Quantitative rate constants for radical reactions in the nanopores of cotton

The understanding of radical reactions in nanostructured materials is important for developing new synthetic procedures and controlling degradation reactions. To develop this area, an easy method for measuring quantitative rate constants of some radical reactions in nanostructures is required. A simple method for measuring the rate constant of dye bleaching, kdye, by organic radicals in such materials is introduced, involving the measurement of microsecond bleaching kinetics by diffuse reflectance spectroscopy, following laser flash creation of the radicals. Using wet and dry cotton as model substrates, we obtained kdye of 2-hydroxy-2-propyl and 1-hydroxy-1-cyclohexyl radicals with reactive red 3 and reactive orange 4 and compared them to solution-phase values. Surprisingly, the reactions in cotton follow simple liquid-phase kinetics and are diffusion-controlled. A cage effect in cotton is also found.     

Determination of the rate constants for reactions of gaseous substrates with adsorbed O− radical anions

We consider the possibility of determining the rate constants for reactions of gaseous substrates with O^– radical anions adsorbed on catalysts from the loss of the substrate from the gas phase. We consider two reaction pathways, including attack on O^– by the substrate from the gas phase and from the surface of the contact catalyst, under conditions of equilibrium distribution of the substrate between the phases.L. M. Litvinenko Institute of Physical Organic Chemistry and Coal Chemistry, National Academy of Sciences of Ukraine, 70 ul. R. Lyuksemburg, Donetsk 340114, Ukraine. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 32, No. 2, pp. 80–82, March–April 1996. Original article submitted June 6, 1995.     

A quantitative study of alkyl radical reactions by kinetic spectroscopy III. Absorption spectrum and rate constants of mutual interaction for the ethyl radical

Intrinsic spectral and kinetic parameters have been measured for the ethyl radical, which was formed in the gas phase by the flash photolysis of azoethane. Absolute values of the extinction coefficient ?(λ) were derived from complementary measurements of the yield of nitrogen and the absorbance of an equivalent concentration of the ethyl radical. The absorption spectrum is broad, structureless, and comparatively weak; ?(247) = 4.8 × 10² l/mol·cm at the maximum, and the oscillator strength is (9.1 ± 0.5) × 10^?3. This is in good qualitative agreement with a spectrum obtained independently using the technique of molecular modulation spectrometry. The biomolecular reactions of mutual interaction were the only significant reactions of the ethyl radical in this system; kinetic analysis of the second-order decline of the absorbance during the dark period yielded a value of k/?(λ) for each experiment. The rate constant for mutual interaction was evaluated from the product of corresponding measurements of k/?(λ) and ?(λ) individual values are independent of the wavelength of measurement, and the mean value is k = (1.40 ± 0.27) × 10¹⁰ l/mol·sec. The rate constant for mutual combination was derived with the aid of product analysis as k₂ = (1.24 ± 0.23) × 10¹⁰ l/mol·sec; it stands in close agreement with the set of “high” values obtained by direct measurement using a variety of methods.     

Elementaey rate constants of radical reactions in cyclohexanol undergoing oxidation

Conclusions The elementary rate constants of the reactions of RO₂ ^. + RH and RO₂ ^. +RO₂ ^. in cyclohexanol undergoing oxidation and in mixtures of cyclohexanol with chlorobenzene were measured by the method of the photochemical aftereffect.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 1737–1743, October, 1966.     

An absolute measurement of the rate constant for ethyl radical combination

An absolute value of k_r of ethyl radicals at 860 ± 17°K of 4.5 × 10⁹ M^?1·sec^?1 was determined under VLPP conditions, where the value of k_r/k_r^∞ should be about 1/2. Thus k_r^∞(M^?1·sec^?1) ～ 10¹⁰ at 860°K. An error of as much as a factor of 2 in k_r would be surprising, but possible. The value of 10¹⁰M^?1·sec^?1 seems to be a factor of from 2 to 5 too high to be compatible with extensive data on the reverse reaction and the accepted thermochemistry. Changes in the heat of formation and entropy of the ethyl radical can change the situation somewhat, but even these changes when applied to the work of Hiatt and Benson [3] indicate that ethyl combination should be ～ 10^9.3 M^?1·sec^?1. More work is necessary if a better value is desired.     

Relative rate constants of the gas-phase decomposition reactions of the s-butoxy radical

s-Butoxy radicals have been generated by reacting fluorine with s-butanol: Over the temperature range 398.6 to 493.3 K the s-butoxy radical decomposes by two different pathways to yield acetaldehyde and propionaldehyde, acetaldehyde being the major product: The ratio k₁/k₂ was found to be temperature dependent. An Arrhenius plot of the data (398.6 to 493.3 K) yields the relative Arrhenius parameters, E₁ - E₂ = ?11.2 ± 0.8 kJ mol^?1 and (A₁/A₂) = 0.59 ± 0.14. The ratio of rate constants k₁/k₂ was shown to be independent of total pressure (80–600 torr) and of the pressure of s-butanol (2–13 torr). The kinetic results for these s-butoxy decomposition reactions are discussed in relation to the literature data and in terms of the thermochemistry of the reactions.     

