Kinetics and mechanism of the reactions of C6H12, C5H10 and C6D12 cycloalkanes with peroxynitrous acid in aqueous solution and the gas phase

The c-C₆H₁₂/c-C₆D₁₂ kinetic isotope effect (KIE), the k₆/k₅ rate constant ratio (c-C₆H₁₂/c-C₅H₁₀), and the temperature dependence of these ratios in the gas-phase reactions of cycloalkanes with peroxynitrous acid and OH radicals are identical. The same result was obtained for the reactions in aqueous solution. These data are in accord with the conclusion that OH^· radicals formed in the homolysis of the HO-ONO bond are the active species in the reactions of HOONO with hydrocarbons in aqueous solution and in the gas phase. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 44, No. 2, pp. 105–110, March–April, 2008.     

Substrate selectivity and mechanism of oxidation of alkanes by peroxynitrous acid in aqueous solution

The relative rate constants (k_rel) were measured for the oxidation of alkanes (RH) by peroxynitrous acid (HOONO) in aqueous solutions at 25 °C. The krel values for the reactions of RH with HOONO and with OH radicals in the series of C₂-C₇ alkanes and the isotopic effects (c-C₆H₁₂/c-C₆D₁₂) agree. However, the k_rel value for methane was lower than for its reaction with OH. Possible reaction mechanisms are discussed. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 5, pp. 295–299, September–October, 2006.     

Kinetics and mechanism for the oxidation of alkanes by peroxynitrous acid in the gas phase: the slow step involving reaction with oh radicals

The relative rate constants (kRH/kEtH), the temperature dependence of these constants at from 5 to 55 °C, and the activation parameters were found for reactions of propane, butane, pentane, hexane, isobutane, cyclopentane, and cyclohexane with peroxynitrous acid (HOONO) in water. The similarity of these results to the data for the reaction of alkanes with OH radicals confirms that the active species in the reactions of HOONO with hydrocarbons in water are OH radicals formed in the homolysis of the HO—ONO bond.     

Kinetics, selectivity, and mechanism of the reactions of arenes with adamantyl carbocations in sulfuric acid

Data on the kinetics, selectivity, kinetic isotope effect, and the effect of the acidity of the medium on the rate of the reactions of benzene and alkylbenzenes in sulfuric acid (59–78 wt.% H₂SO₄) solutions of 1-adamantanol at 30 °C indicate that the direct reagents are the adamantyl carbocations (Ad⁺) that alkylate the arenes. The ortho positions of the benzene ring are not accessible on account of steric hindrances. The rate of attack by the Ad⁺ cation on the accessible para and meta positions of the ring is controlled by the formation of a σ complex. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 1, pp. 14–18, January–February, 2006.     

Kinetics,mechanism, and endo selectivity of Diels–Alder reactions of alkylmonosubstituted ethenes with cyclohexa-1,3-diene in the gas phase

The reactions where Y = CH₃ (M), C₂H₅ (E), i? C₃H₇ (I), and t? C₄H₉ (T) have been studied between 488 and 606 K. The pressures of CHD ranged from 16 to 124 torr and those of YE from 57 to 625 torr. These reactions are homogeneous and first order with respect to each reagent. The rate constants (in L/mol·s) are given by The Arrhenius parameters are used as a test for a biradical mechanism and to discuss the endo selectivity of the reactions.     

Kinetics and mechanisms of the reactions of hypochlorous acid,chlorine, and chlorine monoxide with bromite ion

