroxyl radicals ... 

The alkyl radicals initially generated will usually react exclusively with oxygen. Other reactions such as decomposition, disproportionation, isomerization, recombination and reaction with the fuel will only compete when the partial pressure of oxygen is low or the temperature high (ca. 450 °C). The nature of their reaction with oxygen, particularly in the temperature range 250—400 °C, has led to considerable experimentation and discussion. 

Although much is already known in this field, the possibilities of its exploitation has not been fully explored yet. As an example, the idea of the recombination and disproportionation of low molecular alkyl radicals as an explanation of the crosslinking efficiency of polyolefins may be put forward. Of equal importance is to defme the problems which await a more detailed study as well as to interpret sometimes controversial results and particularly to elaborate new principles of physical crosslinking of polyalkenes or chemically induced crosslinking of oriented macromolecules. 

Recombination (dimerization) reaction of macroradicals may occur parallel to disproportionation in which alkyl radicals decay via transfer of a hydrogen atom from one P-carbon of one of the two radicals to the carbon of the second radical caiying a radical site. 

To establish the effect of structure on the ratio of recombination and disproportionation, the knowledge of reactions of low-molecular analogs may be use. It may be deduced from the ratios of rate constants of disproportionation and combination determined for different types of alkyl radicals that primary alkyl radicals enter combination preferably while tertiary radicals dispoiporti[Pg.153]

An increase of disproportionation at the expense of recombination when ing from primary to secondary and tertiary alkyl radicals may be expressed by the ratio 1 5 25. An almost equal ratio for disproportionation and recombination has been found for reactions of the methyl radical with ethyl, isopropyl and rer/.butyl radicals [13]. 

A reaction of hydrogen atoms with the CHj groups of a surrounding polymer chain and formation of secondary alkyl radicals and molecular hydrogen will compete with this reaction. Secondary aUcyl radicals either recombine or disproportionate but the probability of both reactions is not so different now. Not only disproportionation of... 

Accounting for high pressures (of several tens of torr and higher) and taking the second-order recombination rate constant kj.ec 10 cm mol s (see [219]), the disproportionation rate constant can be measured from the known absolute values and the temperature dependence of A. The results of such calculations for several alkyl radical pairs are given in Table 10. The activation energy for disproportionation reactions is seen to be usually low. 

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. 

Reactions of recombination and disproportionation of alkyl radicals are diffusion-controlled... 

Recombination of alkyl radicals, as that of atoms, occurs practically without an activation energy. In the gas phase at a sufficiently high pressure the recombination of methyl radicals is bimolecular with the rate constant close to (l/4)Zo (where Zo is the frequency factor of bimolecular collisions, and the factor 1/4 reflects the probability of collisions of particles with the antiparallel orientation of s ins). The theoretical estimation of the constant at a collision diameter of 3.5-10 m agrees with the experimental value = 2-10 ° l/(mol-s) (300 K). This k value agrees with the estimation by the theory of absolute reaction rates under the assumption that the free rotation of methyl groups is retained in the transition state. In the liquid the recombination of meth)d radicals is bimolecular with the rate constant of difiiision collisions (see Qiapter 5). For example, in water 2k = 3.2-10 l/(mol s) (298 K). Ethyl radicals react with each other by two methods recombine and disproportionate... 

The kjkc ratio depends slightly on temperature kjkc = 0.087 exp(1.55/R 7). The ratio between recombination and disproportionation depends, naturally, on the structure of the alkyl radical, which can be seen from the presented data (decaline, 303 K)... 

Aliphatic aminyl radicals, as alkyl radicals, enter into disproportionation and recombination reactions. The reaction direction depends on the radical structure. For example, methylaminyl radicals predominantly recombine and diisopropylaminyl radicals disproportionate... 

From simulations of a number of laminar premixed flames of C3- and C4-hydrocarbons, it is known (Warnatz, 1981) that after the initial H-atom abstraction by H, O, or OH, the destiny of the alkyl radicals formed is the central point of interest. Due to the instability of C3- and C4-alkyl radicals with respect to thermal decomposition (see below), the competing reactions with O or O2 and recombination or disproportionation reactions are unimportant in flame propagation. 

The free radical polymerization of HPMA in the presence of mercaptans involves two different initiation mechanisms (Scheme 2) [26]. One is the initiation by RS radicals from chain transfer agent the other appears to be the direct initiation by the primary isobutyronitrile (IBN) radicals formed by the decomposition of AIBN [27]. The RS are formed by either the free radical transfer reaction of alkyl mercaptans with the IBN radicals or the chain transfer reaction of an active polymer chain with the mercaptans. The initiation by the RS radicals produces the ST polymers with a functional group at one end of the polymer chain. The initiation by IBN radicals leads to nonfunctional polymer chains with an IBN end group. The presence of the polymers with IBN end groups effects the purity and the functionality of ST polymers. As expected, the production of nonfunctionalized polymer chains is affected by reaction conditions. The polymerization is mainly terminated by chain transfer reaction with the mercaptans, but other termination mechanisms, such as disproportionation and recombination, take place depending on the reaction conditions [26]. 

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