Chain ion molecular mechanism of ascorbic acid oxidation with hydrogen peroxide. Cu3+ ion as a chain carrier

The kinetics and mechanism of ascorbic acid (DH₂) oxidation have been studied under anaerobic conditions in the presence of Cu²⁺ ions. At 10^?4 ≤ [Cu²⁺]₀ < 10^?3M, 10^?3 ≤ [DH₂]₀ < 10^?2M, 10^?2 ≤ [H₂O₂] ≤ 0.1M, 3 ≤ pH < 4, the following expression for the initial rate of ascorbic acid oxidation was obtained: where χ₂ (25°C) = (6.5 ± 0.6) × 10^?3 sec^?1. The effective activation energy is E₂ = 25 ± 1 kcal/mol. The chain mechanism of the reaction was established by addition of Cu⁺ acceptors (allyl alcohol and acetonitrile). The rate of the catalytic reaction is related to the rate of Cu⁺ initiation in the Cu²⁺ reaction with ascorbic acid by the expression where C is a function of pH and of H₂O₂ concentration. The rate equation where k₁(25°C) = (5.3 ± 1) × 10³M^?1 sec^?1 is true for the steady-state catalytic reaction. The Cu⁺ ion and a species, which undergoes acid–base and unimolecular conversions at the chain propagation step, are involved in quadratic chain termination. Ethanol and terbutanol do not affect the rate of the chain reaction at concentrations up to ≈0.3M. When the Cu²⁺–DH₂–H₂O₂ system is irradiated with UV light (λ = 313 nm), the rate of ascorbic acid oxidation increases by the value of the rate of the photochemical reaction in the absence of the catalyst. Hydroxyl radicals are not formed during the interaction of Cu⁺ with H₂O₂, and the chain mechanism of catalytic oxidation of ascorbic acid is quantitatively described by the following scheme. Initiation: Propagation: Termination:      

Kinetics of the gas-phase reaction of allyl alcohol with iodine. The heat of formation and stabilization energy of the hydroxyallyl radical

The reaction of iodine with allyl alcohol has been studied in a static system, following the absorption of visible light by iodine, in the temperature range 150-190°C and in the pressure range 10-200 torr. The rate-determining step has been found to be and k₃ is consistent with the equation From the activation energy and the assumption E_-3 = 1 ± 1 kcal mol^?1, it has been calculated that kcal mol^?1. The stabilization energy of the hydroxyallyl radical has been found to be 11.4 ± 2.2 kcal mol^?1.     

Reaction of methoxy radicals with oxygen. I. Using dimethyl peroxide as a thermal source of methoxy radicals

Using dimethyl peroxide as a thermal source of methoxy radicals overthe temperature range of 110–160°C, and the combination of methoxy radicals and nitrogen dioxide as a reference reaction: a value was determined of the rate constant for the reaction of methoxy radicals with oxygen: is independent of nitrogen dioxide or oxygen concentration and added inert gas (carbon tetrafluoride). No heterogeneous effects were detected. The value of k₄ is given by the expression In terms of atmospheric chemistry, this corresponds to a value of 10^5.6 M^?1·sec^?1 at 298 K. Extrapolation to temperatures where the combustion of organic compounds has been studied (813 K) produces a value of 10^7.7 M^?1·sec^?1 for k₄. Under these conditions, reaction (4) competes with hydrogen abstraction or disproportionation reactions of the methoxy radical and its decomposition (3): In particular k₃ is in the falloff region under these conditions. It is concluded that reaction (4) takes place as the result of a bimolecular collision process rather than via the formation of a cyclic complex.     

Kinetic studies of free radical reactions by mass spectrometry. II. The reactions of SO + ClO,SO + OClO and SO + BrO

The rates of several novel elementary reactions involving ClO, BrO and SO free radicals in their ground states were studied in a discharge-flow system at 295 K, using mass spectrometry. The rate constant k₂ was determined from the decay of SO radicals in the presence of excess ClO radicals: The SO + OClO overall reaction has a complex mechanism, with the primary step having a rate constant k₅ equal to (1.9 ± 0.7) × 10^?12 cm³ sec^?1: A lower limit for the rate constant of the rapid reaction of SO radicals with BrO radicals was determined:      

Molecular-modulation spectrometry: CH3ONO photolysis and detection of nitroxyl

The method of molecular-modulation spectrometry for studying photochemical reactions has been applied to methyl nitrite photolysis. The infrared absorption of the nitroxyl radical HNO has been observed in the gas phase at 3300 cm^?1. Under the present experimental conditions the steady-state concentration of HNO under steady illumination was 1.1 × 10¹² particles/cc, and the observed modulation amplitude was 4.5 × 10¹⁰ particles/cc. At 25°C and 1 atm of nitrogen, the cross section for infrared absorption by HNO at 3300 cm^?1 is 1.7 × 10^?19 cm². The rate constant ratio b/c was found to be 8.0. From the literature value of the rate constant d , the observed rate constant for the reaction is e = (5 ± 1) × 10^?11 cc/particle sec.     

