Use of homogeneous competition for OH radicals in electrochemical studies of H abstraction from organic molecules

The influence on the N₂O photocurrent of homogeneous competition for OH radicals between two organic solutes which form (as the result of hydrogen abstraction) radicals, one of which is reduced by the electrode, the other being oxidised, is considered theoretically. Such competition can be employed to investigate the kinetics of hydrogen abstraction in a case in which uncompetitive homogeneous destruction of OH radicals by a solute has no effect on the photocurrent. The influence of incomplete oxidation of alcohol radicals when a light pulse of short duration is employed is discussed, together with complications caused by adsorption of the organic solute. Competition for OH radicals between phenol and methanol points to a rate constant for H abstraction from phenol of ca. 2.8 × 10⁹ l mol^?1 s^?1 and to rapid heterogeneous reduction of most of the C₆H₄OH radicals throughout the accessible potential range.     

Degradation of n-butylparaben and 4-tert-octylphenol in H2O2/UV system

The degradation of two endocrine disrupting compounds: n-butylparaben (BP) and 4-tert-octylphenol (OP) in the H₂O₂/UV system was studied. The effect of operating variables: initial hydrogen peroxide concentration, initial substrate concentration, pH of the reaction solution and photon fluency rate of radiation at 254 nm on reaction rate was investigated. The influence of hydroxyl radical scavengers, humic acid and nitrate anion on reaction course was also studied. A very weak scavenging effect during BP degradation was observed indicating reactions different from hydroxyl radical oxidation. The second-order rate constants of BP and OP with OH radicals were estimated to be 4.8×10⁹ and 4.2×10⁹ M^?1 s^?1, respectively. For BP the rate constant equal to 2.0×10¹⁰ M^?1 s^?1was also determined using water radiolysis as a source of hydroxyl radicals.     

Electron transfer,equilibrium, and protonation in the system of cis- and trans-stilbene in 2-propanol

Spectrophotometric pulse radiolysis experiments with cis- and trans-stilbene (S_c and S_t) in 2-propanol show that both isomers react with the solvated electron with a rate constant of 4.5 × 10⁹ M^?1 s^?1. The absorption spectra of the two anion radicals have maxima at 496 and 486 nm, respectively. The absorbances at 400–550 nm disappear exponentially corresponding to a pseudo first order protonation of the anion radicals. The rate constants for the protonation of the cis isomer is 6.4 × 10⁵ and of the trans isomer 0.7 × 10⁵ s^?1. In mixtures of cis- and trans-stilbene the electron transfer

has a forward rate constant of 9 × 10⁷ M^?1 s^?1 while the back reaction has a rate constant of 2.15 × 10⁷ M^?1 s^?1. An equilibrium constant K = 4.2 is calculated.     

Rate constants for the reactions of alkyl radicals with 1,4-cyclohexadiene

An electron paramagnetic resonance (EPR ) technique was used to show that simple alkyl radicals readily abstract hydrogen from 1,4-cyclohexadiene. Rate constants for the reaction were ca. 10⁴–10⁵ M^?1 s^?1 at 300 K and activation energies 5–7 kcal mol^?1. For the stabilized radicals, allyl and benzyl, the rate constants were <10² M^?1 s^?1 at 300 K. The data suggest that 1,4-cyclohexadiene could be used as an effective trap to probe rearrangement reactions of carbon centered radicals and biradicals.     

Bimolecular self-reactions of ketyl radicals of acetophenone and diphenylaminyl radicals

The rate constants of self-reactions of ketyl radicals of acetophenone in n-heptane [2k = (3.2 ± 0.5) × 10⁹ M^?1 s^?1] and diphenylaminyl radicals in toluene [2k = (3.3 ± 0.5) × 10⁷ M^?1 s^?1] have been determined at 298 K using the flash photolysis technique. The rate constant of ketyl radicals is equal to the calculated diffusion constant and, therefore, this reaction is diffusion-controlled. The aminyl radical recombination rate is independent of the viscosity of the toluene/vaseline oil binary mixture (0.55 ? η ? 12 cP) and this reaction is activation-controlled. Reactivity anisotropy averaging due to the cage effect has been considered for ketyl and some other radicals. On the basis of the analysis it has been proposed that ketyl recombination involves formation of not only pinacol, but also iso-pinacols.     

