Thermal unimolecular decomposition of ethyl 2-furoate and its reactivity toward OH radicals: A theoretical study

Mechanism and kinetics of the thermal pyrolysis of ethyl 2-furoate were studied in a wide range of conditions (ie, 500-1500 K and 1-7600 Torr) using the accurate dual-level theory CCSD(T)&MP2(FC)//B3LYP/aug-cc-pVTZ and state-of-the-art Rice-Ramsperger-Kassel-Marcus based master equation rate model including hindered internal rotation and tunneling treatments. The predicted rate coefficients are found to be in accordance with the experimental data. The mechanism details are revealed as (i) the major decomposition channel occurs via a six-center transition state with the barrier energy of 48.3 kcal/mol at 0 K, leading to the formation of C₂H₄ and 2-furoic acid and (ii) pressure has a slightly positive effect on the rate constant, particularly at the high temperature. Also, the reactivity of ethyl 2-furoate toward OH radicals was discussed in the first place.     

Experimental and theoretical studies on the thermal decomposition of metformin

Journal of Thermal Analysis and Calorimetry - This work is a first attempt to understand the mechanism of metformin thermal decomposition under inert conditions. Thermal gravimetric analysis...     

Experimental and theoretical studies on the thermal decomposition of heterocyclic nitrosimines

A series of substituted 2-nitrosiminobenzothiazolines (2) were synthesized by the nitrosation of the corresponding 2-iminobenzothiazolines (6). Thermal decomposition of 2a--f and of the seleno analogue 7 in methanol and of 3-methyl-2-nitrosobenzothiazoline (2a) in acetonitrile, 1,4-dioxane, and cyclohexane followed first-order kinetics. The activation parameters for thermal deazetization of 2a were measured in cyclohexane (Delta H(++) = 25.3 +/- 0.5 kcal/mol, Delta S(++) = 1.3 +/- 1.5 eu) and in methanol (Delta H(++) = 22.5 +/- 0.7 kcal/mol, Delta S(++) = -12.9 +/- 2.1 eu). These results indicate a unimolecular decomposition and are consistent with a proposed stepwise mechanism involving cyclization of the nitrosimine followed by loss of N(2). The ground-state conformations of the parent nitrosiminothiazoline (9a) and transition states for rotation around the exocyclic C==N bond, electrocyclic ring closure, and loss of N(2) were calculated using ab initio molecular orbital theory at the MP2/6-31G* level. The calculated gas-phase barrier height for the loss of N(2) from 9a (25.2 kcal/mol, MP4(SDQ, FC)/6-31G*//MP2/6-31G* + ZPE) compares favorably with the experimental barrier for 2a of 25.3 kcal/mol in cyclohexane. The potential energy surface is unusual; the rotational transition state 9a-rot-ts connects directly to the orthogonal transition state for ring-closure 9aTS. The decoupling of rotational and pseudopericyclic bond-forming transition states is contrasted with the single pericyclic transition state (15TS) for the electrocyclic ring-opening of oxetene (15) to acrolein (16). For comparison, the calculated homolytic strength of the N--NO bond is 40.0 kcal/mol (MP4(SDQ, FC)/6-31G*//MP2/6-31G* + ZPE).     

Chemistry of diazocarbonyl compounds: XXV. Comparative photochemistry of diazo compounds and sulfur ylides of the 1,3-dioxane-4,6-dione series

Photochemical decomposition of 2,2-dialkyl-5-diazo-1,3-dioxane-4,6-diones in the presence of pyridine, methanol, or dimethyl sulfide as carbene traps involves mainly the Wolff rearrangement which is likely to follow a concerted pattern, while the yield of the “carbene” products does not exceed 27–28%. No carbene intermediates are formed in the photolysis of the corresponding dioxo sulfonium ylides under analogous conditions, and the main photochemical process is 1,2-methyl shift (Stevens rearrangement), followed by photochemical transformations of the primary products according to the Norrish type II pattern.     

