Generation Of Methyl Radicals Research Articles

Oxidative coupling of methane over Y 2O 3[sbnd]CaO catalysts

Binary oxides of Y 2O 3 CaO were evaluated as catalysts in the oxidative coupling of methane to C 2+ (sum of C 2H 6, C 2H 4, C 3H 8, and C 3H 6) hydrocarbons. Passing a mixture of CH 4/O 2 and He gases (at 6, 3 and 31 ml/min respectively) in a fixed-bed flow reactor 13% and 7.5% of C 2+ yields were achieved at 750°C and 650°C, respectively, over 0.5 g of 10 mol-% Y 2O 3 CaO catalyst prepared by calcining a coprecipitate of their oxalates at 800°C. The C 2+ yields on 10 mol-% Y 2O 3 CaO, prepared by physical mixing, were lower than those on the coprecipitated catalyst. With increasing Y 2O 3 content in the coprecipitated catalyst, the C 2+ selectivity at 700°C was significantly enhanced even at ca. 3 mol-%, whereas at 600°C such a change was not observed. A similar dependence on the Y 2O 3 content was found in the way both surface areas and basicities decreased. Those changes were attributed to the formation of a solid solution accompanying the production of interstitial oxygen ions. Electron-spin resonance (ESR) studies indicated that the ion is a superoxide ion which is responsible for the generation of methyl radical from methane. At low reaction temperatures, 700°C, it was found that a lattice distortion of Y 2O 3 in the binary oxides also affected the C 2+ selectivity.

AN ESR STUDY OF THE VISIBLE LIGHT PHOTOCHEMISTRY OF GILVOCARCIN V

Photolysis of gilvocarcin (GV) at 405 nm in argon saturated dimethylsulfoxide (DMSO) or 50% DMSO-water solutions in the presence of the sodium salt of 3,5-dibromo-2,6-dideutero-4-nitrosobenzene sulfonic acid (DBNBS-d2) generates the CH3-DBNBS-d2.spin adduct. It is postulated that this spin adduct is produced by photoreduction of DMSO by GV and the consequent formation and trapping of the generated methyl radicals. Gilvocarcin V also photoreduces oxygen and methyl viologen with quantum yields of 0.019 and 0.0012 respectively. The quantum yield for singlet oxygen formation by GV in DMSO, determined by measuring the rate of production of the nitroxyl radical produced by the reaction of 2,2,6,6-tetramethylpiperidinol with singlet oxygen, was found to be 0.15. Thus, GV photochemistry proceeds by both Type I and Type II pathways which could contribute to the reported GV phototoxicity in biological systems.

Oxidative dimerization of methane over well defined lithium-promoted magnesium oxide catalysts

Pure MgO samples having greatly different morphologies and lithium-promoted catalysts derived from these materials have been examined by electron microscopy. In addition, their ability to generate and react with gas-phase methyl radicals has been determined using e.s.r. in combination with a matrix-isolation technique. Lithium caused the MgO to sinter severely at 700 °C, but the resulting material showed improved activity for the generation of methyl radicals. These factors gave rise to catalysts which were moderately active and selective for the oxidative dimerization of CH4 at 700 °C (up to 20% conversion, 66% C2+ selectivity). The results are particularly significant in that catalysts which had been prepared such that the lithium was initially present as Li2O on the surface exhibited almost constant C2+ production as the Li2O was converted to Li2CO3. Thus, neither Li2O nor Li2CO3 is the active phase for the selective conversion of CH4. A catalyst prepared from Li2CO3 and MgO sintered much less extensively, produced more CH3˙ radicals and gave higher yields of C2+ hydrocarbons. A comparison of these catalytic properties suggests that surface area and related morphological factors do not have an adverse effect on the oxidative dimerization reaction, provided the surface can be rendered inert to secondary reactions with methyl radicals.

The Interaction of Laser Generated Methyl Radicals with Cd, Te, and CdTe Surfaces

ABSTRACTThe mechanism of the interaction of methyl radicals with Cd, Te, and CdTe surfaces has been studied in ultrahigh vacuum by Auger electron spectroscopy and thermal desorption mass spectrometry. Methyl radicals generated by the laser photodissociation of acetone at 193 nm efficiently etch both Te and Te-rich CdTe surfaces. However, there is no evidencefor reaction of methyl radicals with Cd or stoichiometric CdTe. A temperature dependencein the rate of methyl radical etching of Te-rich CdTe is related to a competition between acetone scavanging of radicals on the surface and reaction of radicals to form volatile metalorganics. Acetone itself has a small but finite reaction probability with Te and Te-rich CdTe surfaces.