Rate constants for the gas-phase reaction of OH radicals with a series of ketones at 299 ± 2 K

Relative rate constants for the reaction of OH radicals with a series of ketones have been determined at 299 ± 2 K, using methyl nitrite photolysis in air as a source of hydroxyl radicals. Using a rate constant for the reaction of OH radicals with cyclohexane of 7.57 × 10^?12 cm³ molecule^?1 s^?1, the rate constants obtained are (× 10¹² cm³ molecule^?1 s^?1): 2-pentanone, 4.74 ± 0.14; 3-pentanone, 1.85 ± 0.34; 2-hexanone, 9.16 ± 0.61; 3-hexanone, 6.96 ± 0.29; 2,4-dimethyl-3-pentanone, 5.43 ± 0.41; 4-methyl-2-pentanone, 14.5 ± 0.7; and 2,6-dimethyl-4-heptanone, 27.7 ± 1.5. These rate constants indicate that while the carbonyl group decreases the reactivity of C? H bonds in the α position toward reaction with the OH radical, it enhances the reactivity in the β position.     

Rate constants for reactions of perfluorobutylperoxyl radical with alkenes

Perfluorobutylperoxyl radicals were produced by radiolytic reduction of perfluorobutyl iodide in aerated methanol solutions. Rate constants for the reactions of this peroxyl radical with various organic compounds were determined by kinetic spectrophotometric pulse radiolysis. The rate constants for alkanes and alkenes were determined by competition kinetics using chlorpromazine as a reference. The results indicate that hydrogen abstraction from aliphatic compounds takes place with a rate constant that is too slow to measure in our system (<10⁵ M^?1 s^?1), and that abstraction of allylic and doubly allylic hydrogens is slow compared with addition. Addition to alkenes takes place with rate constants of the order of k = 10⁶ ? 10⁸ M^?1 s^?1. Good correlation was obtained between log k and the Taft substituent constants σ* for the various substituents on the double bond. Perfluorobutylperoxyl radical is found to be more reactive than trichloromethylperoxyl and other peroxyl radicals.     

New technique for generating high concentrations of gaseous OH radicals in relative rate measurements

We have developed a technique for generating high concentrations of gaseous OH radicals in a reaction chamber. The technique, which involves the UV photolysis of O₃ in the presence of water vapor, was used in combination with the relative rate method to obtain rate constants for reactions of OH radicals with selected species. A key improvement of the technique is that an O₃/O₂ (3%) gas mixture is continuously introduced into the reaction chamber, during the UV irradiation period. An important feature is that a high concentration of OH radicals [(0.53–1.2) × 10¹¹ radicals cm^?3] can be produced during the irradiation in continuous, steady‐state experiment. Using the new technique in conjunction with the relative rate method, we obtained the rate constant for the reaction of CHF₃ (HFC‐23) with OH radicals, k₁. We obtained k₁(298 K) = (3.32 ± 0.20) × 10^?16 and determined the temperature dependence of k₁ to be (0.48 ± 0.13) × 10^?12 exp[?(2180 ± 100)/T] cm³ molecule^?1 s^?1 at 253–328 K using CHF₂CF₃ (HFC‐125) and CHF₂Cl (HCFC‐22) as reference compounds in CHF₃–reference–H₂O gas mixtures. The value of k₁ obtained in this study is in agreement with previous measurements of k₁. This result confirms that our technique for generating OH radicals is suitable for obtaining OH radical reaction rate constants of ～10^?16 cm³ molecule^?1 s^?1, provided the rate constants do not depend on pressure. In addition, it also needed to examine whether the reactions of sample and reference compound with O₃ interfere the measurement when selecting this technique. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 317–325, 2003     

Effects of ring strain on gas-phase rate constants. 2. OH radical reactions with cycloalkenes

Relative rate constants for the gas-phase reactions of OH radicals with a series of cycloalkenes have been determined at 298 ± 2 K using methyl nitrite photolysis in air as a source of OH radicals. Using a rate constant for the reaction of OH radicals with isoprene of 9.60 × 10^?11 cm³ molecule^?1 s^?1, the rate constants obtained were (X 10¹¹ cm³ molecule^?1 s^?1): cyclopentene 6.39 ± 0.23, cyclohexene 6.43 ± 0.17, cycloheptene 7.08 ± 0.22, 1,3-cyclohexadiene 15.6 ± 0.5, 1,4 cyclohexadiene 9.48 ± 0.39, bicyclo[2.2.1]-2-heptene 4.68 ± 0.39, bicyclo[2.2.1] 2,5 heptadiene 11.4 ± 1.0, and bicyclo[2.2.2] 2 octene 3.88 ± 0.19. These data show that the rate constants for the nonconjugated cycloalkenes studied depend on the number of double bonds and the degree of substitution per double bond, and indicate that there are no obvious effects of ring strain energy on these OH radical addition rate constants. A predictive technique for the estimation of OH radical rate constants for alkenes and cycloalkenes is presented and discussed.     

