Methyl radicals, abstraction reactions

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>. 

An aromatic ring and a double or triple bond in the a-position relative to the C—H bond weaken this bond by virtue of the delocalization of the unpaired electron in its interaction with the iT-bond. The weakening of the C—H bond is very considerable for example, D(C—H) is 422 kJ mol-1 in ethane [27], 368 kJ mol-1 in the methyl group of propene [27] (AD = 54 kJ mol-1), and 375 kJ mol-1 in the methyl group of toluene [27] (AD = 47 kJ mol-1). Such decrease in the strength of the C—H bond diminishes the enthalpy of the radical abstraction reaction and, hence, its activation energy. This effect is illustrated below for the reactions of the ethylperoxyl radical with hydrocarbons ... 

Evidence for the polar character of the transition state is that electron-withdrawing groups in the para position of toluene (which would destabilize a positive charge) decrease the rate of hydrogen abstraction by bromine while electron-donating groups increase it,10 However, as we might expect, substituents have a smaller effect here (p -1,4) than they do in reactions where a completely ionic intermediate is involved, e.g., the SnI mechanism (see p. 344). Other evidence for polar transition states in radical abstraction reactions is mentioned on p. 685. For abstraction by radicals such as methyl or phenyl, polar effects are... 

In the reaction of peracetic acid with acetaldehyde (in the absence of oxygen) the majority of the methyl radicals abstract hydrogen, preferentially from acetaldehyde, to form methane ... 

Except for extreme cases like this, radical reactions are generally not subject to strong polar effects. From the picture of C—H bonding in Chapter 1, we can deduce that the SOMO of a methyl radical is close to halfway between the local cr and cr orbitals of a C—H bond, so that the interactions should be more or less equally the SOMO with the HOMO and with the LUMO. In agreement, methyl radicals abstracting hydrogen atoms are found to be only marginally electrophilic. 

Evans and Polanyi- have noted that in a homologous series of exothermic abstraction reactions (A + BC AB + C), the change in activation energy is related to the change in heat of reaction by the relation AAact = ofA act where a is a constant for a given series. The more recent and more accurate results on methyl radical abstractions would seem, however, to place limits on the quantitative value of the rule. Voevodskii has pro-... 

The methyl radical produced in primary Reaction 2 can abstract a hydrogen atom from an unreacted butene molecule in a chain propagation step or it can add to a butene molecule to provide a pentyl radical, which is the precursor for the observed Cr, products (see Reactions 7 and 8). Methyl radical addition (Reaction 8) is favored at low conversion... 

The pyrolysis Is Initiated by thermal fission of the CHi-0 bond (reaction 1). The resulting oxynaphthyl radical and methyl radical abstract H-atoms from the parent forming naphthol, CH% and the methylene-naphthylether radical (reactions 2 and 3). 

Methylcorrinoids are competent for the efficient methylation of alkyl radicals. Thermolysis of 2 -bis(ethoxycarbonyl)propylcobalamin and methylcobalamin at 70 °C led to formation of cob(II)alamin and the organic products 2-ethyl-2-methylmalonic acid diethyl ester and 2,2-dimethylmalonic acid diethyl ester. The former product was generated with quantitative deuterium incorporation from CDsCobjllljalamin. The proposed mechanism involves homolytic substitution on methylcob(III)alamin by the 2 -bis(ethoxycarbonyl)propyl radical, resulting in net methyl-radical abstraction, a process calculated to be highly exothermic (A7/ -201 kJmoK ). The stereochemical course of the reaction should result in net inversion at the methyl carbon, although this has not been demonstrated. The reaction may serve as a precedent for several biosynthetic methylations, such as the antibiotic thienamycin synthesis. ... 

It is assumed that acetone at such high temperatures decomposes into methyl and acetyl radicals, and the latter further degrades into methyl radical and carbon monoxide. Then methyl radical abstracts one hydrogen atom from acetone to give methane and an acetyl-methyl radical that decomposes into ketene and another methyl radical. Displayed here is the reaction mechanism for the Schmidlin ketene synthesis. 

In a formally related radical abstraction reaction, the cobalt-bound methyl group of methylcobalamin (3) and other methylcorrinoids is rapidly transformed to co(II)corrinoids, such as cob(II)inamide (42+), (giving methyl-cob(III)inamide, 45" ) and cob(II)alamin (Bi2r, 23) (see Fig. 14) [86,134]. Under appropriate conditions (aprotic solvents), this type of reaction is not sensitive to the presence of molecular oxygen and does not involve free methyl radicals [134]. 

The first step is the thermal or photochemical breaking of the chlorine-chlorine bond. This initiation reaction is followed by the first propagation step (Rg. 11.40), the abstraction of a hydrogen atom from methane by a chlorine atom to produce a methyl radical and hydrogen chloride. In the second propagation step, the methyl radical abstracts a chlorine atom from a chlorine molecule to give methyl chloride and another chlorine atom that can carry the chain reaction forward. There are many possible termination reactions Figure 11.40, which shows the overall mechanism, includes only one. 

Benzoyl peroxide has been the most common source of phenyl radicals. But in reaction with thiazoles the benzoyloxy radical abstracts a hydrogen atom from the thiazole nucleus or from a methyl group in the case of methylthiazoles, giving by-products such as dithiazolyls or 2.2 -dithiazolylethane (183). The results obtained with benzoyl peroxide are summarized in Tables III-23, III-24. and III-25. 

In H abstraction, a hydrogen radical reacts with a molecule (primarily a paraffin) and produces a hydrogen molecule and a radical. In the same way, a methyl radical reacts to produce a radical and methane. Similar reactions with other radicals (ethyl and propyl) can also occur. In addition, some radicals like H, CH, etc, are added to olefins to form heavier radicals. 

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].

The competitive method employed for determining relative rates of substitution in homolytic phenylation cannot be applied for methylation because of the high reactivity of the primary reaction products toward free methyl radicals. Szwarc and his co-workers, however, developed a technique for measuring the relative rates of addition of methyl radicals to aromatic and heteroaromatic systems. - In the decomposition of acetyl peroxide in isooctane the most important reaction is the formation of methane by the abstraction of hydrogen atoms from the solvent by methyl radicals. When an aromatic compound is added to this system it competes with the solvent for methyl radicals, Eqs, (28) and (29). Reaction (28) results in a decrease in the amount... 

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. 

It is thus anticipated that compressive stress inhibits while tensile stress promotes chemical processes which necessitate a rehybridization of the carbon atom from the sp3 to the sp2 state, regardless of the reaction mechanism. This tendency has been verified for model ring-compounds during the hydrogen abstraction reactions by ozone and methyl radicals the abstraction rate increases from cyclopropane (c3) to cyclononane (c9), then decreases afterwards in the order anticipated from Es [79]. The following relationship was derived for this type of reactions ... 

Chung and coworkers tried to observe similar species in y-irradiated DMSO-h6 at 77 K, however, repeated attempts were unsuccessful. Besides no free -CH3 radicals were detected in the y-irradiated DMSO-h6. They suggested that this remarkable difference of an all-or-nothing deuterium effect might be connected with the very much larger reactivity of the methyl radical in a subsequent reaction of hydrogen abstraction due to the greater reactivity of the C—H over the C—D bond. 

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