Showing papers on "Disproportionation published in 1981"
TL;DR: In this paper, a mechanism was proposed where oxides of these elements, present in the zeolites, reduce the dimensions of pore openings and channels sufficiently to favor formation and outward diffusion of p-xylene, the isomer with the smallest minimum dimension.
266 citations
TL;DR: In this paper, the authors present a mechanism in which the ascorbate radical ion is in equilibrium with a dimer, which reacts with hydrogen ion, or with other proton donors present including water and buffers (at rates depending upon their acid strengths).
Abstract: Existing data on the kinetics of ascorbate radical decay, together with some new data on the effects of temperature, ionic strength, and presence of phosphate buffers, suggest a mechanism in which the ascorbate radical ion is in equilibrium with a dimer. This dimer reacts with hydrogen ion, or with other proton donors present including water and buffers (at rates depending upon their acid strengths), to form the disproportionation products ascorbate ion and dehydroascorbate acid.
195 citations
TL;DR: In this article, a discussion of redistribution reactions catalyzed by transition-metal complexes is presented, and it is shown that the complete lack of isomerization of the p-tolyl group during the disproportionation of Ar 3 SiH is noteworthy.
Abstract: Publisher Summary This chapter discusses redistributions catalyzed by transition-metal complexes. Redistribution, or disproportionation, constitutes an important class of reactions in organosilicon chemistry. Redistributions on silicon may be initiated by thermolysis or by catalytic activation at somewhat lower temperatures. In the gas phase, methyl is transferred from Me n SiH 4–n , to CH 3 + almost as readily as hydrogen. Silacyclobutanes with metal substituents may also be polymerized to give novel polymers with pendant metal groups. By using chiral silyl hydrides, it has been demonstrated that the exchanges occur with the retention of configuration at silicon. The complete lack of isomerization of the p -tolyl group during the disproportionation of Ar 3 SiH is noteworthy. Similarly, the vinyl group of trimethylvinylsilane is hydrolyzed by a catalytic amount of Zeise's salt [(C 2 H 4 )PtC1 3 ] - in moist acetone. The redistribution reactions of hydridodisilanes of the type R 3 SiSiR 2 H are more varied than those of the corresponding monosilanes. In redistribution reactions catalyzed by transition-metal complexes, di- or polysiloxanes are expected to share some characteristics of both monosilanes and di- or polysilanes. Thus, the availability of the four-membered metallacycle in the catalytic cycle greatly enhances the rate of disproportionation of tetramethyldisiloxane. Some of these catalytic cycles lead to products that appear to arise from divalent silylenoid species.
150 citations
TL;DR: The product produced by the action of o-iodosobenzoic acid upon tryptophanyl bonds has been identified as N-acyldioxindolylalanine, and it is inferred from that structure that the chemical reaction proceeds via a two-step oxidation of the tryPTophanyl residue followed by formation of an iminospirolactone which hydrolyzes, cleaving the peptide chain.
Abstract: o-Iodoxybenzoic acid, a disproportionation product of o-iodosobenzoic acid, has been identified as a contaminant in most preparations of o-iodosobenzoic acid capable of both modifying and cleaving certain tyrosyl residues. A new synthetic approach for the production of o-iodosobenzoic acid containing low amounts of o-iodoxybenzoic acid combined with preincubation of the reagent with p-cresol to destroy the remaining o-iodoxybenzoic acid prior to the reaction with a polypeptide allows preparation of reagent solutions in which tyrosyl residues remain intact during tryptophanyl bond cleavage. In addition, the product produced by the action of o-iodosobenzoic acid upon tryptophanyl bonds has been identified as N-acyldioxindolylalanine. It is inferred from that structure that the chemical reaction proceeds via a two-step oxidation of the tryptophanyl residue followed by formation of an iminospirolactone which hydrolyzes, cleaving the peptide chain. Small peptides ending with dioxindolylalanine can be coupled to aminopropyl glass in high yield and are suitable for solid-phase Edman degradation.
89 citations
TL;DR: In this paper, the amount and dynamic behavior of adsorbed hydrocarbon species formed during the reaction of H2+ CO on Ru/SiO2 were measured by the in situ infrared spectroscopic technique using Fourier-transform methods; the average chain length (the CH2/CH3 ratio), and the dependences of the rate of accumulation of such hydrocarbons on temperature and partial pressures of H 2 and CO gases, together with their reactivities under various reaction conditions, such as hydrogenation.