Decomposition of meta- and para-phenylphenol during ozonation process

The decomposition of meta-phenylphenol (m-PP) and para-phenylphenol (p-PP) in a heterogeneous gas-liquid system using ozone was investigated. The influence of different reaction parameters such as ozone and PP isomers concentration as well as pH and temperature of the reaction mixture on the PP decay rate was determined. The second-order rate constants for the direct reaction of molecular ozone, determined in a homogeneous system, were (5.85 ± 0.35) × 10² M^?1 s^?1 and (8.90 ± 0.33) × 10² M^?1 s^?1 for m-PP and p-PP, respectively. The rate constants for the reaction of m-PP and p-PP with ozone increased with increasing pH. The reaction rate constants with ozone were found to be (1.75 ± 0.02) × 10⁹ M^?1 s^?1 and (1.86 ± 0.02) × 10⁹ M^?1 s^?1 for m-PP and p-PP anions, respectively.     

Photocurrent kinetics during electron emission from metal into electrolyte solution: Part III. H3O+ solutions

The photocurrent kinetics in acid solutions have been investigated. The diffusion coefficients of atoms H?((7±2)×10^?5cm²s^?1) and D?((4±1)×10^?5cm²s^?1) and OH^? and OD^? radicals ((1±0.3)×10^?5cm²s^?1) are found. The rate constants of capture of solvated electrons by H₃O⁺ and D₃O⁺ ions are identical and equal to (8±1)×10⁹M^?1s^?1. From the shape of the kinetic curves it follows that electrochemical desorption of atomic hydrogen occurs from the adsorbed state. The rate constant of this process has been measured. It is shown that the rate constant of electrochemical desorption depends only slightly on the potential.     

Flash photolysis resonance fluorescence investigation of the reactions of OH radicals with OCS and CS2

Rate constants for the reaction of OH radicals with OCS and CS₂ have been determined at 296 K using the flash photolysis resonance fluorescence technique. The values derived from this study are k_{OH + OCS} = (5.66 ± 1.21) × 10^?14 cm³ molecule^?1 s^?1 and k_{OH + CS2} = (1.85 ± 0.34) × 10^?13 cm³ molecule^?1 s^?1, where the uncertainties are 95% confidence limits making allowance for possible systematic errors.     

Rate constants for the reaction of OH radicals with 2-methyl-2-butene over the temperature range 297–425 K

Absolute rate constants, k₂, for the reaction of OH radicals with 2-methyl-2-butene have been determined over the temperature range 297–425 K using a flash photolysis-resonance fluorescence technique. The Arrhenius expression obtained was k₂ = 3.6 × 10^?11 exp [(450 ± 400)/RT] cm³ molecule^?1 s^?1.     

Absolute rate constants for the addition of hydroxymethyl radicals to alkenes in methanol solution

Absolute rate constants and their temperature dependencies were determined for the addition of hydroxymethyl radicals (CH₂OH) to 20 mono- or 1,1-disubstituted alkenes (CH₂ = CXY) in methanol by time-resolved electron spin resonance spectroscopy. With the alkene substituents the rate constants at 298 K (k₂₉₈) vary from 180 M^?1s^?1 (ethyl vinylether) to 2.1 middot; 10⁶ M^?1s^?1 (acrolein). The frequency factors obey log A/M^?1s^?1 = 8.1 ± 0.1, whereas the activation energies (E_a) range from 11.6 kJ/mol (methacrylonitrile) to 35.7 kJ/mol (ethyl vinylether). As shown by good correlations with the alkene electron affinities (EA), log k₂₉₈/M^?1s^?1 = 5.57 + 1.53 · EA/eV (R² = 0.820) and E_a = 15.86 ? 7.38 · EA/eV (R² = 0.773), hydroxymethyl is a nucleophilic radical, and its addition rates are strongly influenced by polar effects. No apparent correlation was found between E_a or log k₂₉₈ with the overall reaction enthalpy. © 1995 John Wiley & Sons, Inc.     