Solid state reactivity of organic compounds with inorganic compounds. IV synthesis and thermal decomposition of uranium oxinate in the solid state

8-hydroxyquinoline (oxine) and uranyl acetate react in the solid state in 13 stoichiometry to give UO₂(C₉H₆NO)₂·C₉H₆NOH. This reaction is diffusion controlled with an activation energy of 44.4 kJ mol^–1. The reaction occurs by the surface migration of 8-hydroxyquinoline, which penetrates the product lattice to react with uranyl acetate. The isothermal decomposition of the solution phase product UO₂Q₂·HQ (Q=C₉H₆NO) obeys the Prout-Tompkins equation with an energy of activation of 53.3 kJ mol^–1.

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Zusammenfassung Die Festkörperreaktion von 8-Hydroxychinolin und Uranylazetat im Verhältnis 13 liefert UO₂(G₉H₆NO)₂·C₉H₆NOH. Die Reaktion ist diffusionsbestimmt und besitzt eine Aktivierungsenergie von 44.4 kJmol^–1. Die Reaktion verläuft durch die Oberflächenmigration von 8-Hydroxychinolin, welches zur Reaktion mit Uranylazetat in das Gitter des Produktes eindringt. Die thermische Zersetzung der Mischphase UO₂Q₂·HQ mitQ=C₉H₆NO unterliegt der Prout-Tompkins-Gleichung mit einer Aktivierungsenergie von 53,3 kJ·mol^–1.

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8- 13, UO₂(C₉H₆NO)₂·C₉H₆NOH. 44,4 ·^–1. 8-, . UO₂ Q ₂ · HQ (Q=C₉H₆NO) - 53,3 ·^–1.

     

A theoretical approach to the role of transfer reactions in the thermal decomposition of polymers

The relative importance of intermolecular and intramolecular transfers is investigated in the thermal degradation of polymers, in particular polystyrene.It is indicated that any mechanism for the pyrolysis behaviour of this polymer must include both modes of transfer. The model is offered as a diagnostic tool for mechanistic investigations of the thermal decompositions of polymers.     

Experimental and theoretical study on the kinetics and mechanism of thermal decomposition of 1,2-dichloroethane

The kinetics and mechanism for the thermal decomposition of 1,2-dichloroethane (EDC) was studied in a flow system over the temperature range of 849–1064 K and pressure range of 10–300 Torr under homogeneous conditions in a tubular quartz reactor. Gas chromatography was used to measure the concentration of products. Four-center HCl elimination was found to be the most important channel in this system. Minor products such as methane, ethylene, acetylene, chloroethane, and chloroprene were identified. Ab initio calculations at the DFT, CASMP2, and QCISD(T) levels of theory were carried out to investigate the mechanism of this system and to calculate necessary parameters to compute the rate constants of different steps. Dependence of formation of vinyl chloride on the total pressure is measured, experimentally.     

The thermal decomposition of hydrocarbons. Part 3. Polycyclic n-alkylaromatic compounds

The pyrolysis of the polycyclic n-alkylaromatic compounds, n-alkylpyrenes, methyl-anthracenes and n-alkylnaphthalenes is reviewed. The types and distributions of the decomposition products and also the mechanisms proposed for their formation are described. The theoretical calculations published on the feasibility of the unique elementary hydrogen transfer step, radical hydrogen transfer, are also discussed.     