Abstract: The amount and dynamic behaviour of adsorbed hydrocarbon species formed during the reaction of H2+ CO on Ru/SiO2 were measured by the in situ infrared spectroscopic technique using Fourier-transform methods; the average chain length (the CH2/CH3 ratio), and the dependences of the rate of accumulation of such hydrocarbons on temperature and partial pressures of H2 and CO gases were investigated, together with their reactivities under various reaction conditions, such as hydrogenation. These adsorbed hydrocarbons grow in a linear form on a limited part of the Ru surface, whereas most of the rest of the surface is covered by adsorbed CO. The adsorbed hydrocarbons are readily dehydrogenated to form carbon and hydrogen on evacuation, are also hydrogenated to form mainly CH4 with some higher hydrocarbons, and exchange their hydrogen with molecular hydrogen. Their reactivities, however, were markedly retarded by adsorbed CO when CO was present in the gas phase.Infrared observations demonstrated that small amounts of carbon deposited by the CO disproportionation reaction or of accumulated hydrocarbon cuts down the amount of weakly held adsorbed CO markedly, which suggests that the sites for the weakly held CO play an important role in carbon deposition, which is followed by its hydrogenation to form reaction products.
77 citations
TL;DR: In this paper, the influence of partial pressures of toluene and hydrogen on the rates of disproportionation has been determined in a dynamic reactor under high pressure over a series of protonic mordenites and over a fluorinated alumina (7 wt % F).
59 citations
TL;DR: The metal-carbon composites obtained by CO disproportionation (2CO→CO2 + C) over an iron-nickel (containing 75 Wt%Ni) are characterized by TEM, X-ray diffraction and krypton adsorption at 77 K as mentioned in this paper.
58 citations
ARCO1
TL;DR: In this article, the course of intermolecular reactions induced by peroxide decomposition in linear polyethylene has been determined and compared with similar reaction systems in which the substrate is a lowmolescular-weight alkane.
Abstract: The course of intermolecular reactions induced by peroxide decomposition in linear polyethylene has been determined and compared with similar reaction systems in which the substrate is a lowmolecular-weight alkane. Efficiency of intermolecular coupling is 40% for an alkane and 25–30% for the polymer system. Competing reactions, which reduce coupling efficiency, are mainly disproportionation of the initially formed radicals and to a minor extent further reactions involving the products from the peroxide decomposition. The low efficiencies found are similar to those well studied when initial radicals are produced by radiolysis in polymer and alkane systems.
55 citations
50 citations
TL;DR: In this paper, it was shown that the increasing basicity along the series MgO, CaO, SrO causes a marked increase in the total adsorptive capacity, and an increase in relative population of polymers with respect to dimers.
Abstract: CO adsorption on CaO and SrO takes place, as on MgO, via a disproportionation reaction leading to both surface carbonates and unusual surface species with a complex vibrational spectrum in the low-frequency range. These are thought to be negatively charged CO polymers, the simplest ones being (CO)2–2. A strong electrostatic interaction between negative species and surface cations accounts for the marked dependence of the infrared signal on the lattice parameter of the solids. The increasing basicity along the series MgO, CaO, SrO causes: (i) a marked increase in the total adsorptive capacity; (ii) an increase in the relative population of polymers with respect to dimers; (iii) an increase in the importance of a Boudouart-like reaction upon desorption.
49 citations
TL;DR: In this paper, a series of catalytic carbons has been prepared by CO disproportionation (2CO→CO2 + C) over an iron nickel catalyst, and the dependence of the rate of carbon deposition on temperature, between 400 and 650°C, and on the CO partial pressure in the reacting CO CO 2 mixtures has been investigated.
TL;DR: In this article, the interaction of carbon monoxide with Ni(100) has been studied by ellipsometry and Auger electron spectroscopy and the rate of oxidation of this surface carbide is, where hc is the carbon 272 eV Auger peak height, n=0.5 and the apparent activation energy Eact =13.3 kcal/mole.
TL;DR: In this paper, the acid-strength distribution of the sites generated at various levels of Na+H+ exchange in synthetic, large-port sodium mordenite has been characterised by measuring the heat of adsorption of NH3 on these sites.
