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.     

The reaction of O(1D) with H2O and the reaction of OH with C3H6

N₂O was photolyzed at 2139 Å to produce O(¹D) atoms in the presence of H₂O and CO. The O(¹D) atoms react with H₂O to produce HO radicals, as measured by CO₂ production from the reaction of OH with CO. The relative importance of the various possible O(¹D )–H₂O reactions is The relative rate constant for O(¹D) removal by H₂O compared to that by N₂O is 2.1, in good agreement with that found earlier in our laboratory. In the presence Of C₃H₆, the OH can be removed by reaction with either CO or C₃H₆: From the CO₂ yield, k₃/k₂ = 75,0 at 100°C and 55.0 at 200°C to within ± 10%. When these values are combined with the value of k₂ = 7.0 × 10^?13exp (–1100/RT) cm³/sec, k₃ = 1.36 × 10^?11 exp (–100/RT) cm³/sec. At 25°C, k₃ extrapolates to 1.1 × 10^?11 cm³/sec.     

Kinetic isotope effect for hydrogen/deuterium abstraction by chlorine atoms from (CH3)2NNO2 and (CD3)2NNO2

The kinetic isotope effect for the abstraction of hydrogen/deuterium from dimethylnitramine and dimethylnitramine-d₆ by chlorine atoms has been studied in the temperature range 273–353 K. The rate constant ratio k_H0/k_D is given by the Arrhenius expression, k_H/k_D=(0.92 ± 0.07)exp(286 ± 250/RT), where R is expressed in cal mol^?1 K^?1. The absolute rate constant for the deuterium abstraction reaction is extrapolated as k_D=(1.50 ± 0.90) × 10^?10 exp(?1,486 ± 370/RT) cm³ molecule^?1 s^?1. The temperature dependence of the kinetic isotope effect was calculated using the conventional transition-state theory, and the obtained values for k_H/k_D and ΔE_{H, D} are in good agreement with the experimental value for a bent transition state geometry, with two new vibrational frequencies of 340 cm^?1 (272 cm^?1) corresponding to the in-plane and out-of-plane motions of hydrogen (deuterium) atoms in the Cl…H…C arrangement. © 1993 John Wiley & Sons, Inc.     

Hydrolyse acide du p-nitrophényl-diazométhane dans les mélanges H2O?D2O: analyse des effets isotopiques

The velocity of the hydrogen ion catalysed hydrolysis of p-nitrophenyl-diazo-methane (I) has been measured in H₂O? D₂O mixtures, giving an isotopic α_i = 0.49. The product isotope effect r = 5.1, determined from product analyses, combined with the (overall) solvent isotope effect k_H/k_D = 2.81, yields the primary kinetic isotope effect (k_H/k_D)^I = 3.8, and the secondary kinetic isotope effect (k_H/k_D)^II = 0.75. The CICH₂COOH-catalysed hydrolysis of I in H₂O? D₂O mixtures gave a straight-line plot of k_n/k_H versus the atomic fraction n of deuterium. With four carboxylic acids, as catalysts, values of about 4.3 for the kinetic (overall) isotope effects were observed.     

The structures of some cycloalkane molecular ions by photodissociation and charge-transfer studies in an ICR-spectrometer

The photodissociation spectra of the molecular ions of some cycloalkanes are compared with those of the corresponding acyclic alkanes and alkenes. It is shown that the molecular ions of cyclohexane and cycloheptane are cyclic whereas the cyclopentane ring opens upon ionisation. This conclusion is supported by a study of the charge-transfer equilibria: C₆D₁₂^± + C₆H₁₂ ? C₆D₁₂ + C₆H₁₂^±, C₅D₁₀^± + C₅H₁₀ ? C₅D₁₀ + C₅H₁₀^±. It is furthermore shown that the maxima in the photodissociation spectra of the molecular ions of saturated hydrocarbons correspond to different dissociation processes.     

Experimental study of the kinetics of reaction of chlorine atoms with tetrahydrofuran and fully deuterated tetrahydrofuran

The overall rate constants for H-abstraction (k_H) from tetrahydrofuran and D-abstraction (k_D) from fully deuterated tetrahydrofuran by chlorine atoms in the temperature range of 298-547 K were determined. In both cases, very weak negative temperature dependences of the overall rate constants were observed, described by the expressions: k_H = (1.55 ± 0.13) × 10⁻¹⁰ exp(52 ± 28/T) cm³ molecule⁻¹ s⁻¹ and k_D = (1.27 ± 0.25) × 10⁻¹⁰exp(55 ± 62/T) cm³ molecule⁻¹ s⁻¹. The experimental results show that the value of the kinetic isotope effect (k_H/k_D), amounting to 1.21 ± 0.10, is temperature independent at 298-547 K.     