Reduction and destruction rates of nitroxide spin probes

A series of nitroxides was tested for rates of one-electron reduction in a chemical, a photochemical, and two biological systems by ESR assays. In all cases, piperidine and hydropyridine nitroxides were reduced consistently more rapidly than pyrroline and pyrrolidine nitroxides. Substituents on the nitroxides also affected reduction rates, although not as greatly as ring structure. One of the reduction systems, consisting of the photosensitizer FMN and the photoreductant EDTA, was used to study both anaerobic reduction and O 2-dependent reoxidation of some of the nitroxides. Reduced piperidine and hydropyridine nitroxides were also oxidized more rapidly than the reduced pyrroline and pyrrolidine nitroxides. Reoxidation subsequent to reduction was partially inhibited by Superoxide dismutase, indicating that Superoxide radicals are involved in the process. Even after prolonged reoxidation, not all of the probe molecules were returned to their oxidized form, implying an irreversible “destruction” of the spin probe concomitant with its chemical reduction. Probe destruction was studied more specifically with a photochemical system for generating methyl radicals, which showed that these carboncentered radicals destroyed different nitroxides at rates which were much less influenced by the nitroxide structures than one-electron reduction was.

Absolute rate coefficients for methyl radical reactions by laser photolysis/time-resolved infrared chemiluminescence: methyl-d3 + HX .fwdarw. methane-d3 + X (X = Br, I)

Absolute rate coefficients are reported for the room temperature reactions of deuterated methyl radicals with HI and HBr, CD/sub 3/ + HI (HBr) ..-->.. CD/sub 3/H + I(Br). Excimer laser photolysis of CD/sub 3/I is used to generate methyl radicals, and time-resolved infrared chemiluminescence from the CH stretch of the CD/sub 3/H products is detected to follow the time evolution of the reaction. The rate constants obtained in this manner are (7.7 +/- 0.7) x 10/sup -12/ cm/sup 3/ molecule/sup -1/ s/sup -1/ for CD/sub 3/ + HI and (4.7 +/- 0.4) x 10/sup -12/ cm/sup 3/ molecule/sup -1/ s/sup -1/ for CD/sub 3/ + HBr. These rate constants are considerably greater than earlier, indirectly measured values and indicate that the activation energy for these light-atom transfer reactions is lower than previously believed. 53 references, 3 figures, 2 tables.

Suppression of unwanted heterogeneous platinum(0)-catalyzed reactions by poisoning with mercury(0) in systems involving competing homogeneous reactions of soluble organoplatinum compounds: thermal decomposition of bis(triethylphosphine)-3,3,4,4-tetramethylplatinacyclopentane

Thermal decomposition of bis(trialkylphosphine)-3,3,4,4-tetramethylplatinacyclopentanes in hydrocarbon solvents yields two major products: 2,2,5p-tntramethylbutane and l-methyl-l-tert:butylcyclopropane. The former appears to be generated by heterogeneous process(es) catalyzed by platinum(0) (colloidal and/or solid) formed during the decomposition; the latter is the product of a homogeneous reaction sequence. The existence of competing heterogeneous and homogeneous reaction pathways complicates the study of this thermal decomposition. The addition of mercury(O) to the system selectively suppresses the heterogeneous reaction by poisoning the bulk platinum(O). This technique has been tested in one well-defined model system-the homogeneous hydrogenation of dineopentylbis(triethylphosphine)platinum(Il) to dihydridobis(triethylphosphine)platinum(Il) and neopentane in the presence of the heterogeneous platinum(0)-catalyzed hydrogenation of l-methylcyclopentene to methylcyclopentane-and found to eliminate the heterogeneous reaction while leaving the homogeneous one essentially unaffected. Application of this technique to the thermal decomposition of 3,3,4,4-tetramethylplatinacyclopentane simplifies the reaction by eliminating 2,2,3,3-tetramethylbutane as a product. Similarly, addition of mercury(0) to solutions containing H2 and (1,5-cyclooctadiene)dimethylplatinum(Il) in cyclohexane suppresses the autocatalytic, heterogeneous platinum-catalyzed conversion of the organoplatinum compound to (inter alia) cyclooctane, methane, and platinum(0). In other reactions examined-especially the high-temperature thermal decompositions of (1,5-cyclooctadiene)dineopentylplatinum(II) and of cis-bis(cyclopentyldimethylphosphine)dimethylplatinum(Il) in hydrocarbon solvents-mercuq/ poisoning does not successfully separate homogeneous and heterogeneous reactions. In the latter instance, an additional complicating processapparent reaction of mercury(O) with the organoplatinum compound-introduces reaction paths which seem to generate methyl radicals. The paper includes a simple preparation of 1,4-dihalo-2,2,3,3-tetramethylbutane and illustrates the use of this material as a reagent for the preparation of 3,3,4,4-tetramethylmetallacyclopentanes via the corresponding di-Grignard reagents.

