Reactions of tert-butoxy radicals with vinyl monomers

Tert-butoxy radicals generated in the photolysis of di-tert-butyl peroxide in benzene at 25°C react with vinyl monomers by double-bond addition and, in most cases, also by competitive hydrogen abstraction. The rate constant for the double-bond addition changes by a factor of 17 between isoprene, which shows the highest reactivity, and the methacrylate derivatives, which are the least reactive of the monomers considered. The fraction of tert-butoxy radicals that react by hydrogen abstraction varies considerably with the monomer structure, ranging from 0.9 (cyclohexyl methacrylate) to less than 0.05 (styrene and conjugated diolefins). In the methacrylate derivatives, most of the hydrogen abstraction takes place, not in the α-methyl group, but in the alkyl chain.     

On the reactivity of diethyl hydroxyl amine toward free radicals

Diethyl hydroxyl amine is an efficient trap for alkyl, alkoxy, and peroxy radicals. The specific rate constant for the reaction of ethyl radicals (gas phase, 25°C), tert-butoxy radicals (benzene solution, 115°C), and poly (peroxystyryl) peroxy radicals (styrene solution, 50°C) were evaluated as 7.2 × 10⁵, 7.7 × 10⁷, and 2.9 × 10⁵ M^?1·sec^?1, respectively. Several possible secondary reactions of the nitroxide radicals are discussed.     

On the mechanism of reaction of tert-butoxyl radicals with dialkylphosphites

It has been shown by ESR spectroscopy that the title reaction involves abstraction of hydrogen from the phosphite, since at ?10°C the reaction has a kinetic deuterium isotope effect, k_H/k_D, or ～3. The rate constant for hydrogen abstraction is c. 2 × 10⁴ M^?1 s^?1. There is no significant addition of alkoxyl radicals to the phosphite.     

Effects of diffusion on the self-termination kinetics of isopropylol radicals in solution

Rate constants for the bimolecular self-reaction of isopropylol radicals [(CH₃)₂?OH] in various solvents are determined as functions of temperature by kinetic electron spin resonance. For hydrocarbon solvents they are well described by theoretical equations for reactions controlled by translational diffusion if diffusion coefficients of 2-propanol, a constant reaction distance, and a spin statistical factor of 1/4 are applied. Deviations from 2k_t ～ D at high diffusion constants agree with trends expected from recent theoretical models. For hydrogen-bonding solvents large negative deviations are observed. They are attributed to steric constraints and slower rotational diffusion of radical–solvent aggregates. The disproportionation-to-combination ratio of isopropylol increases with solvent viscosity. As previously for tert-butyl, this is explained by anisotropic reorientation during encounters. Further, rate data are given for the decarbonylation of the 2-hydroxy-2-methylpropanoyl radical and for several hydrogen abstraction reactions of isopropylol.     

Kinetics and rate constants for the reaction of tri-n-butylgermanium hydride with methyl iodide and carbon tetrachloride. The combination of methyl and trichloromethyl radicals in solution.

The kinetics of the photoinitiated reductions of methyl iodide and carbon tetrachloride by tri-n-butylgermanium hydride in cyclohexane at 25°C have been studied and absolute rate constants have been measured. Rate constants for the combination of CH₃? and CCl₃? radicals are equal within experimental error and are also equal to the values found for the self-reactions of most non-polymeric radicals in low viscosity solvents, i.e. ～1–3 × 10⁹ M^?1 sec^?1. Rate constants for hydrogen atom abstraction by CH₃? and CCl₃? radicals are both ～1?2 × 10⁵ M^?1 sec^?1. Tri-n-butyltin hydride is about 10–20 times as good a hydrogen donor to alkyl radicals as is tri-n-butylgermanium hydride. The strength of the germanium–hydrogen bond, D(n-Bu₃Ge–H) is estimated to be approximately 84 kcal/mole.     

A spectrokinetic study of CH2I and CH2IO2 radicals

The UV absorption spectrum and kinetics of CH₂I and CH₂IO₂ radicals have been studied in the gasphase at 295 K using a pulse radiolysis UV absorption spectroscopic technique. UV absorption spectra of CH₂I and CH₂IO₂ radicals were quantified in the range 220–400 nm. The spectrum of CH₂I has absorption maxima at 280 nm and 337.5 nm. The absorption cross-section for the CH₂I radicals at 337.5 nm was (4.1 ± 0.9) × 10^?18 cm² molecule^?1. The UV spectrum of CH₂IO₂ radicals is broad. The absorption cross-section at 370 nm was (2.1 ± 0.5) × 10^?18 cm² molecule^?1. The rate constant for the self reaction of CH₂I radicals, k = 4 × 10^?11 cm³ molecule^?1 s^?1 at 1000 mbar total pressure of SF₆, was derived by kinetic modelling of experimental absorbance transients. The observed self-reaction rate constant for CH₂IO₂ radicals was estimated also by modelling to k = 9 × 10^?11 cm³ molecule^?1 s^?1. As part of this work a rate constant of (2.0 ± 0.3) × 10^?10 cm³ molecule^?1 s^?1 was measured for the reaction of F atoms with CH₃I. The branching ratios of this reaction for abstraction of an I atom and a H atom were determined to (64 ± 6)% and (36 ± 6)%, respectively. © 1994 John Wiley & Sons, Inc.     

