Methyl radical dimerization

It is interesting to note that among initiators studied, a benzyl ketal type initiator (2,2-dimethoxy-2-phenyl acetophenone) is more active than other initiators of acetophenone type. 2,2-Dimethoxy-2-phenyl acetophenone is more active than BME, probably due to the presence of the extra methoxy group on the -carbon, which gives a more active phenyldimethoxy methyl radical (and it further cleaves to give an even more active methyl radical) than the phenylmethoxy methyl radical from BME (4). It is known that the phenylmethoxy methyl radical dimerizes easily and thus loses some of its role as a radical in a... 

In the case of vitamin B12, eq 19 is well investigated and its rate constant k = 4 x 109 L/mohs for primary radicals.203 Comparison of this value with the rate constant for methyl radical dimerization of 1010 L/mohs191 indicates that eq 19 proceeds at diffusion-controlled rates. Newer approaches may provide greater accuracy in the measurement of rates of recombination of free radicals, providing a better understanding of hydrogen atom abstraction (eq 18).205... 

Isobutyl group N- Methylpyrrolidone, 803 Methyl radical dimerization, 154... 

Katritzky AR, Yang B (1998) Novel heteroatom-Unked analogues of trityl radicals diaryl(benzotriazol-l-yl)methyl radical dimers. J Org Chem 63 1467-1472... 

A typical oxidation is conducted at 700°C (113). Methyl radicals generated on the surface are effectively injected into the vapor space before further reaction occurs (114). Under these conditions, methyl radicals are not very reactive with oxygen and tend to dimerize. Ethane and its oxidation product ethylene can be produced in good efficiencies but maximum yield is limited to ca 20%. This limitation is imposed by the susceptibiUty of the intermediates to further oxidation (see Figs. 2 and 3). A conservative estimate of the lower limit of the oxidation rate constant ratio for ethane and ethylene with respect to methane is one, and the ratio for methanol may be at least 20 (115). 

Some control over the spHt between methyl radical oxidation (to HCHO) and dimerization in heterogeneous oxidation can be achieved by varying conditions (116). For homogeneous oxidation, an efficiency of 70—80% to methanol has been claimed at 8—10% conversions (110). This is the high end of the reported range and is controversial. Even so, such technology appears unlikely to be competitive for regular commercial use until further advances are made (117). The critical need is to protect the products from further oxidation (118). 

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

Entry 3 has only alkyl substituents and yet has a significant lifetime in the absence of oxygen. The tris(/-butyl)methyl radical has an even longer lifetime, with a half-life of about 20 min at 25°C. The steric hindrance provided by the /-butyl substituents greatly retards the rates of dimerization and disproportionation of these radicals. They remain highly reactive toward oxygen, however. The term persistent radicals is used to describe these species, because their extended lifetimes have more to do with kinetic factors than with inherent stability." Entry 5 is a sterically hindered perfluorinated radical and is even more long-lived than similar alkyl radicals. 

The attack on the aromatic nucleus by hydroxyl radicals is probably analogous to that by phenyl and methyl radicals, Eq. (34a,b). Evidence that the first step is the addition of hydroxyl radical to benzene, rather than abstraction of a hydrogen atom, has recently been adduced from a study of the radiolysis of water-benzene mixtures. The familiar addition complex may undergo two reactions to form the phenolic and dimeric products respectively, Eq. (34a,b). Alternative mechanisms for the formation of the dimer have been formulated, but in view of the lack of experimental evidence for any of the mechanisms further discussion of this problem is not justified. 

The two rate constants are related by a numerical factor ki = 2fc2. (Also, see Problem 1.) Obviously one should not report that the rate constant for the dimerization of methyl radicals is X, unless a definition of k is given. 

By means of this expression, the values of Yt yield [A]f, and Eq. (3-28) provides the means for data analysis. Or, with additional algebra, one can express Y, directly, and float both k] and Ye in the calculation. As an example of the application of Eq. (3-28), consider the dimer-monomer equilibration of triphenyl methyl radical 2... 

Majeti11 has studied the photochemistry of simple /I-ketosulfoxides, PhCOCH2SOCH3, and found cleavage of the sulfur-carbon bond, especially in polar solvents, and the Norrish Type II process to be the predominant pathways, leading to both 1,2-dibenzoylethane and methyl methanethiolsulfonate by radical dimerization, as well as acetophenone (equation 3). Nozaki and coworkers12 independently revealed similar results and reported in addition a pH-dependent distribution of products. Miyamoto and Nozaki13 have shown the incorporation of protic solvents into methyl styryl sulfoxide, by a polar addition mechanism. 