The reaction between BrO2(-) and excess HOCl (p[H+] 6-7, 25.0 degrees C) proceeds through several pathways. The primary path is a multistep oxidation of HOCl by BrO(2)(-) to form ClO(3)(-) and HOBr (85% of the initial 0.15 mM BrO(2)(-)). Another pathway produces ClO(2) and HOBr (8%), and a third pathway produces BrO(3)(-) and Cl(-) (7%). With excess HOCl concentrations, Cl(2)O also is a reactive species. In the proposed mechanism, HOCl and Cl(2)O react with BrO(2)(-) to form steady-state species, HOClOBrO(-) and ClOClOBrO(-). Acid facilitates the conversion of HOClOBrO(-) and ClOClOBrO(-) to HOBrOClO(-). These reactions require a chainlike connectivity of the intermediates with alternating halogen-oxygen bonding (i.e. HOBrOClO(-)) as opposed to Y-shaped intermediates with a direct halogen-halogen bond (i.e. HOBrCl(O)O(-)). The HOBrOClO(-) species dissociates into HOBr and ClO(2)(-) or reacts with general acids to form BrOClO. The distribution of products suggests that BrOClO exists as a BrOClO.HOCl adduct in the presence of excess HOCl. The primary products, ClO(3)(-) and HOBr, are formed from the hydrolysis of BrOClO.HOCl. A minor hydrolysis path for BrOClO.HOCl gives BrO(3)(-) and Cl(-). An induction period in the formation of ClO(2) is observed due to the buildup of ClO(2)(-), which reacts with BrOClO.HOCl to give 2 ClO(2) and Br(-). Second-order rate constants for the reactions of HOCl and Cl(2)O with BrO(2)(-) are k(1)(HOCl) = 1.6 x 10(2) M(-1) s(-1) and k(1)(Cl)()2(O) = 1.8 x 10(5) M(-)(1) s(-)(1). When Cl(-) is added in large excess, a Cl(2) pathway exists in competition with the HOCl and Cl(2)O pathways for the loss of BrO(2)(-). The proposed Cl(2) pathway proceeds by Cl(+) transfer to form a steady-state ClOBrO species with a rate constant of k(1)(Cl2) = 8.7 x 10(5) M(-1) s(-1).     

Kinetics of some reactions of vinyl and isopropyl radicals in the gas phase

Vinyl and isopropyl radicals were generated by the pyrolysis of azoisopropane in the presence of acrolein at 473–563 K. Reaction products were analyzed by gas chromatography. Rate constant ratios k₂/k₁ = 0.02 ± 0.01 and k₄/k₃ = 0.01 ± 0.005 are suggested for the following reactions: The rate constant ratio of reactions (7) and (c) obeys the Arrhenius equation The Arrhenius equation was derived for (k₈ + k₉).     

Steric retardation of SN2 reactions in the gas phase and solution

The gas-phase S(N)2 reactions of chloride with ethyl and neopentyl chlorides and their alpha-cyano derivatives have been explored with B3LYP, CBS-QB3, and PDDG/PM3 calculations. Calculations predict that the steric effect of the tert-butyl group raises the activation energy by about 6 kcal/mol relative to methyl in both cases. Solvent effects have been computed with QM/MM Monte Carlo simulations for DMSO, methanol, and water, as well as with a polarizable continuum model, CPCM. Solvents cause a large increase in the activation energies of these reactions but have a very small differential effect on the ethyl and neopentyl substrates and their cyano derivatives. The theoretical results contrast with previous conclusions that were based upon gas-phase rate measurements.     

Kinetics,mechanism, and stereochemistry of Diels-Alder reactions of carbonyl-substituted ethenes with cyclohexa-1,3-diene in the gas phase

The kinetics of the Diels–Alder additions of CH₂ ? CHCHO, CH₂? C(CH₃)CHO, and CH₂? CHC(CH₃)O to cyclohexa-1,3-diene (CHD) have been studied in the gas phase. The stereochemistry and the mechanism of these reactions are discussed. In contrast with other Diels–Alder additions involving CHD as diene, a biradical mechanism does not fit the experimental results.     

Kinetics,mechanism, and stereochemistry of Diels-Alder reactions of CN-substituted ethenes with cyclohexa-1, 3-diene in the gas phase

The kinetics of the Diels-Alder additions of CH₂ = CHCN, CH₂ = C(CH₃) CN, and cis- and trans-CH₃CH = CHCN to cyclohexa-1, 3-diene have been studied in the gas phase. The stereochemistry of these reactions is discussed. In terms of a biradical mechanism, a minimum value of 4.1 ± 0.8 kcal mol^?1 for the stabilizing effect of a CN group vis-à-vis a methyl group is shown to fit the experimental activation energies.     