Regioselectivity and Deuterium Isotope Effects in Geraniol Hydroxylation by the Cytochrome P-450 Monooxygenase from Catharanthus roseus (L.) G. DON

The hydroxylation of geraniol ( 8 ) by cytochrome P-450 (P-450_Cath.) from the subtropical plant Catharanthus roseus (L.) G. DON was optimised to give 8-hydroxygeraniol ( 9 ) as the single product in 35% yield. Incubations of different ¹³C- and ²H-labelled geraniols revealed that H-abstraction is completely regioselective in favour of the CH₃ group trans to the chain at C(6) of 8 . An intramolecular isotope effect k_H/k_D = 8.0 was determined, suggesting that H-abstraction is one of the major rate-contributing steps; however, the intermolecular isotope effect was surprisingly inverse at low conversion k_H/k_D = 0.50, indicating the existence of rate-contributing steps preceding the first irreversible, isotope-sensitive reaction in the sequence.     

Kinetic H/D isotope effects for gas phase hydroxylation of benzene and chlorobenzene between 520–1080 K. Hydroxyl radical versus O(3P) atom attack

Gas phase slow combustion of (chloro)benzene in O₂/N₂ mixtures, and induced by addends such as tert butylhydroperoxide, cyclohexane, or methanol, leads to (chloro)-phenol as the only important aromatic product. Using C₆H₆/C₆D₆ mixtures, formation of phenol/perdeuterophenol was studied between 520–1080 K. The temperature dependence of this product ratio was found to obey the Arrhenius expression for the intermolecular isotope effect log k_H/k_D = ?0.14 ± 0.03 + (1240 ± 80)/2.303RT (R in cal/mol K). Essentially the same result was obtained for the intramolecular isotope effect, measuring the change in isomer distribution for the chlorophenols formed from p-deuterio-chlorobenzene versus those for chlorobenzene. These results are in accordance with H(D)-abstraction by ·OH, via a linear transition state, as the first and (relative) rate determining step. Whereas above 1000 K, at reduced pressure, the intramolecular isotope effect continues to prevail, C₆H₆/C₆D₆ do not show differences in rate of formation of C₆H₅OH/C₆D₅OH. Under these conditions, the only effective reaction of arene to phenol appears to be set in by addition of O(³P).     

Kinetic isotope effect in the reaction of oh radical with acetone-D6

The discharge-flow method with resonance fluorescence detection of OH radicals was applied to obtain the rate constant value of k _D = 1.95 ± 0.14 (1σ) 10¹⁰ cm³ mol^-1s^-1 at 298 K. Combination with k _H from our previous study gives the kinetic isotope effect of k _H / k _D = 5.33 ± 0.41. OH + CH₃C(O)CH₃ → Products (H) OH + CD₃C(O)CD₃ → Products(D) This revised version was published online in June 2006 with corrections to the Cover Date.     

Relative-rate kinetic study of the reaction of bromine atom with neopentane

The rate coefficient ratio ofk ₁/k ₂=0.83±0.21 has been determined for the reactions Br+neo-C₅H₁₂ (1) and Br+C₂H₆ (2) by applying the relative-rate kinetic method atT=298 K.     

Radical-type addition of benzyl bromide to vinyl chloride: the kinetics and activation energy of the process

Radical telomerization of vinyl chloride with benzyl bromide and the competitive reaction of benzyl bromide with vinyl chloride and trimethylvinylsilane have been studied. The relative rate constant for the addition of C₆H₅C · H₂ to vinyl chloride,k _rel (with respect to trimethylvinylsilane), is close to unity, whereas the activation energy of the addition of C₆H₅C^.H₂ to vinyl chloride is considerably lower (by 7 kcal mol^–1) than in the reaction involving trimethylvinylsilane. The possible fragmentation of the radical-adduct C₆H₅CH₂CH₂C^.HCl was suggested as one of the possible reasons of underestimation ofk _rel. The activation energy was estimated by the MPDO/3 method.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 886–888, May, 1993.     

Der geschwindigkeitsbestimmende Schritt bei der Decarboxy-lierung von 2,4-Dihydroxybenzoesäure in mässig konzentrierter wässeriger Salzsäure

The decarboxylation kinetics of 2,4-dihydroxybenzoic acid have been studied in 0.1–8 N aqueous HCl at 50°. At low HCl concentrations, the observed first order rate constant, k, increases with increasing acidity of the solution. In solutions with 3.5–6 N HCl, k remains constant. The D₂O solvent isotope effect decreases from ^kH₂O/^kD₂O = 2.0 in 1N HCl to 1.3 in 5 N HCl, and it remains unchanged at 1.3 if the HCl concentration is increased further to 8 N. It is concluded that an increase of the acidity of the solution causes a change of the rate determining step from slow proton transfer to rate limiting C? C bond cleavage.     

