Reactions of methyl radicals

Step 3 Reaction of methyl radical with molecular chlorine... 

The most advanced treatment leading to the absolute determination of equilibrium constants will be demonstrated with the two following simple reactions of methyl radical ... 

Studies have been carried out on the methylated complex [H3C-Niin(17)(H20)]2+, which is obtained from the reaction of methyl radicals (generated by pulse radiolysis) with [Ni(17)]2+. The volumes of activation are consistent with the coherent formation of Ni—C and Ni—OH2 bonds, as expected for the generation of a Ni111 complex from a square planar Ni11 precursor.152 The kinetics of reactions of [H3C-Niin(17)(H20)] + involving homolysis, 02 insertion and methyl transfer to Crn(aq) have been determined, and intermediates have been considered relevant as models for biological systems.153 Comparing different alkyl radicals, rate constants for the... 

This may be of significance in connetion with the biosynthesis of acetate from carbon dioxide, because the next step, the fixation of carbon monoxide, was demonstrated by B. Krautler. He irradiated methyl cobalamin under Co at 30 atm and obtained the acyl cobalamin as the product. Interestingly, a radical mechanism was iproposed, involving the reaction of methyl radicals with CO to give acyl radicals, which then recombine with the cobalt complex /55/. 

The trapping of allyl radicals with other open-shell species can be studied in all reactions in which a sufficiently high concentration of radicals is created and in which the concentration of nonradical trapping agents is low. This prerequisite has been met in Kolbe electrolysis reactions, in which radicals are generated by one-electron oxidation of carboxylate anions. One of the simplest systems, the reaction of methyl radicals with... 

Table 13 B3LYP/6-31G energies of optimized geometries for reaction of methyl radical with methoxyamidyl S0 102e...

The hydrogen abstraction from alkoxyamidyl 102 has been modeled at B3LYP/ 6-31G level by the reaction of methyl radical (R = CH3 ), with methoxy-formamidyl 102e giving 103e and methane. Energies are presented in Table 13. 

Fig. 21. Volume profile for the reaction of methyl radicals with the nitrilotriacetate complex of Co(II).

Steacie, E. W. R., and A. F. Trotman-Dickenson The Reactions of Methyl Radicals. IV. The Abstractions of Hydrogen Atoms from Cyclic Hydrocarbons, Butynes, Amines, Alcohols, Ethers and Ammonia. J. chem. Physics 19, 329 (1951)-... 

These are radical-radical reactions or reactions of methyl radicals with a product of a radical-radical reaction (owing to concentration effects) and are considered less important than reactions (3.72) and (3.86). However, reactions (3.72) and (3.86) are slow, and reaction (3.92) can become competitive to form the important methoxy radical, particularly at high pressures and in the lower-temperature region of flames (see Chapter 4). 

The reaction of benzyl radicals with 02 through an intermediate adduct may not be possible, as was found for reaction of methyl radical and 02 (indeed,... 

Washida, N., and K. D. Bayes. The reaction of methyl radicals with atomic oxygen. Chem. Phys. Lett. 23 373-375, 1973. 

Scheme 6.14 Backside and frontside substitution reactions of methyl radical with disilane...

Hoyland, J. R., MINDO/2 calculations of the reaction of methyl radicals with ethylene and butadiene, Theor. Chim. Acta Bert.) 22, 229 (1971). 

Nitrosomethane (1) is known to be less stable than its isomer formaldoxime 2 and original attempts to isolate this species failed owing to its facile isomerization to the oxime 2. Already Bamberger and Seligman considered in 1903 that it would be difficult to isolate nitrosomethane after oxidation of methylamine due to its rapid isomerization to 2. Hence, 2 is always present in the synthesis of the nitrosomethane. Nitrosomethane is produced in the pyrolysis or photolysis of tcrf-butyl nitrite and by the reaction of methyl radicals with nitric oxide. Early results were confusing since the final product obtained is dimeric nitrosomethane. It was first isolated in 1948 by Coe and Doumani from the photolysis of gaseous ferf-butyl nitrite according to the overall reaction shown in equation 2. 

Overview of Rate Constants for the Reaction of Methyl Radicals with LM 11 1 CH3 for Several Systems... 

The past decade has been an extremely fruitful one in the field of quantitative free radical kinetics. Two papers can be identified as the starting point of much of this work. The first of these is the acetone photolysis study by Noyes and Dorfman1 which gave confidence to the use of acetone as a reproducible source of methyl free radicals in a fairly simple kinetic environment. The second is the study of Gomer and Kistiakow-sky2 of the absolute rate of recombination of CH3 radicals. The latter study made it possible to give absolute values for the Arrhenius parameters for the reactions of alkyl free radicals with stable molecules. It also opened the way for putting the reactions of methyl radicals with other alkyl radicals on an absolute basis. 