Formation of alkyl nitrates from the reaction of branched and cyclic alkyl peroxy radicals with NO

The yields of C₅ and C₆ alkyl nitrates from neopentane, 2-methylbutane, 2-methylpentane, 3-methylpentane, and cyclohexane have been measured in irradiated CH₃ONONO-alkane-air mixtures at 298 ± 2 K and 735-torr total pressure. Additionally, OH radical rate constants for neopentyl nitrate, 3-nitro-2-methylbutane, 2-nitro-2-methylpentane, 2-nitro-3-methylpentane, and cyclohexyl nitrate, relative to that for n-butane, have been determined at 298 ± 2 K. Using a rate constant for the reaction of OH radicals with n-butane of 2.58 × 10^?12 cm³ molecule^?1 s^?1, these OH radical rate constants are (in units of 10^?12 cm³ molecule^?1 s^?1): neopentyl nitrate, 0.87 ± 0.21; cyclohexyl nitrate, 3.35 ± 0.36; 3-nitro-2-methylbutane, 1.75 ± 0.06; 2-nitro-2-methylpentane, 1.75 ± 0.22; and 2-nitro-3-methylpentane, 3.07 ± 0.08. After accounting for consumption of the alkyl nitrates by OH radical reaction and for the yields of the individual alkyl peroxy radicals formed in the reaction of OH radicals with the alkanes studied, the alkyl nitrate yields (which reflect the fraction of the individual RO₂ radicals reacting with NO to form RONO₂) determined were: neopentyl nitrate, 0.0513 ± 0.0053; cyclohexyl nitrate, 0.160 ± 0.015; 3-nitro-2-methylbutane, 0.109 ± 0.003; 2-nitro-2methylbutane, 0.0533 ± 0.0022; 2-nitro-2-methylpentane, 0.0350 ± 0.0096; 3- + 4-nitro-2-methylpentane, 0.165 ± 0.016; and 2-nitro-3-methylpentane, 0.140 ± 0.014. These results are discussed and compared with previous literature values for the alkyl nitrates formed from primary and secondary alkyl peroxy radicals generated from a series of n-alkanes.     

Gas‐Phase Reaction of Monomethylhydrazine with Ozone: Kinetics and OH Radical Formation

The gas‐phase reaction of monomethylhydrazine (CH₃NH? NH₂; MMH) with ozone was investigated in a flow tube at atmospheric pressure and a temperature of 295 ± 2 K using N₂/O₂ mixtures (3–30 vol% O₂) as the carrier gas. Proton transfer reaction–mass spectrometry (PTR‐MS) and long‐path FT‐IR spectroscopy served as the main analytical techniques. The kinetics of the title reaction was investigated with a relative rate technique yielding k_MMH+O3 = (4.3 ± 1.0) × 10^?15 cm³ molecule^?1 s^?1. Methyldiazene (CH₃N?NH; MeDia) has been identified as the main product in this reaction system as a result of PTR‐MS analysis. The reactivity of MeDia toward ozone was estimated relative to the reaction of MMH with ozone resulting in k_MeDia+O3 = (2.7 ± 1.6) × 10^?15 cm³ molecule^?1 s^?1. OH radicals were followed indirectly by phenol formation from the reaction of OH radicals with benzene. Increasing OH radical yields with increasing MMH conversion have been observed pointing to the importance of secondary processes for OH radical generation. Generally, the detected OH radical yields were definitely smaller than thought so far. The results of this study do not support the mechanism of OH radical formation from the reaction of MMH with ozone as proposed in the literature.     

Kinetics and mechanism of atmospheric reactions of partially fluorinated alcohols

Gas-phase reactions typical of the Earth’s atmosphere have been studied for a number of partially fluorinated alcohols (PFAs). The rate constants of the reactions of CF₃CH₂OH, CH₂FCH₂OH, and CHF₂CH₂OH with fluorine atoms have been determined by the relative measurement method. The rate constant for CF₃CH₂OH has been measured in the temperature range 258–358 K (k = (3.4 ± 2.0) × 10¹³exp(?E/RT) cm³ mol^?1 s^?1, where E = ?(1.5 ± 1.3) kJ/mol). The rate constants for CH₂FCH₂OH and CHF₂CH₂OH have been determined at room temperature to be (8.3 ± 2.9) × 10¹³ (T = 295 K) and (6.4 ± 0.6) × 10¹³ (T = 296 K) cm³ mol^?1 s^?1, respectively. The rate constants of the reactions between dioxygen and primary radicals resulting from PFA + F reactions have been determined by the relative measurement method. The reaction between O₂ and the radicals of the general formula C₂H₂F₃O (CF₃CH₂? and CF₃?HOH) have been investigated in the temperature range 258–358 K to obtain k = (3.8 ± 2.0) × 10⁸exp(?E/RT) cm³ mol^?1 s^?1, where E = ?(10.2 ± 1.5) kJ/mol. For the reaction between O₂ and the radicals of the general formula C₂H₄FO (? HFCH₂O, CH₂F?HOH, and CH₂FCH₂?) at T = 258–358 K, k = (1.3 ± 0.6) × 10¹¹exp(?E/RT) cm³ mol^?1 s^?1, where E = ?(5.3 ± 1.4) kJ/mol. The rate constant of the reaction between O₂ and the radicals with the general formula C₂H₃F₂O (?F₂CH₂O, CHF₂?HOH, and CHF₂CH₂?) at T = 300 K is k = 1.32 × 10¹¹ cm³ mol^?1 s^?1. For the reaction between NO and the primary radicals with the general formula C₂H₂F₃O (CF₃CH₂? and CF₃?HOH), which result from the reaction CF₃CH₂OH + F, the rate constant at 298 K is k = 9.7 × 10⁹ cm³ mol^?1 s^?1. The experiments were carried out in a flow reactor, and the reaction mixture was analyzed mass-spectrometrically. A mechanism based on the results of our studies and on the literature data has been suggested for the atmospheric degradation of PFAs.     