TL;DR: The dependence on reaction medium of the kinetics of three ground-state elementary reactions occurring in the iron-methylene blue photoredox system has been investigated by studying the relaxation of the photostationary state and by flash photolysis as discussed by the authors.
Abstract: The dependence on reaction medium of the kinetics of three ground-state elementary reactions occurring in the iron-methylene blue photoredox system has been investigated by studying the relaxation of the photostationary state and by flash photolysis. The rate constants which have been evaluated include 2k/sub 6/, for disproportionation of semimethylene blue (S), k/sub -6/, for syn proportionation (the oxidation of leucomethylene blue (L) by methylene blue (MB)), and k/sub 10/, for the oxidation of leucomethylene blue by ferric ion. Variations of media include nature of the solvent and anions, ionic strength, and concentration of acid. Values of the equilibrium constant K/sub 6/ = k/sub -6/2k/sub 6/ = (S)/sup 2//(L)(MB) have been derived from the kinetic data and used in conjunction with potentiometric data to determine values of the one-electron standard reduction potentials, epsilon/sup o'//sub MB/S/ and epsilon/sup o'//sub S/L/ in several media. As in the iron-thionine photoredox system, the half-reduced dye, S, is a minor component of the photostationary state and oxidation of leuco dye by ferric ion appears to proceed via a metastable association complex of the reactants. Mechanistic interpretations of some of the medium effects are suggested.
TL;DR: A theoretical analysis of chemical reactions in the early stages of anthracene pyrolysis is presented in this paper, which makes extensive use of thermochemical and kinetic estimation methods as well as fundamental principles of free radical kinetics.
TL;DR: In this article, a 5.5% Ni Al 2 O 3 catalyst was investigated for the disproportionation of carbon monoxide by infrared spectroscopy and temperature-programmed desorption and the reaction was found to be of first order with respect to the surface concentration of CO below 200 °C.
TL;DR: In this article, the quantum yield for formation of the semiquinone depends upon pH, the results being attributed to the relative efficiency with which the various acid/base forms of the hydroquinone react with quinone.
Abstract: — Irradiation of anthraquinone-2,6-disodium sulphonate in aqueous solution leads to formation of hydroxyquinone and semiquinone, which undergoes disproportionation to form the fully reduced hydroquinone. The quantum yield for formation of the semiquinone depends upon pH, the results being attributed to the relative efficiency with which the various acid/base forms of the hydroquinone react with quinone. The photoreaction does not involve production of hydroxyl radicals and luminescence studies suggest that the quantum yield for formation of the triplet state is unity. Indeed, flash photolysis studies indicate the presence of a short-lived triplet (τ < 1 μs) which reacts with water to form a photosolvolate (τ= 5 μs) which, in turn, decays to form the observed products. These findings are in agreement with a mechanism previously proposed by Stonehill.
TL;DR: In this paper, the cracking of n -heptane has been studied at 400, 450, and 470 °C over a CrHNaY zeolite catalyst using a tubular packed-bed reactor.
TL;DR: In this article, the 13.13 C nmr spectra of various thioglucose derivatives have been assigned and a mechanism for the formation of the sulfinic acid derivative from AuSTg is proposed.
TL;DR: In this article, it was suggested that the attack by the aldehyde hydrate anion is at the metal to which hydride transfer initially occurs, and that the carboxylate (or carboxylic acid) and of the alcohol (or alkoxide) thus occur in the coordination sphere of the complex.