The rate of reaction of methyl radicals is in excellent agreement with the predictions of the Smoluchowski theory (see Chap. 2, Sect. 2.6). Consequently, it appears that geminate radicals move towards and away from each other at a diffusion-limited rate. Once an encounter pair is formed, reaction is very rapid (primary recombination). Furthermore, the encounter pair is held together for a considerable time (< 0.1ns in mobile solvents) because the surrounding solvent molecules hinder their separation (solvent caging). There is much evidence which lends some support for this view the most important influences on the recombination probability are listed below. 

The slow combustion reactions of acetone, methyl ethyl ketone, and diethyl ketone possess most of the features of hydrocarbon oxidation, but their mechanisms are simpler since the confusing effects of olefin formation are unimportant. Specifically, the low temperature combustion of acetone is simpler than that of propane, and the intermediate responsible for degenerate chain branching is methyl hydroperoxide. The Arrhenius parameters for its unimolecular decomposition can be derived by the theory previously developed by Knox. Analytical studies of the slow combustion of methyl ethyl ketone and diethyl ketone show many similarities to that of acetone. The reactions of methyl radicals with oxygen are considered in relation to their thermochemistry. Competition between them provides a simple explanation of the negative temperature coefficient and of cool flames. 

I have spent some time trying to explore the experimental basis for such a reaction, and at the moment I feel that there is no good experimental foundation for writing it. From a structural point of view, it appears to be a highly unlikely reaction. The simplest example of such a reaction would be the reaction of methyl radicals with oxygen to produce formaldehyde, plus hydroxyl radical (Reaction 8)... 

The reaction of methyl radicals with substituted thiophenes has been studied (Scheme 60) (71ACS2183). The radicals were generated by the Fe2+ catalyzed action of H202 on DMSO. The decomposition of diacetyl peroxide in thiophenes gives low yields of a-methylated products (73TL637). 

The product of the reaction of methyl radicals with NO was generally assumed to be nitrosomethane, CH3NO, even before it was found. 

The rate of the reaction of methyl radicals with 03 has been studied from 243 to 384 K by monitoring the decay of methyl in the presence of excess 03 [99], With the temperature dependence of the rate constant it was estimated that less than 1% of the methyl radicals in the stratosphere react with ozone. The reactions of a series of alkyl radicals (CH3, C2HS, n-C3H7, i -C3H7, and t-C4H9) with ozone were investigated at 298 K [100]. The rate coefficients were found to correlate with the difference between the ionization potential of each radical, and the electron affinity of 03. 

Ulanski P, Merenyi G, Lind J, Wagner R, von Sonntag C (1999) The reaction of methyl radicals with hydrogen peroxide. J Chem Soc Perkin Trans 2 673-676 Ulanski P, Bothe E, Hildenbrand K, von Sonntag C (2000) Free-radical-induced chain breakage and depolymerization of polyfmethacrylic acid). Equilibrium polymerization in aqueous solution at room temperature. Chem Eur J 6 3922-3934... 

The fluorine atoms in trifluoroethylene enhance the rate of attack by the nucleophilic methyl radicals, but retard attack by the electrophilic trifluoromethyl radicals. Methyl radicals add preferentially to the most substituted end of trifluoroethylene, in direct contradiction of the resonance stabilization hypothesis. In fact we can see that the reaction of methyl radicals... 

One feature of the correlations is the scatter in the points for unsubstituted alkyl radicals, and this is particularly serious for the reaction of methyl radicals with ethylene. The experimental A-factor of this process is probably the most accurately known of any radical addition, and AS°9S is also very well established yet the point lies well away from the line through the other data. A possible explanation may be that in methyl radical, and other nucleophilic alkyl radical additions, the transition state is more like the reactants, so that the correlation with /15°98, a quantity calculated from product properties, is less likely. The early nature of the transition state in methyl radical reactions is... 

Durham, R. W., and E. W. R. Steacie The reaction of methyl radicals with nitric oxide, and the rate of combination of methyl radicals. J. chem. Physics 20, 582 (1952). 

Carbonyl radicals in a polymer arise through a radical oxidation of aldehyde groups. The comparison of the rate of transfer reaction of methyl radical to tertiary alkanes and to aldehydes is more favorable for aldehydes which are by one to two orders more reactive [89]. The concentration of aldehyde groups in an oxidized polymer is usually considerably lower than that of alkane bonds C—H. The course of decarbonylation becomes therefore significant only in such a case when further oxidation occurs in highly oxidized microregions. As a random reaction taking place in the polymer bulk it is less probable. 

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