The addition of methyl radicals to hexafluoroacetone

The reaction of methyl radicals (Me) with hexafluoroacetone (HFA), generated from ditertiary butyl peroxide (dtBP), was studied over the temperature range of 402–433 K and the pressure range of 38–111 torr. The reaction resulted in the following displacement process taking place: where TFA refers to trifluoroacetone. The trifluoromethyl radicals that were generated abstract a hydrogen atom from the peroxide: such that k_6a is given by: where θ = 2.303RT kcal/mol. The interaction of methyl and trifluoromethyl radicals results in the following steps: Product analysis shows that k₁₇/kk = 2.0 ± 0.2 such that k₁₇ = 10^10.4±0.5M^?1 · s^?1. The rate constant k₅ is given by: It is concluded that the preexponential factor for the addition of methyl radicals to ketones is lower than that for the addition of methyl radicals to olefins.     

H2S-promoted thermal isomerization of cis-2-pentene to 1-pentene and trans-2-pentene around 800 K

H₂S accelerates the thermal isomerization of cis-2-pentene (P2c) to 1-pentene (P1) and trans-2-pentene (P2t) to around 800 K. This effect is interpreted on the basis of a free radical mechanism in which 2-pentenyl and thiyl radicals are the main chain carriers. P1 formation is essentially explained by the competing processes: P2t formation is due to addition-elimination processes: the importance of which has been evaluated against process (?4μ): The following ratios of rate constants have been measured and are discussed: (RT in cal mol^?1).     

A shock tube study of reactions of atomic oxygen with isocyanic acid

Reactions of atomic oxygen with isocyanic acid (HNCO) have been studied in incident and reflected shock wave experiments using HNCO/N₂O/Ar mixtures. Quantitative time-histories of the NH(X³Σ^?) and OH(X²Π_i) radicals were measured behind the shock waves using cw, narrow-linewidth laser absorption at 336 nm and 307 nm, respectively. The second-order rate coefficients of the reactions: and were determined from early-time NH and OH formation rates, with least-squares two-parameter fits of the results given by: and cm³ mol^?1 s^?1. The minimum and maximum rate constant factors (?,F) define the lower and upper uncertainty limits, respectively. An upper limit on the rate coefficient of was determined to be: .     

The self-inhibited pyrolysis of isobutane

The pyrolysis of isobutane was investigated in the ranges of 770° to 855°K and 20 to 150 Torr at up to 4% decomposition. The reaction is homogeneous and strongly self-inhibited. A simple Rice-Herzfeld chain terminated by the recombination of methyl radicals is proposed for the initial, uninhibited reaction. Self-inhibition is due to abstraction of hydrogen atoms from product isobutene giving resonance-stabilized 2-methylallyl radicals which participate in termination reactions. The reaction chains are shown to be long. It is suggested that a previously published rate constant for the initiation reaction (1) is incorrect and the value k₁ = 10^16.8 exp (?81700 cal mol^?1/RT)s^?1 is recommended. The values of the rate constants for the reactions (4i) (4t) (8) are estimated to be and From a recalculation of previously published data on the pyrolysis of isobutane at lower temperatures and higher pressures, the value k_11c, = 10^9.6 cm³ mol^?1 s^?1 is obtained for the rate constant of recombination of t-butyl. A calculation which is independent of any assumed rate constants or thermochemistry shows that the predominant chain termination reaction is the recombination of two methyl radicals in the conditions of the present work and the recombination of two t-butyl radicals in those of our previous study at lower temperatures and higher pressures.     

The determination of the arrhenius parameters for the reaction Cl + C2F5I → ICl + C2F5 using a competitive method

The reactions have been studied competitively over the range of 28–182°C by photolysis of mixtures of Cl₂ + C₂F₅I+ CH₄. We obtain where θ = 2.303RT J/mol. The use of published data on reaction (2) leads to log (k₁cm³/mol sec) = (13.96 ± 0.2) ? (11,500 ± 2000)/θ.     