Determination of absolute rate constants for free radical polymerization of ethyl α-fluoroacrylate and characterization of the polymer

The absolute rate constants for propagation (k_p) and for termination (k_t) of ethyl α-fluoroacrylate (EFA) were determined by means of the rotating sector method; k_p = 1120 and k_t = 4.8 × 10⁸ L/mol.s at 30°C. The monomer reactivity ratios for the copolymerizations with various monomers were obtained. By combining the k_p values for EFA from the present study and those for common monomers with the monomer reactivity ratios, the absolute values of the rate constants for cross-propagations were also evaluated. Reactivities of EFA and poly(EFA) radical, being compared with those of methyl acrylate and its polymer radical, were found to be little affected by the α-fluoro substitution. Poly(EFA) prepared with the radical initiator was characterized by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Although the glass transition temperature obtained by DSC for poly(EFA) resembled that of poly(ethyl α-chloroacrylate), its TGA thermogram showed fast chain de polymerization to EFA that was distinct from complicated degradation of poly(ethyl α-chloroacrylate).     

Establishment of the steady state in the flow pyrolysis of neopentane; determination of individual rate constants

An induction period, interpreted as due to the initial establishment of the steady-state CH₃ concentration, has been observed in the flow pyrolysis of neopentane. Independent rate constants have been calculated for the initiation and termination reactions and for one propagation reaction.     

Determination of the elementary rate constants of autoacceleration reactions on polymers

Equations describing the course of an autoacceleration reaction on a polymer in the low conversion region were derived and used to obtain relationships between the curvature of the conversion curve at the beginning and the elementary rate constants k₁ and k′₂. Calculations carried out by using our relationships are compared with the procedure suggested by Kawabe and Yanagita, and the applicability of the individual methods for the determination of the elementary rate constants is discussed.     

Relative rate constants for the reactions of trichloropropenyl radical. Application of the two-parameter Taft equation

The relative rate constants for the hydrogen atom abstraction by CCl₃CH?CH· radical from CH₂Cl₂, CHCl₃, CH₃COCH₃, CH₃CN, C₆H₅CH₃, C₆H₅OCH₃, CH₃CHO, and CH₃OH in the liquid phase at 20°C have been measured. It was shown that these reaction rate constants are correlated by the two-parameter Taft equation with ρ* = 0.726 ± 0.096, r* = 1.22 ± 0.16. A relationship between r* and bond dissociation energy D(R? H) has been found for the abstraction reactions of different free radicals.     

Rate constants for the reactions of hydroxyl radical with several fluoroethers by the relative rate method

Relative rate experiments were used to measure ratios of chemical kinetics rate constants as a function of temperature for the reactions of OH with eight fluoroethers, including CF3OCF2CHF2, CF3OCF2CHFCF3, CHF2CF2OCHF2, CF3CHFCF2OCH2CF3, (CF3)2CHOCHF2, CF2HCF2OCH2CF3, CHF2CF2OCHFCF3, and CF3CH2OCH2CF3. The temperature ranges were about 270-400 K. Each compound was measured against at least two references. Results are compared with previous data where available. An approach using model compounds for the approximate estimation of rate constants for the fluoroethers is discussed. Observed temperature dependences for fluoroethers from the present work and some literature work are shown to be accurately predictable, based on a previously determined correlation of k298K with the pre-exponential factor, A, in the Arrhenius equation k = Ae(-E/RT).     

Absolute rate constants of alkene addition reactions of a fluorinated radical in water

Absolute rate constants of *R(f)SO(3)(-) radical addition to a series of water-soluble alkenes containing ionic, carboxylate substituents were measured by laser flash photolysis experiments in water. The observed rate constants were all considerably larger than those of structurally similar analogues in a nonpolar organic solvent, with rate factors of 3-9-fold being observed. It is concluded that such rate enhancements derive at least in part from stabilization of the polar transition state for addition of the electrophilic fluorinated radical to alkenes by the polar solvent water.     

Determination of absolute rate constants of addition of ethyl radicals to olefins

Theoretical and Experimental Chemistry -     

Nucleophilic character of the tert-butyl radical. Absolute rate constants for the reactions with substituted toluenes

The decay of photochemically generated tert-butyl radicals is studied at 48°C in 11 m- and p-substituted toluenes by time-resolved electron spin resonance spectroscopy. It is governed by the second-order self-termination perturbed by a pseudo-first-order reaction of the radical with the toluenes. The first-order lifetimes yield the rate constants k_A for hydrogen transfer from toluenes to tert-butyl. Substituent effects on the rate constants confirm the nucleophilic character of the radical.     