Rate constants for the elementary reactions between CN radicals and CH4, C2H6, C2H4, C3H6, and C2H2 in the range: 295 ⩽ T/K ⩽ 700

Pulsed laser photolysis, time-resolved laser-induced fluorescence experiments have been carried out on the reactions of CN radicals with CH₄, C₂H₆, C₂H₄, C₃H₆, and C₂H₂. They have yielded rate constants for these five reactions at temperatures between 295 and 700 K. The data for the reactions with methane and ethane have been combined with other recent results and fitted to modified Arrhenius expressions, k(T) = A′(298) (T/298)ⁿ exp(?θ/T), yielding: for CH₄, A′(298) = 7.0 × 10^?13 cm³ molecule^?1 s^?1, n = 2.3, and θ = ?16 K; and for C₂H₆, A′(298) = 5.6 × 10^?12 cm³ molecule^?1 s^?1, n = 1.8, and θ = ?500 K. The rate constants for the reactions with C₂H₄, C₃H₆, and C₂H₂ all decrease monotonically with temperature and have been fitted to expressions of the form, k(T) = k(298) (T/298)ⁿ with k(298) = 2.5 × 10^?10 cm³ molecule^?1 s^?1, n = ?0.24 for CN + C₂H₄; k(298) = 3.4 × 10^?10 cm³ molecule^?1 s^?1, n = ?0.19 for CN + C₃H₆; and k(298) = 2.9 × 10^?10 cm³ molecule^?1 s^?1, n = ?0.53 for CN + C₂H₂. These reactions almost certainly proceed via addition-elimination yielding an unsaturated cyanide and an H-atom. Our kinetic results for reactions of CN are compared with those for reactions of the same hydrocarbons with other simple free radical species. © John Wiley & Sons, Inc.     

Thermal reactions of ethane-d6–acetylene and D2–acetylene mixtures behind shock waves

A study of the thermal decomposition of an acetylene–ethane-d₆ mixture indicates that the rate constant for hydrogen abstraction from acetylene by methyl is more than 20 times less than for abstraction from ethane. Isotopic exchange is initiated by a rapid reaction between product D atoms and C₂H₂. A series of experiments involving the reactions of a D₂–acetylene mixture indicated that a molecular exchange process was also occurring, and it was shown that d[C₂HD]/dt = k[D₂]^0.7[C₂H₂]^0.3, effective activation energy = 15.8 kcal/mol. This mechanism made an insignificant contribution to isotope exchange in C₂H₂–C₂D₆ mixtures.     

The Reaction of Sulfenic Acids with Peroxyl Radicals: Insights into the Radical‐Trapping Antioxidant Activity of Plant‐Derived Thiosulfinates

Sulfenic acids play a prominent role in biology as key participants in cellular signaling relating to redox homeostasis, in the formation of protein‐disulfide linkages, and as the central players in the fascinating organosulfur chemistry of the Allium species (e.g., garlic). Despite their relevance, direct measurements of their reaction kinetics have proven difficult owing to their high reactivity. Herein, we describe the results of hydrocarbon autoxidations inhibited by the persistent 9‐triptycenesulfenic acid, which yields a second order rate constant of 3.0×10⁶ M ^?1 s^?1 for its reaction with peroxyl radicals in PhCl at 30 °C. This rate constant drops 19‐fold in CH₃CN, and is subject to a significant primary deuterium kinetic isotope effect, k_H/k_D=6.1, supporting a formal H‐atom transfer (HAT) mechanism. Analogous autoxidations inhibited by the Allium‐derived (S)‐benzyl phenylmethanethiosulfinate and a corresponding deuterium‐labeled derivative unequivocally demonstrate the role of sulfenic acids in the radical‐trapping antioxidant activity of thiosulfinates, through the rate‐determining Cope elimination of phenylmethanesulfenic acid (k_H/k_D≈4.5) and its subsequent formal HAT reaction with peroxyl radicals (k_H/k_D≈3.5). The rate constant that we derived from these experiments for the reaction of phenylmethanesulfenic acid with peroxyl radicals was 2.8×10⁷ M ^?1 s^?1; a value 10‐fold larger than that we measured for the reaction of 9‐triptycenesulfenic acid with peroxyl radicals. We propose that whereas phenylmethanesulfenic acid can adopt the optimal syn geometry for a 5‐centre proton‐coupled electron‐transfer reaction with a peroxyl radical, the 9‐triptycenesulfenic is too sterically hindered, and undergoes the reaction instead through the less‐energetically favorable anti geometry, which is reminiscent of a conventional HAT.     