The reactions of alkoxy radicals with O2. II. n-C3H7O radicals

n-C₃H₇ONO was photolyzed with 366 nm radiation at ?26, ?3, 23, 55, 88, and 120°C in a static system in the presence of NO, O₂, and N₂. The quantum yields of C₂H₅CHO, C₂H₅ONO, and CH₃CHO were measured as a function of reaction conditions. The primary photochemical act is and it proceeds with a quantum yield ?₁ = 0.38 ± 0.04 independent of temperature. The n-C₃H₇O radicals can react with NO by two routes The n-C₃H₇O radical can decompose via or react with O₂ via Values of k₄/k₂ ? k_4b/k₂ were determined to be (2.0 ± 0.2) × 10¹⁴, (3.1 ± 0.6) × 10¹⁴, and (1.4 ± 0.1) × 10¹⁵ molec/cm³ at 55, 88, and 120°C, respectively, at 150-torr total pressure of N₂. Values of k₆/k₂ were determined from ?26 to 88°C. They fit the Arrhenius expression: For k₂ ? 4.4 × 10^?11 cm³/s, k₆ becomes (2.9 ± 1.7) × 10^?13 exp{?(879 ± 117)/T} cm³/s. The reaction scheme also provides k_4b/k₆ = 1.58 × 10¹⁸ molec/cm³ at 120°C and k_8a/k₈ = 0.56 ± 0.24 independent of temperature, where      

The kinetics of the reactions of C2F5 radicals with Br2 and HBr

A mixture of Br₂ + HBr + C₂F₅I was photolyzed in the vapor phase. The reaction forms C₂F₅ radicals which are removed by Competitive studies over the range of 74–146°C gave ratios of k₁₀/k₉, and these were combined with values obtained previously by different methods at higher temperatures upto 515°C to give where θ = 2.303RT J/mol. A value is assigned to the activation energy E₁₀, and this, with other data, leads to at 25°C. This result is in excellent agreement with two previous independent determinations.     

Pyrolysis of 2,4-dimethylhexene-l and the stability of isobutenyl radicals

2,4-Dimethylhexene-l has been decomposed in single-pulse shock tube experiments. Rate expressions for the initial reactions are and sec^?1 at 1.5–5 atm and 1050°K. This leads to ΔH^°_f300 (CH₂ = C(CH₃)CH₂) = 124 kJ/mol, or an allylic resonance energy of 50 kJ/mol. Rate expressions for the decomposition of the appropriate olefins which yield isobutenyl radicals and methyl, ethyl, isopropyl, n-propyl, t-butyl, and t-amyl radicals, respectively, are presented. The rate expression for the decomposition of isobutenyl radical is (at the beginning of the fall-off region). For the combination of isobutenyl and methyl radicals, the rate constant at 1020°K is Combination of this number and the calculated rate expression for 2-methylbutene-1 decomposition gives S. (1100) = 470 J/mol °K. This yields It is demonstrated that an upper limit for the rate of hydrogen abstraction by isobutenyl from toluene is      

A quantitative study of alkyl radical reactions by kinetic spectroscopy. II. Combination of the methyl radical with the oxygen molecule

The kinetics of the reaction have been studied, using the technique of flash photolysis and kinetic spectroscopy to follow the methyl radical concentration. The order of the reaction lies between 2 and 3 throughout the range of pressure from 25 to 380 torr at 22°C, and the results are consistent with a single reaction sequence: The limiting values of the third-order rate coefficients at low pressures are (3.6±0.3) × 10¹¹ 1.² mole^?2 sec^?1 when M is neopentane, and (0.94 ± 0.03) × 10¹¹ 1.² mole^?2 sec^?1 when M is nitrogen. The limiting value of the second-order rate coefficient at high pressures is (3.1 ± 0.3) × 10⁸ 1. mole^?1 sec^?1. The rate constant for the independent second-order reaction is shown to be not much greater than 2 × 10⁵ 1. mole^?1 sec^?1, so that this reaction does not complete significantly with the combination reaction. This new interpretation is contrary to currently accepted views.     