Determination of propagation and termination rate constants for some methacrylates in their radical polymerizations

Rotating sector determinations of k_p and 2k_t for ten methacrylates undergoing radical polymerization were carried out at 30°C. Ester groups in the monomers were: isopropyl, ethyl, β-cyclohexylethyl, methyl, γ-phenylpropyl, β-phenylethyl, β-methoxyethyl, benzyl, β-chloroethyl, and phenyl. Values of k_p obtained were 121, 126, 1190, 141, 149, 228, 249, 1250, 254, and 411 l./mole-sec., respectively; values of 2k_t × 10^?6 were 4.52, 7.35, 32.8, 11.6, 0.813, 1.88, 9.30, 41.9, 6.71, and 11.9 l./mole-sec., respectively. Omitting the data for the β-cyclohexylethyl and benzyl esters, a Taft correlation, log k_p = (0.70 ± 0.18)σ* + 2.2, was established, where σ* denotes Taft's polar substitutent constants for the above-mentioned ester groups. The steric substituent constants E_s were found to have no influence on k_p. Combination of k_p with r₂ data from copolymerization studies with styrene or methyl methacrylate as M₁ comonomer revealed that the more reactive monomer gave rise to the more reactive polymer radical. Monomer viscosities and molar volumes of the ester groups were found to correlate with 2k_t.     

Temperature dependence of the rate constants for reactions of the carbonate radical with organic and inorganic reductants

Rate constants have been measured by pulse radiolysis for the reactions of the carbonate radical, CO₃^·?, with a number of organic and inorganic reactants as a function of temperature, generally over the range 5 to 80°C. The reactants include the substitution-inert cyano complexes of Fe^II, Mo^IV, and W^IV, the simple inorganic anions SO₃^2?, ClO₂^?, NO₂^?, I^?, and SCN^?, several phenolates, ascorbate, tryptophan, cysteine, cystine, methionine, triethylamine, and allyl alcohol. The measured rate constants ranged from less than 10⁵ to 3 × 10⁹ M^?1 s^?1, the activation energies ranged from ?11.4 to 18.8 kJ mol^?1, and the pre-exponential factors ranged from log A = 6.4 to 10.7. The activation energies for the metal complexes and inorganic anions generally decrease with increasing driving force for the reaction, as expected for an outer sphere electron transfer. For highly exothermic reactions, however, the activation energy appears to increase, probably reflecting the temperature dependence of diffusion. For many of the organic reactants, the activation energies were low and independent of driving force, suggesting that the oxidation is via an inner sphere mechanism.     

Determination of the activation rate constants of alkyl halide initiators for atom transfer radical polymerization

The second-order activation rate constants k_A for low-molar-mass alkyl halides catalyzed by cuprous halide complexes Cu(I)X/2L (X = Cl or Br; L = 4,4′-diheptyl-2,2′-bipyridine) were determined by the nitroxide capping method along with ¹H NMR. The k_A for 1-phenylethyl bromide, a typical initiator for atom transfer radical polymerization (ATRP), with the Cu(I)Br complex was found to be close to the known value of the k_A for a polystyryl bromide, being large enough for the initiation to be completed at an early stage of polymerization. It was also found that k_A strongly depends on the kind of halogen and the steric factor of the alkyl halide in question.     

Ion–molecule reactions by chemical ionization: Determination of rate constants

Tungsten hexacarbonyl was studied by chemical ionization mass spectrometry using carbon monoxide as reagent gas. The variation of the relative abundances of ions as a function of reactant gas pressure was used for evaluating the relative rate constants for consecutive reactions. The results are in agreement with the rate constant ratios obtained from ion trap mass spectrometric data.     

Dibromosilylene: Determination of the absolute rate constants for its gas-phase reactions with oxygen and nitric oxide

Following the newly-discovered UV absorption spectrum of Br₂Si, its reactions with oxygen and nitric oxide in the gas phase have been quantitatively investigated using the flash photolysis-kinetic absorption spectroscopy technique. The room temperature bimolecular rate constants are:      

Absolute rate constants for some hydrogen atom abstraction reactions by a primary fluoroalkyl radical in water

A combination of laser flash photolysis and competitive kinetic methods have been used to measure the absolute bimolecular rate constants for hydrogen atom abstraction in water from a variety of organic substrates including alcohols, ethers, and carboxylic acids by the perfluoroalkyl radical, *CF(2)CF(2)OCF(2)CF(2)SO(3)(-) Na(+). Comparison, where possible, of these rate constants with those previously measured for analogous reactions in the non-polar organic solvent, 1,3-bis(trifluoromethyl)benzene (J. Am. Chem. Soc, 1999, 121, 7335) show that the alcohols react 2-5 times more rapidly in the water solvent and that the ethers react at the same rate in both solvents. A transition state for hydrogen abstraction that is more reminiscent of an "intimate ion pair" than a "solvent separated ion pair" is invoked to explain these modest solvent effects.