Atomization mechanisms and gas phase reactions in graphite furnace atomic absorption spectrometry

Summary The amounts of carbon monoxide as well as the total amounts of hydrocarbons generated in different types of graphite tubes were investigated under various experimental conditions. Depending on whether or not a matrix like 50 g of sodium nitrate was added the amount of carbon monoxide formed during atomization at 1,700 K in a pyrocoated tube was in the range 60 to 600 nmoles when using a thermal pretreatment temperature of 1,200 K. The corresponding values for an uncoated tube were 250 to 1,300 nmoles. The effect of carbon monoxide on the atomization behaviour of silver, bismuth, chromium, copper and lead was investigated experimentally and the results were evaluated by means of thermodynamically based models. In accordance with theoretical predications, only lead, bismuth and chromium, which are assumed to be atomized by oxide decomposition, showed substantial shifts in their appearance temperatures in different gas mixtures, and changes in activation energies.

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Atomisierungsmechanismen und Gasphasenreaktionen in der Graphitofen-AAS

Zusammenfassung Die Mengen an CO sowie die Gesamtmengen an Kohlenwasserstoffen, die in verschiedenen Graphitrohrtypen erzeugt werden, wurden unter unterschiedlichen Versuchsbedingungen untersucht. Abhängig von der Zugabe einer Matrix von 50 g Natriumnitrat ergab sich eine während der Atomisierung bei 1 700 K im mit Pyrolysegraphit überzogenen Rohr gebildete CO-Menge im Bereich von 60 bis 600 nMol, wenn die thermische Vorbehandlung bei 1 200 K vorgenommen wurde. Die entsprechenden Werte für ein nicht überzogenes Rohr lagen bei 250 bis 1 300 nMol. Die Wirkung von CO auf das Atomisierungsverhalten von Ag, Bi, Cr, Cu und Pb wurde untersucht und die Ergebnisse wurden mit Hilfe thermodynamischer Modelle ausgewertet. In Übereinstimmung mit theoretischen Annahmen zeigten nur Pb, Bi und Cr, von denen eine Atomisierung durch Oxidzersetzung angenommen wird, wesentliche Verschiebungen der Erscheinungstemperaturen in verschiedenen Gasgemischen sowie Veränderungen der Aktivierungsenergien.

     

Heavy water reactions with alkaline-earth metal dications in the gas phase: Kinetics at room temperature

Room temperature rate coefficients and product distributions are reported for the reactions initiated in D₂O with dications of the alkaline-earth metals Mg, Ca, Sr and Ba. The measurements were performed with a selected-ion flow tube (SIFT) tandem mass spectrometer and electrospray ionization (ESI). Mg²⁺ reacts with water by a fast electron transfer leading to charge separation with a rate coefficient of 1.4 × 10⁻⁹ cm³ molecule⁻¹ s⁻¹. Ca²⁺ reacts with D₂O in a first step to form the adduct Ca²⁺(D₂O), with an effective bimolecular rate coefficient of 2.3 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, which then undergoes rapid charge separation by deuteron transfer to form CaOD⁺ and D₃O⁺ in a second step with k = 7.9 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹. The CaOD⁺ ion reacts further by clustering up to five more D₂O molecules. Sr²⁺ clusters up to eight D₂O molecules and Ba²⁺ up to seven D₂O molecules, with the first addition of D₂O being rate determining in each case and the last addition being distinctly slower, as might be expected from a transition in the occupation of the added water molecules from an inner to an outer hydration shell.     

Profiling mechanisms of alkane hydroxylase activity in vivo using the diagnostic substrate norcarane

Mechanistically informative chemical probes are used to characterize the activity of functional alkane hydroxylases in whole cells. Norcarane is a substrate used to reveal the lifetime of radical intermediates formed during alkane oxidation. Results from oxidations of this probe with organisms that contain the two most prevalent medium-chain-length alkane-oxidizing metalloenzymes, alkane omega-monooxygenase (AlkB) and cytochrome P450 (CYP), are reported. The results--radical lifetimes of 1-7 ns for AlkB and less than 100 ps for CYP--indicate that these two classes of enzymes are mechanistically distinguishable and that whole-cell mechanistic assays can identify the active hydroxylase. The oxidation of norcarane by several recently isolated strains (Hydrocarboniphaga effusa AP103, rJ4, and rJ5, whose alkane-oxidizing enzymes have not yet been identified) is also reported. Radical lifetimes of 1-3 ns are observed, consistent with these organisms containing an AlkB-like enzyme and inconsistent with their employing a CYP-like enzyme for growth on hydrocarbons.     