Recombination of 1,1-dimethylpropyl peroxy radicals in polar solvents

The kinetics of 1,1-dimethylpropyl peroxy radicals recombination in polar solvents—water, methanol, and their mixtures—was studied by EPR spectroscopy in combination with the stopped-flow method, and the rate constants of this reaction were determined. Peroxyl radicals were generated by mixing solutions of Ce⁴⁺ sulfate and 1,1-dimethylpropyl hydroperoxide. The observed EPR signal of the peroxyl radical is a singlet with a g-factor of 2.015 ± 0.001, and a line width of ΔH = (1.36 ± 0.02) × 10^?3 T for methanol and ΔH = (9.7 ± 0.2) × 10^?4 T for water. The measured rate constants of (CH₃)₂C(O₂^·)CH₂CH₃ radical recombination at 298 K are 2k_t = (3.9 ± 0.4) × 10⁴ L mol^?1 s^?1 for water and 2k_t = (5.2 ± 0.5) × 10³ L mol^?1 s^?1 for methanol. A linear relationship between ln(2k_t) and the Kirkwood function (ε?1)/(2ε + 1), where e is the dielectric constant of the medium, has been established, indicating an important role of nonspecific solvation in the recombination of tertiary peroxyl radicals.     

Kinetics of the gas-phase reactions of β-phellandrene with OH and NO3 radicals and O3 at 297 ± 2 K

Rate constants for the gas-phase reactions of the biogenically emitted monoterpene β-phellandrene with OH and NO₃ radicals and O₃ have been measured at 297 ± 2 K and atmospheric pressure of air using relative rate methods. The rate constants obtained were (in cm³ molecule^?1 s^?1 units): for reaction with the OH radical, (1.68 ± 0.41) × 10^?10; for reaction with the NO₃ radical, (7.96 ± 2.82) × 10^?12; and for reaction with O₃, (4.77 ± 1.23) × 10^?17, where the error limits include the estimated uncertainties in the reference reaction rate constants. Using these rate constants, the lifetime of β-phellandrene in the lower troposphere due to reaction with these species is calculated to be in the range of ca. 1–8 h, with the OH radical reaction being expected to dominate over the O₃ reaction as a loss process for β-phellandrene during daylight hours.     

Kinetics and mechanism for the oxidation of 1,1,1-trichloroethane

The reaction mechanisms for oxidation of CH₃CCl₂ and CCl₃CH₂ radicals, formed in the atmospheric degradation of CH₃CCl₃ have been elucidated. The primary oxidation products from these radicals are CH₃CClO and CCl₃CHO, respectively. Absolute rate constants for the reaction of hydroxyl radicals with CH₃CCl₃ have been measured in 1 atm of Argon at 359, 376, and 402 K using pulse radiolysis combined with UV kinetic spectroscopy giving ??(OH + CH₃CCl₃) = (5.4 ± 3) 10^?12 exp(?3570 ± 890/RT) cm³ molecule^?1 s^?1. A value of this rate constant of 1.3 × 10^?14 cm³ molecule^?1 s^?1 at 298 K was calculated using this Arrhenius expression. A relative rate technique was utilized to provide rate data for the OH + CH₃ CCl₃ reaction as well as the reaction of OH with the primary oxidation products. Values of the relative rate constants at 298 K are: ??(OH + CH₃CCl₃) = (1.09 ± 0.35) × 10^?14, ??(OH + CH₃CClO) = (0.91 ± 0.32) × 10^?14, ??(OH + CCl₃CHO) = (178 ± 31) × 10^?14, ??(OH + CCl₂O) < 0.1 × 10^?14; all in units of cm³ molecule^?1 s^?1. The effect of chlorine substitution on the reactivity of organic compounds towards OH radicals is discussed.