Spin-trapping with 2-methyl-2-nitrosopropane: photochemistry of carbonyl-containing compounds. Methyl radical formation from dimethyl sulfoxide

The photochemical reactions of several carbonyl-containing compounds investigated by spin-trapping with 2-methyl-2-nitrosopropane revealed different modes of scission depending on the structure of the initial compound. Thus, in photo-Fries rearrangements, the acyl radical was detected. 1,3-Diphenyl-2-propanone decarbonylated to yield the benzyl radical. Finally, valerophenone yielded the radicals expected by γ-hydrogen abstraction. In a dark reaction, dimethyl sulfoxide reacts with NaOH to generate methyl radicals. The latter result suggests the need for caution in the use of dimethyl sulfoxide with 2-methyl-2-nitrosopropane for the detection of hydroxyl radicals.

Initiation mechanisms in radical polymerizations : Reaction of cumyloxy radicals with methyl methacrylate and styrene

Cumyloxy (1-methyl-1-phenylethoxy) radicals have been generated by thermolysis (60�) of dicumyl hyponitrite in methyl methacrylate and styrene. The carbon-centred radicals formed by interaction of cumyloxyl with the respective monomers were trapped as stable adducts of 1,1,3,3-tetramethyl-isoindolin-2-yloxyl. Extensive hydrogen atom abstraction and methyl radical generation as well as double-bond addition were observed in methyl methacrylate. Styrene underwent only double-bond addition by both cumyloxy and methyl radicals. Some possible implications of these results for polymer structure are discussed. A kinetic study of the decomposition of dicumyl hyponitrite in cyclohexane at various temperatures gave k=7.7 × 1014exp(-13600/T) s-1 for the rate constant. Rate constants for the addition of cumyloxyl to methyl methacrylate (k ≈ 2 × 104 dm3 mol-1 s-1) and styrene (k≈2 × 105 dm3 mol-1 s-1) at 60�have been estimated.

Methyl radical reactions with isopropanol and methanol, their ethers and their deuterated derivatives

The reactions of methyl radicals with three isotopically different isopropanols three isotopically different methanols and the undeuterated ethers (diisopropyl and dimethyl ethers) have been investigated. The photolysis of acetone or acetone-d6 has been used to generate methyl radicals or trideuteromethyl radicals. An isotopic exchange reaction between acetone and alcohols makes it necessary to work with isotopically compatible pairs. Values for Arrhenius parameters, based on Shepp's rate-constant for the combination of methyl radicals, are as follows : [graphic omitted] Primary isotope effects in the alkyl groups of these alcohols are larger than expected solely on the basis of zero-point-energy difference : at 150°, (kCH/kCD) is ca. 8 for methanol and ca. 6.5 for isopropanol. These values, which resemble those for ethanol and for isobutane, taken together with the corresponding activation-energy differences, suggest that tunnelling may again be important. A secondary reaction, disproportionation between the radicals · C(CH3)2OH and CH3, is important in attack on isopropanol. It gives rise to an apparent enhancement of reactivity of the hydroxyl groups, to a spurious secondary isotope effect and, unless allowance is made, to a low value for the A-factor. The corresponding reaction does not appear to be significant in methanol or ethanol systems. In dimethyl ether, the reactivity of each methyl group is the same as in methanol but in diisopropyl ether there is steric hindrance and the alpha hydrogen atoms are less accessible than in the alcohol.

Generation of Methyl Radicals by Decomposition of Bibenzyl Compounds Containing α-Methoxy Substituents

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTGeneration of Methyl Radicals by Decomposition of Bibenzyl Compounds Containing α-Methoxy SubstituentsG. E. Hartzell and E. S. HuyserCite this: J. Org. Chem. 1964, 29, 11, 3341–3344Publication Date (Print):November 1, 1964Publication History Published online1 May 2002Published inissue 1 November 1964https://doi.org/10.1021/jo01034a051RIGHTS & PERMISSIONSArticle Views212Altmetric-Citations16LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (510 KB) Get e-Alerts Get e-Alerts