Mechanisms of nucleophilic reactions of carbonyl compounds in the gas phase and in a solution

The effect of substituents on nucleophilic addition at the C=O bond, which occurs by the mechanism of intramolecular proton transfer, has been studied by the quantum-chemical MNDO/H method. The effect of nucleophiles and substituents at the carbonyl C atom in the gas phase is opposite to that in solution. Strengthening of the bond between the nucleophile and the carbonyl compound as the result of the transfer of electron density to the carbonyl C atom results in the stabilization of the tetrahedral bipolar adduct. In the formation of an adduct with a strong nucleophile the geometry of the transition state (TS) is closer to that of the reaction product, whereas in the case of a weak nucleophile it is similar to that of the initial reagents. Attack by a weak nucleophile and electron-donating groups at the carbonyl C atom favor the situation in which the reaction system achieves a TS earlier and proton transfer occurs with a low activation barrier.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 227–230, February, 1994.     

Kinetics and mechanisms of the reactions of chlorine atoms with ethane,propane, and n-butane

Absolute (flash photolysis) and relative (FTIR-smog chamber and GC) rate techniques were used to study the gas-phase reactions of Cl atoms with C₂H₆ (k₁), C₃H₈ (k₃), and n-C₄H₁₀ (k₂). At 297 ± 1 K the results from the two relative rate techniques can be combined to give k₂/k₁ = (3.76 ± 0.20) and k₃/k₁ = (2.42 ± 0.10). Experiments performed at 298–540 K give k₂/k₁ = (2.0 ± 0.1)exp((183 ± 20)/T). At 296 K the reaction of Cl atoms with C₃H₈ produces yields of 43 ± 3% 1-propyl and 57 ± 3% 2-propyl radicals, while the reaction of Cl atoms with n-C₄H₁₀ produces 29 ± 2% 1-butyl and 71 ± 2% 2-butyl radicals. At 298 K and 10–700 torr of N₂ diluent, 1- and 2-butyl radicals were found to react with Cl₂ with rate coefficients which are 3.1 ± 0.2 and 2.8 ± 0.1 times greater than the corresponding reactions with O₂. A flash-photolysis technique was used to measure k₁ = (5.75 ± 0.45) × 10⁻¹¹ and k₂ = (2.15 ± 0.15) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ at 298 K, giving a rate coefficient ratio k₂/k₁ = 3.74 ± 0.40, in excellent agreement with the relative rate studies. The present results are used to put other, relative rate measurements of the reactions of chlorine atoms with alkanes on an absolute basis. It is found that the rate of hydrogen abstraction from a methyl group is not influenced by neighboring groups. The results are used to refine empirical approaches to predicting the reactivity of Cl atoms towards hydrocarbons. Finally, relative rate methods were used to measure rate coefficients at 298 K for the reaction of Cl atoms with 1- and 2-chloropropane and 1- and 2-chlorobutane of (4.8 ± 0.3) × 10⁻¹¹, (2.0 ± 0.1) × 10⁻¹⁰, (1.1 ± 0.2) × 10⁻¹⁰, and (7.0 ± 0.8) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, respectively. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 43–55, 1997.     

Theoretical study of the kojic acid structure in gas phase and aqueous solution

Density functional calculations at the B3LYP/6‐311++G** level have been performed to determine the ground‐state conformational preference for kojic acid, a widely used skin‐whitening, antibrowning, and antibacterial agent. It is found that the gas phase consists almost entirely of the 5‐hydroxy‐2‐(hydroxymethyl)‐4H‐pyran‐4‐one tautomer, although several rotamers of this are prevalent. In aqueous solution, however, other tautomers are also present. The validity of the calculations is confirmed by the observed FTIR, NMR, and UV–vis spectra, which show good correspondence with the theoretical spectra. The electronic interactions are interpreted in terms of charge and bond order analysis as well as the composition of the HOMO and LUMO. The calculations show that kojic acid has partial aromatic character and is a good nucleophile. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011