Some reactions of the isopropyl radical

The decomposition of ethane sensitized by isopropyl radicals was studied in the temperature range of 496–548°K. The rate of formation of n-butane, isopentane, and 2,3-dimethylbutane were measured. The expression k₁/k₂^½ was found to be where k₁ and k₂ are rate constants of The decomposition of propylene sensitized by isopropyl radicals was studied between 494 and 580°K by determination of the initial rates of formation of the main products. The ratio of k₁₃/k₂^1/2 was evaluated to be where k₁₃ is the rate constant for The isomerization of the isopropyl radical was investigated by studying the decomposition of azoisopropane. The decomposition of the iso-C₃H₇ radical into C₂H₄ and CH₃ was followed by measuring the rate of formation of C₂H₄. On the basis of the experimental data, obtained in the range of 538–666° K, k₁₅/k₂^½ was found: where k₁₅ is the rate constant of      

High temperature reaction rate coefficients derived from N-atom ARAS measurements and excimer photolysis of NO

Mixtures of NO and NO/H₂ in Ar were shock-heated and photolyzed with an ArF excimer laser. Measurements in these experiments of N-atom profiles using atomic resonance absorption spectrophotometry (ARAS) permitted the determination of two rate coefficients. The rate coefficient for the reaction was found to be 4.29 × 10¹³ exp(?787/T) cm³ mol^?1 sec^?1 (±20% at 1400 K to ±10% at 3500 K). This is the first direct high temperature measurement of this rate coefficient in the exothermic direction. The rate coefficient for the reaction was found to be 1.60 × 10¹⁴ exp(?12650/T) (±35% from 1950 to 2850 K). To our knowledge, this is the first direct measurement of this rate coefficient. A study of the N-atom ARAS absorption behavior revealed a noticeable pressure dependence, as well as a weak temperature dependence, in the Beer-Lambert law absorption coefficient. Proper consideration of these effects is important when the N-atom ARAS diagnostic is used for absolute concentration measurements.     

A shock tube study of H + HNCO → NH2 + CO

The reaction of atomic hydrogen with isocyanic acid (HNCO) to produce the amidogen radical (NH₂) and carbon monoxide, has been studied in shock-heated mixtures of HNCO dilute in argon. Time-histories of the ground-state NH₂ radical were measured behind reflected shock waves using cw, narrowlinewidth laser absorption at 597 nm, and HNCO time-histories were measured using infrared emission from the fundamental v₂-band of HNCO near 5 μm. The second-order rate coefficient of reaction (2(a)) was determined to be: cm³ mol^?1 s^?1, where f and F define the lower and upper uncertainty limits, respectively. An upper limit on the rate coefficient of was determined to be:      

Reactions of alkoxy radicals with O2. I. C2H5O radicals

C₂H₅ONO was photolyzed with 366 nm radiation at ?48, ?22, ?2.5, 23, 55, 88, and 120°C in a static system in the presence of NO, O₂, and N₂. The quantum yield of CH₃CHO, Φ{CH₃CHO}, was measured as a function of reaction conditions. The primary photochemical act is and it proceeds with a quantum yield ?_1a = 0.29 ± 0.03 independent of temperature. The C₂H₅O radicals can react with NO by two routes The C₂H₅O radical can also react with O₂ via Values of k₆/k₂ were determined at each temperature. They fit the Arrhenius expression: Log(k₆/k₂) = ?2.17 ± 0.14 ? (924 ± 94)/2.303 T. For k₂ ? 4.4 × 10^?11 cm³/s, k₆ becomes (3.0 ± 1.0) × 10^?13 exp{?(924 ± 94)/T} cm³/s. The reaction scheme also provides k_8a/k₈ = 0.43 ± 0.13, where      

Thermal decomposition of trimethylene sulfone and 3-methyl sulfolane

Trimethylene sulfone and 3? methyl sulfolane have been pyrolyzed using a modification of the toluene flow method and a comparative rate technique. The main decomposition reactions are where k₁=10^16.1±0.3 exp(?28,100±500/T) sec^?1 and k₂=10^16.1±0.4 exp(?33,200±750/T) sec^?1.     

Thermal dimerization of 1,3-butadiene: Kinetics of the formation of cis,cis-cycloocta-1,5-diene

The thermal reaction of 1,3-butadiene (BD) has been studied between 464 and 557°K at pressures between 49 and 450 torr. The products are 4-vinylcyclohexene (VCH) and cis, cis-cycloocta-1,5-diene (COD), and their formations are second order. The rate constant (in 1/mol · sec) for VCH is given by and that for COD by The thermal reaction of COD has also been studied. The temperature was varied from 505 to 586°K and the pressure from 15 to 51 torr. The rate constants (in sec^?1) for the formations of VCH and BD are given by A biradical mechanism seems to fit these results. The heat of formation and the entropy of COD are estimated.