The thermal decomposition of biacetyl

The thermal decomposition of biacetyl has been studied at small percentage conversion over the temperature range 375-417°C. For these conditions, an almost quantitative mass balance was obtained by gas-chromatographic analysis. The following equation was obtained for the overall reaction Between 240° and 277°C, the decomposition of biacetyl initiated by methyl radicals has also been studied. As source of radicals, the thermolysis of azomethane was used. Moreover, the Arrhenius parameters of the following reactions were determined: where A is in sec^?1 for reaction (1) and in cm³mole^?1 sec^?1 for reactions (3) and (4); E is in kcal/mole. Evidence is provided that the displacement reaction (4) proceeds by a two step mechanism.     

The thermal and initiated thermal decomposition of biacetyl-d6

The thermal and methyl radical initiated decomposition of biacetyl-d₆ were investigated at conversions 5% over the temperature range of 387-412°C and 251–292°C, respectively. The deuterated sample contained 10.5% biacetyl-d₅. The reaction products were analyzed by mass spectrometry. The following data could be determined from product ratios: where E is in kcal/mol With reasonable assumptions and literature data Arrhenius parameters for elementary steps were derived and discussed in particular with respect to isotopic effects in H/D-abstraction reactions.     

A calculation of thermal degradation initiated by random scission,unsteady radical concentration

Changes in molecular weight distribution and in sample volume were calculated for thermal degradation of a polymer. The thermal degradation scheme consists of random scission initiation, depropagation and disproportionation termination reactions. An unsteady radical concentration was considered. There are two parameters, normalized zip length z/x₀ and radical number per initial chain length z′x₀, describing the thermal degradation scheme with an unsteady radical concentration. The effects of the initial number-average molecular weight and order of the disproportionation termination reaction on changes in molecular weight, the sample volume and polydispersity are not significant as long as these two parameters have the same value for each polymer sample. Molecular weights of a degrading sample calculated from the steady state radical concentration tend to be over-estimated and sample volumes tend to be underestimated compared to those calculated with an unsteady radical concentration. The validity of approximations used in the calculation assuming a steady state radical concentration is examined by comparing with results calculated with an unsteady radical concentration for various values of the two parameters. An unrealistically large build-up of monomer radicals is found for both calculations based on the steady state and the unsteady radical concentrations. Two special treatments of monomer radicals can dissipate the build-up of monomer radicals: (1) their immediate vaporization, or (2) an enhanced rate of the termination reaction for the monomer radicals. As a guide, the model based on an unsteady radical concentration is preferred, if the value of z′x₀ exceeds 0.1.     

Study of the radical products of the thermal decomposition of nitrocellulose

The thermal transformations of nitrocellulose are accompanied by the formation of RN0₂ ^– radicals and allyl radicals. A mechanism for the formation of these radicals was proposed.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2641–2644, November, 1989.     

Photopolymerization initiated by carbonyl compounds. VII. Solvent effects on biacetyl photoinitiation

Biacetyl is solvated by methanol or water giving an α-substituted alkanone. Irradiation of the solvated species at 300 nm in the presence of vinyl acetate or vinyl pyrrolidone, produces a significant amount of polymer. The photoprocesses leading to active free radicals are considered to be a type I photocleavage of the α-substituted alkanone.     

Theoretical study of the thermal decomposition of the 5-methyl-2-furanylmethyl radical

The thermal decomposition of the 5-methyl-2-furanylmethyl radical (R(1)), the most important primary radical formed during the combustion and thermal decomposition of 2,5-dimethylfuran (a promising next-generation biofuel), was studied using CBS-QB3 calculations and master equation (ME)/RRKM modeling. Because very little information is available in the literature, the detailed potential energy surface (PES) was investigated thoroughly. Only the main pathways, having a kinetic influence on the decomposition of R(1), were retained in the final ME/RRKM model. Among all the channels studied, the ring-opening of the 5-methyl-2-furanylmethyl radical, followed by ring enlargement to form cyclohexadienone molecules is predicted to be the easiest decomposition channel of R(1). The C(6) cyclic species formed can undergo unimolecular reactions to yield phenol and to a lesser extent cyclopentadiene and CO. Our calculations predict that these species are important products formed during the pyrolysis of 2,5-dimethylfuran (DMF). Other channels involved in the decomposition of R(1) lead directly to the formation of linear and cyclic unsaturated C(5) species and constitute an additional source of cyclopentadiene and CO. High-pressure limit rate constants were computed as well as thermochemical properties for important species. ME/RRKM analysis was performed to probe the influence of pressure on the rate coefficients and pressure dependent rate coefficients were proposed for pressures and temperatures ranging, respectively, from 10(-2) bar to 10 bar and 1000 to 2000 K.     

High-energy ionising radiation initiated decomposition of acetovanillone

Hydroxyl radical, hydrated electron and hydrogen atom intermediates of water radiolysis react with acetovanillone with rate coefficients of (1.05±0.1)×10¹⁰, (3.5±0.5)×10⁹ and (1.7±0.2)×10¹⁰mol^?1 dm³ s^?1. Hydroxyl radical and hydrogen atom attach to the ring forming cyclohexadienyl type radicals. The hydroxyl–cyclohexadienyl radical formed in hydroxyl radical reaction in dissolved oxygen free solution partly transforms to phenoxyl radical. In the presence of O₂ phenoxyl radical formation and ring destruction are observed. Hydrated electron in O₂ free solution attaches to the carbonyl oxygen and undergoes protonation yielding benzyl type radical. In air saturated 0.5 mmol dm^?3 solution using 15 kGy dose most part of acetovanillone is degraded, for complete mineralisation five times higher dose is required. The experiments clearly show that acetovanillone can be efficiently removed from water by applying irradiation technology.     