Incubation period in the 2,2,4,4‐tetramethyl‐1‐piperidinyloxy‐mediated thermal autopolymerization of styrene: Kinetics and simulations

Mechanisms and simulations of the induction period and the initial polymerization stages in the nitroxide‐mediated autopolymerization of styrene are discussed. At 120–125 °C and moderate 2,2,4,4‐tetramethyl‐1‐piperidinyloxy (TEMPO) concentrations (0.02–0.08 M), the main source of radicals is the hydrogen abstraction of the Mayo dimer by TEMPO [with the kinetic constant of hydrogen abstraction (k_h)]. At higher TEMPO concentrations ([N?] > 0.1 M), this reaction is still dominant, but radical generation by the direct attack against styrene by TEMPO, with kinetic constant of addition k_ad, also becomes relevant. From previous experimental data and simulations, initial estimates of k_h ≈ 1 and k_ad ≈ 6 × 10^?7 L mol^?1 s^?1 are obtained at 125 °C. From the induction period to the polymerization regime, there is an abrupt change in the dominant mechanism generating radicals because of the sudden decrease in the nitroxide radicals. Under induction‐period conditions, the simulations confirm the validity of the quasi‐steady‐state assumption (QSSA) for the Mayo dimer in this regime; however, after the induction period, the QSSA for the dimer is not valid, and this brings into question the scientific basis of the well‐known expression k_th[M]³ (where [M] is the monomer concentration and k_th is the kinetic constant of autoinitiation) for the autoinitiation rate in styrene polymerization. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6962‐6979, 2006     

Polymerization of vinyl acetate retarded by 4-methyl-2,6-di-tert-butylphenol: Kinetic and ESR studies

4-Methyl-2,6-di-tert-butylphenol strongly retards the free radical polymerization of vinyl acetate initiated by azobisisobutyronitrile. The chain transfer constant, estimated from rate data, is 0.020 ± 0.004 at 35°C and does not vary significantly with temperature. Molecular weight data lead to transfer constants of 0.023, 0.020, and 0.024 at 35, 45, and 55°C, respectively. A mean kinetic isotope effect of 9.8 ± 1.0 is observed for the phenol deuterated at the OH group, showing that the main attack of poly(vinyl acetate) radicals on the phenol involves hydrogen abstraction from this group. The activation energy for hydrogen abstraction is estimated to be 7.8 kcal/mole, and the rate constant at 50°C is 160 ± 40 1./mole-sec. The stationary concentration of 4-methyl-2,6-di-tert-butylphenoxyl in the polymerization mixture is proportional to the phenol concentration and is independent of the initiator concentration, as shown by electron spin resonance studies. Cross termination of poly(vinyl acetate) and phenoxy radicals occurs to a greater extent than mutual termination of these radicals. The rate constant for cross termination is close to 1 × 10⁸ 1./mole-sec at 50°C; the activation energy for cross termination is 2.9 ± 1.3 kcal/mole.     

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.     

Mechanisms for the Breakdown of Halomethanes through Reactions with Ground‐State Cyano Radicals

One route to break down halomethanes is through reactions with radical species. The capability of the artificial force‐induced reaction algorithm to efficiently explore a large number of radical reaction pathways has been illustrated for reactions between haloalkanes (CX₃Y; X=H, F; Y=Cl, Br) and ground‐state (²Σ⁺) cyano radicals (CN). For CH₃Cl+CN, 71 stationary points in eight different pathways have been located and, in agreement with experiment, the highest rate constant (10⁸ s^?1 M ^?1 at 298 K) is obtained for hydrogen abstraction. For CH₃Br, the rate constants for hydrogen and halogen abstraction are similar (10⁹ s^?1 M ^?1), whereas replacing hydrogen with fluorine eliminates the hydrogen‐abstraction route and decreases the rate constants for halogen abstraction by 2–3 orders of magnitude. The detailed mapping of stationary points allows accurate calculations of product distributions, and the encouraging rate constants should motivate future studies with other radicals.     