Self- and cross-disproportionation-to-combination rations for cyclopentyl and cyclohexyl radicals and their deuterated analogues

Hydrogen, cycloalkene, and bicycloalkyl were found to be the principal products which account for ≈?97% of all products formed in the gas-phase radiolysis of water vapor containing low concentrations of cycloalkanes. From the ratios of cycloalkene-to-bicycloalkyl yields extrapolated to the zero dose, the self- and cross-disproportionation-to-recombination rate constant ratios Δ = k_d/k_c were determined for the following 12 reactions: Δ(c-C₅H₉, c-C₅H₉) = 0.73; Δ(c-C₅D₉, c-C₅D₉) = 0.58; Δ(c-C₆H₁₁, cC₆H₁₁) = 0.59; Δ(c-C₆D₁₁, c-C₆D₁₁) = 0.46; Δ(c-C₅H₉, c-C₆H₁₁) = 0.28; Δ(c-C₅D₉, c-C₆H₁₁) = 0.28; Δ(c-C₅H₉, c-C₆D₁₁) = 0.24; Δ(c-C₅D₉, c-C₆D₁₁) = 0.24; Δ(c-C₆H₁₁, c-C₅H₉) = 0.33; Δ(c-C₆H₁₁, c-C₅D₉) = 0.25; Δ(c-C₆D₁₁, c-C₅H₉) = 0.35; and Δ(c-C₆D₁₁, c-C₅D₉) = 0.28, where in the case of the cross-disproportionation the symbol Δ(R₁,R₂) is used to represent k_d/k_c for the disproportionation in which radical R₁ captures a hydrogen (deuterium) atom from radial R₂. The geometrical mean rule holds in the cross-combination reactions of cyclopentyl and cyclohexyl radicals. The kinetic isotope effect in the disproportionation reaction was determined as 1.24 ± 0.06.     

Laser-induced kinetics: Arrhenius parameters for t-C4H9 + XI → i-C4H9X + I (X = H,D) and the Heat of Formation of the t-butyl radical

The metathesis reaction of DI with t-C₄H₉ generated by 351-nm photolysis of 2,2′-azoisopropane was studied in a low-pressure reactor (VLP? Knudsen cell) in the temperature range of 302–411 K. The data obeyed the following Arrhenius relation when combined with recent data by Rossi and Golden gathered by the same technique (t-C₄H₉ by thermal decomposition of 2,2′-azoisobutane): log k₂^D(M^?1s^?1) = 9.60 – 1.90/θ, where θ = 2.303RT kcal/mol for 302 K < T > 722 K. The metathesis reaction of HI with t-C₄H₉ was studied at 301 K and resulted in k₂^H(M^?1·s^?1) = (3.20 ± 0.62) × 10⁸. An analogous Arrhenius relation was calculated for the protiated system if the small primary isotope effect k₂^H/k₂^D was assumed to be √2 at 700 K. It was of the following form: log k₂^H(M^?1·s^?1) = 9.73 – 1.68/θ. Preliminary data of Bracey and Walsh indicate that earlier Arrhenius parameters determined for the reverse reaction are somewhat in error. Their value of log k₁(M^?1·s^?1) = 11.5 – 23.8/θ yields 7delta;H_f,300⁰(t-butyl) = 9.2 kcal/mol and S₃₀₀⁰(t-butyl) = 74.2 cal/mol7°K when taken in conjuction with this study.     

Kinetics and mechanism of the oxidation of primary alcohols by sodium N-chloroethylcarbamate

The oxidation of primary alcohols by sodium N-chloroethylcarbamate in acid solution, results in the formation of corresponding aldehydes. The reaction is first order with respect to the oxidant and alcohol. The rate increases with an increase in acidity. The oxidation of α,α-dideuterioethanol exhibited a primary kinetic isotope, k_H/k_D = 2.11 at 298 K. The value of solvent isotope effect k(H₂O)/k(D₂O) = 2.23 at 298 K. Addition of ethyl carbamate does not affect the rate. (EtOC(OH)NHCl)⁺ has been postulated as the reactive species. Plots of (log k₂ + Ho) against (Ho + log[H⁺]) are linear with the slope, ?, having values from 1.78–1.87. This suggested a proton abstraction by water in the rate-determining step. The rates of oxidation of alcohols bearing both electron-withdrawing and electron-donating groups are more than that of methanol. A concerted mechanism involving transfer of a hydride ion from the C? H bond of the alcohol tothe oxidant and removal of a proton from the O? H group by a water molecule has been proposed.     

Kinetics of proton transfer in ice via the pH-jump method: Evaluation of the proton diffusion rate in polycrystalline doped ice

The value of the proton diffusion coefficient D_H⁺ in ice was extracted from the diffusion-controlled rate k_D of the proton recombination reaction RO⁻ + H₃O⁺ → k_D ROH + H₂O in polycrystalline doped ice. At −10°C, D_H⁺ was estimated to lie between 3.5×10⁻⁶ and 1.3×10⁻⁵ cm² s⁻¹, well below the corresponding value of (4.1 ± 0.1)×10⁻⁵ cm² s⁻¹ found in supercooled water.