The free radical initiated polymerization of diethylfumarate

The kinetics of polymerization of diethylfumarate initiated by 1,1′ azo-bis-isobutyronitrile have been studied at temperatures between 60 and 80°. Difficulty in isolating the polymer was overcome by using a petroleum ether-methanol two-phase system to separate the monomer and polymer, and by precipitating the polymer from the methanol phase with an aqueous salt solution. The density of the polymer varied from 1·183 g/cm³ at 80° to 1·211 g/cm³ at 20°, giving volume contractions of approximately 16 per cent for complete conversion of monomer into polymer. The initiator exponent was about 0·45, i.e. slightly lower than normally obtained from free radical initiated vinyl polymerization. The overall energy of activation for the polymerization was 89 ± 2 kJ/mole. Constants for chain transfer with monomer, determined from rate and molecular weight measurements at 60, 70 and 80, were in the region of 0·005 0·017.     

Computational studies on the thermal decomposition and isomerization of CHBr2O radical

The unimolecular decomposition study of dibromomethoxy radical, CHBr₂O, and its isomeric hydroxy dibromomethyl radical, CBr₂OH, is carried out using ab initio electronic molecular structure methods. Three kinds of reaction pathways are examined, C–H and C–Br bond scissions, intramolecular three-center HBr elimination and isomerization. Based on the ab initio results, energy-specific rate coefficients k(E) and thermal rate constants k(T,P) are evaluated using RRKM theory and master equation numerical analysis. Relevance to existing experimental evidence is discussed.     

Fluorometric determination of biacetyl

The fluorometric determination of biacetyl is described. 3,4-Diaminoanisole reacts with biacetyl in alkaline ethanol-water solution to give strongly fluorescing 2,3-dimethyl-6-methoxy-quinoxaline. Variables such as solvent composition, quenching by acetic and sulphuric acid, heating time and interference by other carbonyl compounds present are discussed. The method may be suitable for the determination of biacetyl in foodstuffs.     

Co-precipitation of metal dimethylglyoximates generated from biacetyl monoxime or biacetyl and hydroxylamine

A study has been made of some aspects of the co-precipitation of gold and platinum(II) and (IV) with palladium(II) precipitated from homogeneous solution initially containing biacetyl and hydroxylamine in 0.3M hydrochloric acid. The co-precipitation of nickel(II) with palladium(II) and vice versa from near neutral solutions initially containing biacetyl and hydroxylamine was also explored; the solid is enriched in the minor constituent in both cases. As with biacetyl monoxime-hydroxylamine systems, both nickel(II) and palladium(II) promote formation of dimethylglyoxime from such solutions between pH 7 and 8. Factors leading to this unusual behaviour in co-precipitation are discussed and a qualitative explanation tentatively advanced. The co-precipitation of nickel and palladium(II) is different when biacetyl monoxime solutions are used; the former comes out of solution faster than the latter irrespective of their concentration ratio. This disparity in the reaction rates can be used to separate nickel directly from palladium(II) by precipitation or extraction of the dimethylglyoximate into chloroform.     

The thermal decomposition of chlorodifluoromethane

The kinetics of pyrolysis of chlorodifluoromethane have been studied in a flow system between 670° and 750° using partial press of CF₂HCl between 17 mm and 200 mm Hg. The reaction obeys first order kinetics at low conversions, but is retarded by hydrogen chloride. The major features of the reaction are explained by the following mechanism:     

The thermal decomposition of vermiculite

Vermiculite shows some promising potentials, both as ion-exchange material and thermal insulation. The thermal decomposition of vermiculite from Sokli, Finland, has been studied by various thermoanalytical methods, including high-temperature X-ray diffraction and high-pressure DTA. The major phases occurring during successive heating are: 15Å → 11.8Å → 10.1Å → 9.74Å → 9.3Å (all d₀₀₂ spacings). Reaction enthalpies and activation energies are given for most of the reactions. For the evaluation of kinetic parameters, high-temperature X-ray diffraction is suggested as a powerful tool, superior to most conventional methods.     

The thermal decomposition of n-hexane

The kinetics of the pyrolysis of n-hexane was studied in a conventional static reactor over a temperature range of 650–840 K. The overall reaction is essentially first order with the kinetic parameters A = 10^13.92 s^?1 and E_A = 260.3 kJ/mol. The distributions of the main products were analyzed by gas chromatography. A reaction model involving 240 elementary reactions was developed to describe the experimental rate data. The agreement of the model with experimental data was surprisingly good over a wide range of temperatures and pressures and up to medium extents of conversion. Methods for sensitivity studies based upon the quasi-stationary-state assumption (QSSA) were developed, and for a number of more detailed effects, such as self-inhibition, explanations could be given. It was also shown that the hexyl isomerization reactions influence strongly the product distribution. The outstanding capability of kinetic modeling with computer simulations in handling complex kinetic systems is demonstrated.