Reactivity of monoenic and conjugated diene systems present in unsaturated olefin terpolymers—I. Reactions with t-butoxy radicals (CH3)3 CO•

The relative rate constants for the hydrogen abstraction and the double bond addition reactions of t-butoxy radicals (CH₃)₃ CO^? with the model compounds 5-ethylidennorbornane (I), dihydrodicyclopentene (II), isopropylidendicyclopentene (III) and methylcyclopentadienylnorbornylmethane (IV) have been determined by using as a reference reaction the hydrogen abstraction for iso-octane. With (I), (III) and (IV) the predominant process is the hydrogen abstraction, whilst for (II) both mechanisms are important. The results have been applied for the elucidation of some aspects of the initiating mechanism of peroxide-induced cross-linking of EPDM and EPTM terpolymers containing (I)-(IV) pendants.     

The reaction of acrolein with acetylperoxyl radicals in the gas‐phase

A rate constant for the epoxidation of acrolein by acetylperoxyl radicals has been determined to be k₄ = (1.3 ± 0.9) × 10⁴ dm³mol⁻¹s⁻¹ at 383 K, which is anomalously fast in comparison with the epoxidation of alkenes. Abstraction of the acyl hydrogen atom from acrolein by acetylperoxyl radicals at 393 K was found to be at least 60 times slower than from acetaldehyde and at least three orders of magnitude slower than abstraction of the acyl hydrogen atom of the epoxide of acrolein. The fast rate for epoxidation of acrolein and the slow rate for hydrogen abstraction provide an explanation for the anomalously slow rate for the autoxidation of acrolein and suggests that acrolein formed during the autoxidation of alkene will react further to give its epoxide, and not exclusively by abstraction of the acyl hydrogen atom as was previously accepted. © 1999 John Wiley & Sons, Inc., Int J Chem Kinet 31: 277–282, 1999     

An absolute measurement of the rate constant for t-butyl radical combination

The rate constant for tert-butyl radical recombination has been measured near 700°K by the very-low-pressure pyrolysis (VLPP) technique and was found to be 10^8.8±0.3 M^?1·sec^?1 with neglibible temperature dependence. The thermochemical parameters for tert? butyl radicals were varied within reasonable limits to bring into agreement the data for the decomposition of 2,2,3,3-tetramethyl butane and the recombination of tert-butyl radicals. The revised thermochemistry also makes the gas-phase results and liquid-phase results compatible.     

Liquid-phase autoxidation of organic compounds at elevated temperatures. Absolute rate constant for intermolecular hydrogen abstraction in hexadecane autoxidation at 120–190°C

The absolute rate constants for the intermolecular hydrogen abstraction reactions of secondary hydrogens by secondary alkylperoxy radicals in hexadecane autoxidation, k₃, have been determined in the temperature range of 120–190°C using the stirred flow reactor technique. Absolute rate constants determined in this study for hexadecane are in good agreement with those determined for other hydrocarbons in liquid phase, on a per hydrogen basis, at lower temperatures. Arrhenius parameters for k₃/H derived from this study are A = 10^8.6 M^?1 s^?1 and E_a = 16.0 kcal/mol. The values of these parameters provide experimental confirmation for previous estimates made from both lower temperature reactions in the liquid phase and higher temperature reactions in the gas phase. © 1994 John Wiley & Sons, Inc.     

The free-radical reaction of tri-n-butyltin hydride with disulfides

The kinetics and absolute rate constants for the free-radical chain reaction of tri-n-butyltin hydride with di-t-butyl disulfide have been measured in cyclohexane at 30°. The rate controlling step for chain propagation involves the cleavage of the disulfide bond by an attacking tributyltin radical. The rate constant for this bimolecular homolytic substitution at sulfur is ～8 × 10⁴ Mole^?1 sec^?1. Chain termination involves the self-reaction of two tributyltin radicals. The rate constants for attack of tributyltin radicals on some other disulfides and on elemental sulfur have also been measured. The results are compared with literature data for homolytic substitutions on these compounds by a variety of radicals which have their unpaired electron centered on carbon.     

Effect of polymer structure on ease of hydrogen abstraction by cumyloxy radicals

Vulcanizates of five elastomers having different chemical structures were prepared with recipes comprising dicumyl peroxide. Cumyl alcohol/acetophenone molar ratios were determined by gas–liquid chromatography (GLC) on acetone extracts of vulcanizates. Diphenyl ether was used as the internal standard. The ease of hydrogen abstraction by cumyloxy radicals, reflected by the cumyl alcohol/acetophenone ratio, decreases in the following order: cis-1,4-polyisoprene > poly(propylene oxide) > poly(vinyl n-butyl ether) > poly-1-heptene ? EPR. This order is in qualitative agreement with the order based on calculated relative rates of hydrogen abstraction by methyl and tert-butoxy radicals and literature data on oxygen absorption of some of these polymers as a result of hydrogen abstraction by peroxy radicals.     

