Methyl radical chain reaction

Chlorination of Methane. Methane can be chlorinated thermally, photochemicaHy, or catalyticaHy. Thermal chlorination, the most difficult method, may be carried out in the absence of light or catalysts. It is a free-radical chain reaction limited by the presence of oxygen and other free-radical inhibitors. The first step in the reaction is the thermal dissociation of the chlorine molecules for which the activation energy is about 84 kj/mol (20 kcal/mol), which is 33 kJ (8 kcal) higher than for catalytic chlorination. This dissociation occurs sufficiendy rapidly in the 400 to 500°C temperature range. The chlorine atoms react with methane to form hydrogen chloride and a methyl radical. The methyl radical in turn reacts with a chlorine molecule to form methyl chloride and another chlorine atom that can continue the reaction. The methane raw material may be natural gas, coke oven gas, or gas from petroleum refining. 

Recently, Maruta et al. [112] have found that methanol extracts of roots of burdock show a significant antioxidant activity in an in vitro lipid peroxidation assay, and have isolated five caffeoylquinic acid derivatives (CQAs) from the roots of burdock (Arctium lappa L ), an edible plant in Japan. Antioxidant activities of DCQAs and related compounds have been investigated by measuring the hydroperoxidation of methyl linolate via radical chain reaction. This study indicates that in this particular system caffeic acid and CQAs are more effective than a-tocopherol. These results approximately agree with our findings [38], Additionally, CQAs as the principle antioxidative substance in burdock root have been characterized. 

Treatment of adamantyl iodide (21) and methyl acrylate with H3P02/yV-ethylpiperidine in the presence of Et3B induces a radical chain reaction to form the addition product (22) (eq. 12.6e). 

Neat A-(l -methyl-4-pentenyl)hydroxylamine underwent facile cyclization to the corresponding Y-hydroxypyrrolidine 1 on wanning briefly to 50- 60 °C, via a radical chain reaction involving the nitroxide radical. A-(l-Methyl-5-hexenyl)hydroxylamine cyclized to give A-hydroxypipe-ridine 2 only in refluxing xylene under high dilution conditions, this is necessary to avoid formation of byproducts. The cyclization was facilitated by the presence of a-methyl substituents in the hydroxylamine. Transannular cyclization of A-[(3-cyclohexenyl)methyl]hydroxylamine was not successful. Since the isolation of pure samples of the water-soluble and easily oxidized hydroxylamines was not a satisfactory procedure, the crude reaction mixtures were subjected to reduction with a zinc/acetic acid/acetic anhydride system to isolate acetylated cyclic amines. 

Visible light irradiation of l,2-dihydro-2-thioxo-l-pyridinyl jV-(4-alkenyl)-A-alkylcarbamates (PTOC carbamates) produces substituted pyrrolidines by a radical chain reaction (see Section 7.2.5.1). In the absence of hydrogen donors, the intermediate pyrrolidinylmethyl radical reacts with the PTOC carbamate itself to afford 2-[(2-pyridinylthio)methyl]pyrrolidines, e.g., 1, 2, 4 and 521,22. On the other hand, in the presence of a good transfer reagent, another functionality can be introduced. In the presence of diphenyl diselenide, the phenylseleno-sub-stituted products were obtained in good yield, e.g., 3 and 622. In every case, however, a low degree of diastereoselectivity of the cyclization products is observed. 

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 methyl-radical chain given above is presumed to be the most important part of the reaction. However, the presence of hydrogen as a product implicates many other reactions such as... 

The CH300 radical is much less reactive than the CH3 radical, and can do little to continue the chain. By combining with a methyl radical, one oxygen molecule breaks a chain, and thus prevents the formation of thousands of molecules of methyl chloride this, of course, slows down the reaction tremendously. After all the oxygen molecules present have combined with methyl radicals the reaction is free to proceed at its normal rate. 

Clearly, the ArS group had become separated from the rest of the molecule and the most likely explanation was a radical chain reaction (Chapter 39) with the light producing a small amoimt of ArS to initiate the chain. The para-methyl group acts as a label. The whole system is in equilibrium and the more highly substituted alkene is the product. 

In many synthetically useful radical chain reactions, hydrogen donors are used to trap adduct radicals. Absolute rate constants for the reaction of the resulting hydrogen donor radicals with alkenes have been measured by laser flash photolysis techniques and time-resolved optical absorption spectroscopy for detection of reactant and adduct radicals Addition rates to acrylonitrile and 1,3-pentadienes differ by no more than one order of magnitude, the difference being most sizable for the most nucleophilic radical (Table 8). The reaction is much slower, however, if substituents are present at the terminal diene carbon atoms. This is a general phenomenon known from addition reactions to alkenes, with rate reductions of ca lOO observed at ambient temperature for the introduction of methyl groups at the attacked alkene carbon atom . This steric retardation of the addition process either completely inhibits the chain reaction or leads to the formation of rmwanted products. 

Copolymers of vinyl chloride and methyl vinyl ketone undergo chain scission with concomitant rapid decreases in tensile strength and elongation when exposed to near ultraviolet li t and solar radiation. Free radicals formed by the homol3rtic scission of the acyl group apparently deplete the stabilizers used and lead to rapid discoloration of the polymer, presumably by the usual radical chain reaction involving the production of HCl and conjugated double bonds. 

The location of the oxime group in the product in Scheme 4 depends upon where the methyl group is located, but the yields can be high (79%). Irradiation of 2-methoxyfuran also induces side-chain dissociation and the formation of lactones (i.e., methylbutenolides).21 Here the abundant formation of ethane testifies to the participation by methyl radicals. Such reactions are too complex for synthetic purposes. This is true also of 2-acetoxyfuran photochemistry in which the photo-Fries reaction is not observed.21 Curiously, furan itself sensitizes the photo-Fries reaction in benzenoid esters.22... 

By-products. The presence in the products of small quantities of compounds which would arise from combination of free radical intermediates can provide evidence for a free radical process. For example, the explosive reaction of methane with fluorine gives mainly hydrogen fluoride and a mixture of mono-, di-, tri- and tetrafluoromethanes, but small quantities of fluorinated ethanes, including C2F6, are also produced. These two-carbon products cannot be readily explained on the basis of possible molecular reactions (see reaction 6.16), but would arise naturally as combination products of the fluorinated methyl radicals produced in a radical chain reaction sequence (reaction 6.17). 

The maleimide group in BMI can undergo a wide range of possible reactions, either in the neat resin or copolymerized, with other monomers. The predominant reaction is the free radical chain reaction of the double bond ([20, 21] and references therein), which, due to the difunctionality of BMI monomers, results in a crosslinked three-dimensional network. Maleimides have been shown to undergo copolymerization with a number of monomers including methyl methacrylate [22, 23], styrene [22-24], acrylonitrile [22] and... 

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