In the methyl radical, the reaction takes place in the direction of SO (2pn of central carbon) extension, that is to say, the direction perpendicular to the molecular plane. Walsh 76> correlated the remarkable localization of SO at the nitrogen atom in NO 2 to the experimental results indicating that NO 2 abstracts hydrogen from other molecules to form HNO2 rather than HONO, combines with NO to form ON—NO2, dimerizes to produce O2N—NO2, and so forth. Also he pointed out that the SO MO of C1CO is highly localized at the carbon atom, which is connected with the production of CI2CO in the reaction with CI2. The SO extension of NO 2 is schematically shown below 103>. 

The hexakis(methyl isocyanide) dimers, [Pt2(CNMe)6], undergo photolytic cleavage of the Pt—Pt bond to give 15-electron radicals, Pt(CNMe)3.94 Mixtures of platinum and palladium dimers give rise to heteronuclear complexes under photolytic conditions. Mixtures of normal and deuterium-labeled methyl isocyanide complexes reveal that the metal-ligand bonds undergo thermal redistribution.94... 

The color of the quinonoid compounds that may be obtained by disproportionation can be sufficiently like that of the radicals to cause confusion if visual observation or broad-band spectrophotometry is used.11 For example, Preckel and Selwood, using paramagnetism as a measure of the amount of radical, reported that solutions of triphenyl-methyl derivatives more or less rapidly lost their paramagnetism. The decomposed solutions were still highly colored, but the color was no longer dependent on the temperature as it is in the case of a radical-dimer equilibrium mixture. What is more striking, and an even more subtle and dirtier trick on the part of nature, is the fact that Preckel and Selwood s non-paramagnetic solutions were still rapidly bleached by exposure to the air. It is clear that radical-like reactivity is not a safe criterion for the presence of radicals. It is also clear that the ebullioscopic method is particularly unsatisfactory in view of the excellent chance for decomposition. 

In order to find out whether captodative substitution of a methyl radical can lead to persistency, the rate of disappearance by bimolecular selfreaction was measured for typical sterically unhindered captodative radicals (Korth et al., 1983). The t-butoxy(cyano)methyl radical, t-butylthio(cyano)-methyl radical and methoxy(methoxycarbonyl)methyl radical have rate constants for bimolecular self-reactions between 1.0 x 10 and 1.5 X 10 1 mol s Mn the temperature range —60 to - -60°C. The dilTusion-controlled nature of these dimerizations is supported by the Arrhenius activation parameters. Thus, it has to be concluded that there is no kinetic stabilization for captodative-substituted methyl radicals. On the other hand, if captodative-substituted radicals are encountered which are kinetically stabilized (persistent) or which exist in equilibrium with their dimers, then other influences than the captodative substitution pattern alone must be added to account for this phenomenon. 

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. 

A related procedure, which may be of value from the preparative standpoint, involves the preparation of /rans-nitrosomethane dimer by adding a solution of diacetyl peroxide in sec-butyl nitrite to warm sec-butyl nitrite [50]. From the product of the reaction it has been assumed that this preparation involves the generation of free methyl radicals which react with the nitrite to give nitrosomethane and alkoxy radicals. The latter disproportionate to ketones and alcohols, while the nitroso compound dimerizes. 

There are two other compounds which appear in the complex mixture of photolysis products and whose presence have yet to be explained. The first of these is the dimer of trifluoroacetone and the second is 2,2-difluoropropane. The latter may be formed by the addition of methyl radicals to CF2 which may be formed by the molecular decomposition of the ketone or by the disproportionation of trifluoromethyl radicals. 

Neither the relative number of benzylic hydrogens nor the base strength accounts for the slow oxidation rate of the methylnaphthalenes. Formation of radicals in the presence of aromatic hydrocarbons can lead to radical attack on the aromatic ring. Addition of phenyl or methyl radical to the ring gives a cyclohexadienyl radical that may disproportionate or dimerize, or undergo hydrogen abstraction by another radical (3, 9,13). 

Comparative studies indicate that abstraction of hydrogen atoms from the methyl group of 1-methylpyrrole is easier than from the methyl group of toluene. Subsequent dimerization and polymerization of the 1-pyrrolylmethyl radicals, initiated by the addition of r-butoxy radicals, occurs and low yields (0.5%) of l,2-bis(l-pyrrolyl)ethane and l-methyl-3-(l-pyrrolylmethyl)pyrrole have been isolated from the tars (B-77MI30500). The radical dimerization of pyrrole initiated with r-butoxy radicals produces 2-(2-pyrrolyl)-A1-pyrroline (B-77MI30500). 

The structure of dimer (2) was characterized by NMR. Thus, one triphenylmethyl radical reacts at the para-position of a phenyl group in another triphenylmethyl radical, not the central sp3 carbon (to form hexaphenylethane), to form dimer (2). However, rra(p-methylphenyl)methyl radical does not dimerize. So, the electronic effect in free radicals is quite large. 

---