Experimental and theoretical approaches of molecular geometry and mesophase behaviour relationship of laterally substituted azopyridines

New 10 laterally chloro azopyridine-based derivatives, with terminal alkoxy chain length varies between 6 and 16 carbons, were prepared. The mesophase behaviour was investigated for prepared homologues by differential scanning calorimetry and polarised light microscopy. Each homologous series differ from each other by the lateral attached Cl group in the ortho- or meta-position with respect to the ester carbonyl core in the central benzene ring. Elemental analyses, FT-IR, ¹H NMR and C¹³ NMR spectroscopy were used for molecular structure confirmation of the prepared compounds. All prepared homologues were found to be nematogenic. Density functional theory theoretical calculations used to confirm the experimental data and the results are consistent with the experimental investigations. It was found that the nematic range and stability is influenced by the length of the alkoxy chain and the orientation of the lateral chloro group. Moreover, an experimental and theoretical comparative study between the Cl-lateral isomers with respect to the previously reported methyl lateral derivatives as well as the laterally neat one in determination of the type, the stability of the mesophase and the molecular geometry.     

On the possibilities of theoretical analysis of kinetics of the thermal decomposition of solids

The main reasons for changes in the environment surrounding us are discussed on the basis of thermodynamics of irreversible processes. Subsequently, relations between thermodynamics of irreversible processes and chemical kinetics are shown, then the possibilities of theoretical determination of rate constants on the framework of the modified RRKM theory are presented. These latter considerations are supplemented by a discussion concerning the possibilities of determining the activation barriers and structural changes (necessary to account for entropy changes upon reaction) in molecules kept on the surface of crystalline phases by combination of quantum chemistry methods for isolated molecules with those reflecting the influence of the environment (i.e. interaction within the lattice). Finally, the future of theoretical methods in examining the reactivity of solid state systems is briefly discussed.     

Fragmentation of organosulfur compounds upon electron impact. Part III. Metastable decomposition of the molecular ions of methyl thioglycolate and ethyl thioglycolate

The metastable decompositions of the molecular ions of methyl thioglycolate (1) and ethyl thioglycolate (2) were investigated by means of mass analyzed ion kinetic energy (MIKE) spectra and deuterium labeling. The loss of methanol is the only metastable decomposition of 1^+·. This fragmentation occurs via two distinct pathways. The molecular ions of 2 decompose in a variety of ways, i.e., the losses of water, ethene, ethanol or ?₂H₃O₂. All of these decompositions, except the loss of ethene, occur through two distinct mechanisms. During the loss of ?₂H₃O₂, the ethyl group or ethene migrates from the oxygen to the sulfur atom. The loss of H?S, which corresponds to the loss of H?O with a concomitant double hydrogen transfer observed in the case of methyl glycolate (3), does not participate in the metastable decomposition of 1^+· and 2^+·. This is due to the energetic favorableness of the loss of methanol.     

Experimental and theoretical investigation of reversible interconversion, thermal reactions, and wavelength-dependent photochemistry of diazo Meldrum's acid and its diazirine isomer, 6,6-dimethyl-5,7-dioxa-1,2-diaza-spiro[2,5]oct-1-ene-4,8-dione

The photochemical or thermal decomposition of diazo Meldrum's acid (1) in methanolic solutions yields ketoester 3a, the product of the Wolff rearrangement, while products produced from the singlet carbene were not detected. This observation, combined with the analysis of activation parameters for the thermal decomposition of 1, as well as with the results of DFT B3PW91/6-311+G(3df,2p) and MP2/aug-cc-pVTZ//B3PW91/6-311+G(3df,2p) calculations, allows us to conclude that the Wolff rearrangement of 1 is a concerted process. The outcome of the photolysis of diazo Meldrum's acid depends on the wavelength of irradiation. Irradiation with 254 nm light results in an efficient (Phi(254) = 0.34) photo-Wolff reaction, while at 355 nm, the formation of diazirine 2 becomes the predominant process (Phi(350) = 0.024). This unusual wavelength selectivity indicates that Wolff rearrangement and isomerization originate from different electronically excited states of 1. The UV irradiation of diazirine 2 leads to the loss of nitrogen and the Wolff rearrangement, apparently via a carbene intermediate. This process is accompanied by a reverse isomerization to diazo Meldrum's acid. Triplet-sensitized photolysis of both isomers results in the formation of Meldrum's acid, the product of a formal reduction of 1 and 2. Mild heating of diazirine 2 produces quantitative yields of diazo Meldrum's acid. The activation parameters for thermal reactions of diazo 1 and diazirino 2 isomers were determined in aqueous and dioxane solutions.     