The mechanism of cooxidation of isobutyraldehyde and octene-2

The mechanism of isobutyraldehyde-octene-2 cooxidation at 20°C has been investigated. The ratio of cis to trans epoxides in the reaction products shows that, at aldehyde concentrations lower than 1.0M, the epoxide is formed mainly by a radical route. The difference in the ΔH of formation of cis and trans epoxides is around 0.8 kcal/mole at 20°. The isobutyraldehyde involved in the radical epoxidation chain has been found almost quantitatively to be isopropylhydroperoxide, which is formed through the decarboxylation of i-PrCO₂· radicals, addition of oxygen, and abstraction of hydrogen atoms from the aldehyde. A rate constant of about 14 M^?1 sec^?1 at 20° has been determined for the latter reaction. The chain length for the cooxdination reaction decreases from 75 to 20 as the isobutyraldehyde concentration goes from 1.0 to 0.3M. The termination step seems to involve mainly the interaction of two i-PrO₂ · radicals. The cooxidation of octene-2 with pivalaldehyde follows a similar mechanism, but the chain length is about ten times higher under the same experimental conditions.     

Rate constants for hydrogen abstraction by resonance stabilized radicals in high temperature liquids

Benzylic H-atom abstraction rates by diphenylmethyl radicals from a series of donors were determined in nonpolar liquids at elevated temperatures. Relative rates were converted to absolute rates via available equilibrium constant data for the dimerization of diphenylmethyl radicals. Abstraction by diphenylmethyl from 1, 2, 3, 4-tetrahydronaphthalene (tetralin) was studied over the temperature range 489–573 K. Its Arrhenius expression is 10^9.9±0.3 exp{?(10183 ± 373)/T} M^?1 s^?1. Abstraction from other donors was studied at 548 K. Rate constant values ranged from a low of 3.6 M^?1 s^?1 for toluene to a high of 3000 M^?1 s^?1 for 9, 10-dihydroanthracene. Similar reactions with the fluorenyl radical were also studied. In this case, relative rates were converted to absolute rates with an equilibrium constant for fluorenyl dimerization determined from the observed homolysis rate of the dimer and an assumed recombination rate. In addition, forward and reverse rate measurements yielded the equilibrium constant for hydrogen transfer between fluorenyl and diphenylmethyl. At 548 K, fluorenyl is favored by a factor of 13 over diphenylmethyl.     

Insight into the Mechanism of the Initial Reaction of an OH Radical with DNA/RNA Nucleobases: A Computational Investigation of Radiation Damage

Earlier theoretical investigations of the mechanism of radiation damage to DNA/RNA nucleobases have claimed OH radical addition as the dominating pathway based solely on energetics. In this study we supplement calculations of energies with the kinetics of all possible reactions with the OH radical through hydrogen abstraction and OH radical addition onto carbon sites, using DFT at the ωB97X‐D/6‐311++G(2df,2pd) level with the Eckart tunneling correction. The overall rate constants for the reaction with adenine, guanine, thymine, and uracil are found to be 2.17×10^?12, 5.64×10^?11, 2.01×10^?11, and 5.03×10^?12 cm³ molecules^?1 s^?1, respectively, which agree exceptionally well with experimental values. We conclude that abstraction of the amine group hydrogen atoms competes with addition onto C₈ as the most important reaction pathway for the purine nucleobases, while for the pyrimidine nucleobases addition onto C₅ and C₆ competes with the abstraction of H₁. Thymine shows favourability against abstraction of methyl hydrogens as the dominating pathway based on rate constants. These mechanistic conclusions are partly explained by an analysis of the electrostatic potential together with HOMO and LUMO orbitals of the nucleobases.     

Gas phase kinetic study of the thermal reactions of t-butoxy radicals with germane,phosphine, and trimethylsilane

The hydrogen abstraction reactions of tert-butoxyl with germane, phosphine, and trimethylsilane were studied by pyrolyzing di-t-butyl peroxide in the temperature range of 403 to 458 K. From the reported rate constant for the tert-butoxy radical decomposition in the literature, the absolute rate constants were determined from the study of competitive reaction between t-butoxy radical decomposition and hydrogen abstraction reaction. (t-BuO + RH → t-BuOH + R, where RH = GeH₄, PH₃, and HSiMe₃) The Arrhenius parameters are found to be as follows: