Rate constants for the reaction of the bromomethyne radical with alkynes

Absolute rate constants were measured for the gas phase reactions of the CBr($\text{～}\text{X}$²Π) radical produced in the flash photolysis of CHBr₃ with a series of alkynes. The rate of addition to the triple bond followed the trend established for addition to double bonds and a linear correlation between log k and ionization potentials reflects the electrophilic nature of the reaction. In comparison with published data on CCl, S(³P₂) the reactivity follows the trend CBr ≈ CCl ≈ S(³P₂) > O(³P₂).     

Rate constants for anilidyl radical cyclization reactions

[reaction: see text] N-Aryl-5,5-diphenyl-4-pentenamidyl radicals (3) were produced by 266 nm laser-flash photolysis of the corresponding N-(phenylthio) derivatives, and the rate constants for the cyclizations of these radicals were measured directly. The 5-exo cyclization reactions were fast (k(c) > 2 x 10(5) s(-1)), and radicals 3 generally behaved as electrophilic reactants with a Hammett correlation of rho = 1.9 for five of the six radicals studied. However, the p-methoxyphenyl-substituted radical 3f cyclized much faster than expected from the Hammett analysis. Variable temperature studies of parent radical 3a (aryl = phenyl) gave an Arrhenius function with log k = 9.2 - 4.4/2.3RT (kcal/mol). The rate constant for the reaction of p-ethylphenyl-substituted anilidyl radical 3b with Bu(3)SnH at 65 degrees C was k(T) = 4 x 10(5) M(-1) s(-1).     

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.     

Rate constants for some hydrogen abstraction reactions of the carbonate radical

Rate constants have been measured in aqueous solutions for the reactions of the carbonate radical, CO₃^˙?, with several saturated alcohols and one cyclic ether as a function of temperature. Arrhenius pre-exponential factors ranged from 2×10⁸ to 1×10⁹ ?? mol^?1 s^?1 and activation energies ranged from 16 to 29 kJ mol^?1. The results suggest that the reactions are not pure hydrogen abstraction, but involve an additional interaction of the radical with the ? OH or ? O? linkage. © 1993 John Wiley & Sons, Inc.     

Rate constants for the gas-phase reactions of selected dibasic esters with the OH radical

Using a relative rate method, rate constants have been measured for the gas-phase reactions of the OH radical with the dibasic esters dimethyl succinate [CH₃OC(O)CH₂CH₂C(O)OCH₃], dimethyl glutarate [CH₃OC(O)CH₂CH₂CH₂C(O)OCH₃], and dimethyl adipate [CH₃OC(O)CH₂CH₂CH₂CH₂C(O)OCH₃] at 298±3 K. The rate constants obtained were (in units of 10⁻¹² cm³ molecule⁻¹ s⁻¹): dimethyl succinate, 1.4±0.6; dimethyl glutarate, 3.3±1.1; and dimethyl adipate, 8.4±2.5, where the indicated errors include the estimated overall uncertainty of ±25% in the rate constant for cyclohexane, the reference compound. The calculated tropospheric lifetimes of these dibasic esters due to gas-phase reaction with the OH radical range from 1.4 days for dimethyl adipate to 8.3 days for dimethyl succinate for a 24 h average OH radical concentration of 1.0×10⁶ molecule cm⁻³. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet: 30: 471–474, 1998     

Geometric parameters of the transition state in radical reactions of antioxidants

An algorithm for calculating interatomic distances in a transition state of radical abstraction reactions through the enthalpy of a reaction is developed. The algorithm is based on a combination of quantum-chemical calculations with the calculation by the intersecting-parabolas method. Using this method and experimental data (enthalpies and activation energies of reactions), interatomic distances in a reaction center are calculated for the reactions of antioxidants with a symmetrical transition state of the N...H...N and O...H...O types and a nonsymmetrical transition state of the N...H...C, O...H...C, and O...H...N types. These distances are compared with other characteristics of the reactions and reactants. The influence of such factors as the enthalpy of the reaction, triplet repulsion in the transition state, electronegativity of atoms in the reaction center, and steric repulsion of reactants on the geometric parameters of the transition state is examined. The results obtained are used to calculate the increments characterizing the influence of various factors on interatomic distances of radical reactions considered.Translated from Kinetika i Kataliz, Vol. 45, No. 6, 2004, pp. 877–896.Original Russian Text Copyright © 2004 by Denisova, Denisov.     

Kinetic and thermodynamic parameters for the equilibrium reactions of phenols with the dpph. radical

The kinetics and energetics of the reversible reaction of phenols with the dpph. radical have been studied; steric shielding of the divalent N by the o-NO2 in dpph. seems to be the main cause of the entropic barriers of this reaction.     

Rate constants of OH radical reactions in gas phase with some fluorinated compounds: A correlation with molecular parameters

For a number of hydrofluorocarbons (HFCs), E_He^z has been found to have a linear correlation with each of the following: (i) log (k/n); (ii) A/n; and (iii) E_a/R, where E_H = HOMO energy of the molecule, z = average fractional positive charge on the abstractable hydrogen atom in the molecule, k = rate constant of the gas-phase H abstraction reaction of the molecule with OH radical at 298 K, n = number of abstractable H atoms in the molecule, A = preexponential factor, and E_a/R = activation temperature of the said reaction. These correlations have been used to estimate the temperature dependent rate constants for the reactions of OH radical with CF₃CF₂CH₂CH₂CF₂CF₃, CF₃CH₂CF₂CH₂CF₃, CF₃CF₂CH₂CH₂F, CF₃CH₂CH₃, CF₃CH₂CHF₂, CF₃CHFCH₂F, and CHF₂CHFCHF₂ as {6.97 × 10⁻¹³ exp(1481/T)}, {5.43 × 10⁻¹³ exp(1754/T)}, {7.95 × 10⁻¹³ exp(l308/T)}, {8.0 × 10⁻¹³ exp(1300/T)}, {7.03 × 10⁻¹³ exp(1470/T)}, {7.33 × 10⁻¹³ exp(1417/T)}, and {8.09 × 10⁻¹³ exp(1285/T)}, respectively. These have not yet been measured experimentally. Linear correlation between E_He^z and log (k/n) has also been observed for nine halogen substituted acetaldehydes. On the other hand, E_H is found to have a better linear correlation with log (k/n) than E_He^z in the case of fluorinated ethers and alcohols where the available experimental data are at present limited. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 187–194, 1997.     

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).     

Kinetic parameters of radical reactions of 2-mercaptobenzothiazole with quinone imines

The chain reaction of N,N′-diphenyl-1,4-benzoquinone diimine with 2-mercaptobenzothiazole was studied by two methods developed earlier for the nonchain reaction of N-phenyl-1,4-benzoquinone monoimine with 2-mercaptobenzothiazole. In the methods used, the kinetic scheme of the reaction is simplified by creating conditions under which the rates of all stages except radical generation and decay can be neglected. One of the methods was updated. For the nonchain reaction of N-phenyl-1,4-benzoquinone monoimine with 2-mercaptobenzothiazole, both methods gave close results; for the chain reaction of N,N′-diphenyl-1,4-phenylenediamine with 2-mercaptobenzothiazole, the results differed by approximately one order of magnitude.     

Rate constants for concurring radical reactions in solution obtained by kinetic electron spin resonance

It is shown how kinetic electron spin resonance spectroscopy with intermittent radical generation can be used to obtain rate constants of various simultaneous reactions in systems containing more than one kind of transient radicals. The technique is applied to reactions of tert-butyl [(CH₃)₃?] and isopropylol [(CH₃)₂?OH] radicals generated by photolysis of di-tert-butyl ketone and acetone in 2-propanol/acetone mixtures. It yields the rates of generation of the two radicals, the rate constants for their self- and crossterminations and for the reaction of tert-butyl with 2-propanol. The extent of diffusion control of the termination constants is discussed.     

Rate constants for hydrogen abstraction reactions of the sulfate radical,SO4−. Alcohols

Rate constants have been determined for the reactions of SO₄^? with a series of alcohols, including hydrated formaldehyde. The SO₄^? radical was produced by the laser-flash photolysis of persulfate, S₂O₈^2?. Rate constants for the reactions of SO₄^? with alcohols range from 1.0 × 10⁷ for methanol to 3.4 × 10⁸ M^?1 s^?1 for 1-octanol. Rate constants for the reactions of SO₄^? with deuterated methanol and ethanol are lower by about a factor of 2.5. For methanol, ethanol, and 2-propanol, the temperature dependence of the rate constant was determined over the range 10–45°C.     

Rate constants for silylene reactions

It is only since 1985 that the absolute rate constanss have been measured for some reactions of divalent silylene species. In this article the absolute rate constant data reported to date for the reactions of SiH₂, SiMe₂, SiMePh, SiHCl, SiCl₂, SiF₂ and SiBr₂ are reviewed and, where possible, mechanistic pathways discussed. The reactivity of silylenes is, in general, much higher than had previously been estimated on the basis of relative rate studies.     

Substituent parameters for radical reactions Cerium(IV) — substituted toluenes — acrylonitrile system

Summary In the present study, a new approach for the determination of ', that is, radical substituent parameter, has been described. The technique is based on two equations, one reported byYamamoto andOtsu and the other byStreitwieser andPerrin. The system used for the determination of the radical substituent parameter (') was Ce(IV) — substituted toluenes — acrylonitrile system. To explain inhibition, it has been suggested that the reaction scheme involves initiation of polymerization by Ce(IV) and termination by primary radical of the substrate. The kinetic expression based on the reaction scheme formulated could explain the unusual observation of a negative intercept found in the plot of [M]/R _p versus 1/[M]. Whereas a plot of log /₀ against ⁺ did not give a straight line, a good linear plot with zero intercept was obtained when log /₀ was plotted against (⁺ + '). This proves the validity of ' values which have been determinded.     

Stereoselective radical reactions

For many years, the reputed lack of selectivity in some free-radical reactions has resulted in this class of reaction being overlooked in the stereoselective synthesis of important target molecules. More recently, however, the situation has changed as new and milder methods of radical generation have been developed, which have helped researchers to gain a better understanding of the key factors that influence selectivity in radical transformations. As a consequence, the use of radicals in stereoselective synthesis is increasing and there are a number of important intra- and intermolecular additions, where high levels of stereoselectivity have been achieved in the formation of carbon-carbon bonds.     

Substituent parameters for radical reactions: Cerium(IV)-substituted acetophenones–acrylonitrile system

The present investigation describes a new approach for the determination of radical substituent parameters σ. The technique has been developed by an ingenious combination of two equations, one proposed by Yamamoto and Otsu and the other by Streitwieser and Perrin. The σ values for different substituents were first evaluated by application of this newly developed technique to the Ce(IV)-substituted toluenes–acrylonitrile system. The validity of these values was then tested by application to another system, the Ce(IV)-substituted acetophenones–acrylonitrile system, which is the subject of the present study.