Abstract: Acetaldehyde is converted into ethanol and acetic acid (1 : 1) by dilute neutral aqueous solutions of pentamethyl-cyclopentadienyl-rhodium or -iridium complexes, e.g.[{Rh(C5Me5)}2(OH)3]+. Areneruthenium complexes catalyse a similar reaction but more acetic acid than ethanol is formed; the ratios change with added ligand. The reaction is first-order in aldehyde and half-order in the binuclear catalysts [{Rh(C5Me5)}2(OH)3]Cl or [{Ru(C6Me6)}2(OH)3]Cl. When the reactions using [{Rh(C5Me5)}2(OH)3]OH were quenched with PF6– or when aqueous solutions of [{Rh(C5Me5)}2(OH)3][PF6] were reacted with acetaldehyde, µ-hydrido-complexes [{Rh(C5Me5)}2H(O2CMe)2][PF6] and [{Rh(C5Me5)}2H2(O2CMe)][PF6] were isolated. Addition of acetaldehyde to [{Ru(C6Me6)}2(OH)3][PF6] gave [{Ru(C6Me6)}2(O2CMe)3][PF6]. In base (pH 12.8) the disproportionation of acetaldehyde was very much (>104 fold) accelerated in the presence of catalyst and gave alcohol and acetate in a 1 : 1 ratio for all catalysts. The rhodium-catalysed reaction competed successfully with the aldol condensation even at high base strength (1.5 mol dm–3 NaOH) but the ruthenium- and iridium-catalysed reactions were slower. Other aldehydes (EtCHO, PhCHO, MeCHCHCHO) reacted similarly. Hydride complexes [{Rh(C5Me5)}2H(O2CR)2][PF6] could again be isolated from the neutral regime reactions and addition of alkali caused significant accelerations in rate. The mechanism is suggested to be related to that of the Cannizzaro reaction, except that the attack by the aldehyde hydrate anion is at the metal to which hydride transfer initially occurs. Formation of the carboxylate (or carboxylic acid) and of the alcohol (or alkoxide) thus occur in the co-ordination sphere of the complex. A variety of non-half-sandwich complexes of Rh and Ru as well as the metals themselves were essentially inactive.
TL;DR: In this paper, the mesophase formation is observed at relatively low temperatures and after the removal of the catalyst, the obtained mesophases seem to be stable in a wide temperature interval.
TL;DR: In this article, the catalytic action of A1C13 on naphthalene and other polynuclear hydrocarbons was studied at temperatures between 85 and 120°C.
IBM1
TL;DR: In this paper, it was shown that the chemisorption of O2, H2O, and H2 on LaNi5 surfaces induces a strong, weak and no surface segregation, resp.
Abstract: 2014 Surface segregation with the disproportionation into La and Th oxide plus precipitations of elemental Ni play an important role in the understanding of the easy hydrogen absorption of LaNi5 and of the catalytic properties of ThNi5. By means of core-level X-ray photoelectron spectroscopy we show that the chemisorption of O2, H2O, and H2 on LaNi5 surfaces induces a strong, weak and no surface segregation, resp. Precoverage with SO2 blocks further segregation, whereas exposure to H2S does not. The oxygen induced segregation of ThNi5 at room temperature is much weaker than in LaNi5, but is very extensive at 200 °C. J. Physique 42 (1981) 1025-1028 JUILLET 1981,
TL;DR: A series of kinetically unstable mononuclear anionic formyl complexes have been prepared by the action of Li(C 2 H 5 ) 3 BH on neutral metal carbonyl precursors as discussed by the authors.
TL;DR: This article showed that trans-enal arises by an almost concerted out-of-plane cleavage which forms a 1,6-biradical in the perfect geometry for disproportionation to transenal.
TL;DR: In this article, it was shown that pentamethylcyclopentadiene (A) in liquid solution with u.v. light results in homolysis of the ring C-H bond to give the pentammethylcyclopentadienyl radical (B), which self-react at a diffusion-controlled rate (2kt 2 × 109 I mol 1 s−1 s −1 in hexane at 25°C) by two different routes.
Abstract: Irradiation of pentamethylcyclopentadiene (A) in liquid solution with u.v. light results in homolysis of the ring C–H bond to give the pentamethylcyclopentadienyl radical (B)[a(15 H) 6.4, a(13C) 3.5 G], and atomic hydrogen which [graphic omitted] abstracts hydrogen from a second molecule of (A) to give molecular hydrogen and a second radical (B). The radicals (B) self-react at a diffusion-controlled rate (2kt 2 × 109 I mol–1 s–1 in hexane at 25°C) by two different routes. The first, which is thermally and photolytically reversible, is the combination to give the dehydrodimer (C), and the second, which is irreversible, is the disproportionation to give the parent cyclopentadiene (A) and the tetramethylfulvene (D).
TL;DR: In this article, the photochemistry of two series of β-phenyl substituted acyclic β, γ-unsaturated ketones 1 and 2 upon direct irradiation with γ 310nm has been investigated, viz 1c-1h and 2b+2c.