The influence of activated carbon on the thermal decomposition of sodium ethyl xanthate

The thermal decomposition of SEX in a nitrogen atmosphere was studied by coupled thermogravimetry-Fourier transform infrared spectroscopy (TG-FTIR), and by pyrolysis-gas chromatography-mass spectrometry (py-GC-MS). The TG curve exhibited two discrete mass losses of 45.8% and 17.8% respectively, at 200 and 257–364°C. The evolved gases identified as a result of the first mass loss were carbonyl sulfide (COS), ethanol (C₂H₅OH), ethanethiol (C₂H₅SH), carbon disulfide (CS₂), diethyl sulfide ((C₂H₅)₂S), diethyl carbonate ((C₂H₅O)₂CO), diethyl disulfide ((C₂H₅)₂S₂), and carbonothioic acid, O, S, diethyl ester ((C₂H₅S)(C₂H₅O)CO). The gases identified as a result of the second mass loss were carbonyl sulfide, ethanethiol, and carbon disulfide. Hydrogen sulfide was detected in both mass losses by py-GC-MS, but not detected by FTIR. The solid residue was sodium hydrogen sulfide (NaSH).SEX was adsorbed onto activated carbon, and heated in nitrogen. Two discrete mass losses were still observed, but in the temperature ranges 100–186°C (7.8%) and 186–279°C (11.8%). Carbonyl sulfide and carbon disulfide were now the dominant gases evolved in each of the mass losses, and the other gaseous products were relatively minor. It was demonstrated that water adsorbed on the carbon hydrolysed the xanthate to cause the first mass loss, and any unhydrolysed material decomposed to give the second mass loss.Mr. N. G. Fisher would like to thank the A. J. Parker CRC for Hydrometallurgy for the provision of a PhD scholarship.     

Experimental results and theoretical considerations on thermogravimetric decomposition reactions of chemical compounds under high vacuum

Investigations on thermogravimetric decomposition reactions under high vacuum have demonstrated that special attention should be paid to such factors as the sample size, the crucible form, the thickness of the layer in the crucible, the particle size of the probe material, etc. It is shown how the various influences can be eliminated by observing certain starting or test conditions. In addition to experiments on these specific influences, a comparison has been made of the degradation speeds of single crystals and crystal powders of chemical compounds.Results, which are complementary to the thermogravimetric measurements can be obtained with qualitative or quantitative determination of the gaseous decomposition products by mass spectrometric analysis. From these measurements the course of decomposition is followed by recording the partial pressure at the various individual masses. This method is particularly suitable for the study of thermal degradations which produce more than one decomposition product in the same step.     

Stability of diazocarbonyl compounds under the conditions of gas chromatography and chromatography-mass spectrometry analysis

The gas chromatographic analysis of alkyldiazoacetates N₂CHCO₂R (R = CH₃ ? C₄H₉), ??-aliphatic diazoketones RCOCHN₂ (R = C₃H₇, C₅H₁₁, and C₉H₁₉), and aryl-substituted diazoketones Ph (CH₂) _n COCHN₂ (n = 0?C2) is shown to be possible when their retention temperatures are below the boiling points of compounds of this series at atmospheric pressure without decomposition (about 140°C). At higher temperatures occurs partial or complete decomposition of ??-diazoketones in chromatographic columns to form ketenes. Among the impurities in the reaction mixtures at the diazotization of corresponding alkyl glycinates were identified for the first time the nitrate esters of glycolic acid O₂NOCH₂CO₂R, as well as the dimeric products. All diazocarbonyl compounds and the impurities were characterized by mass spectra. For the first time their gas chromatographic retention indices were determined.