Influence of substituents on rate constants for recombination of secondary and primary peroxy radicals

A linear correlation has been found between the logarithm of rate constants (2k_t) for the recombination of secondary peroxy radicals (ROO) and the constants of substituents, R:1g 2k_t=(6.84±0.03)+(2.60±0.10) . Rate constant for the recombination of primary peroxy radicals proves to be independent of the substituent nature: 1g 2k_t=8.5±0.4.

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(2k_t) (ROO) R: 1g 2k_t=(6,84±0,03)+(2,60±0,10) . , 1g 2k_t=8,5±0,4.

     

Determination of the rate constants of the reaction of pentaerythritol tetravalerate peroxy radicals with polyfunctional phosphites at high temperature

The effectiveness of a number of aromatic phosphites as inhibitors of the high-temperature autooxidation of pentaerythritol tetravalerate was studied, and the rate constants of the reaction of its peroxide radicals with aromatic phosphites at 200°C were determinedInstitute of Chemistry, Bashkir Science Center, Urals Branch of the Russian Academy of Sciences, 450054 Ufa. Institute of Organic Chemistry, Academy of Sciences of the Uk-raine, 252000 Kiev. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 2, pp. 289–291, February, 1992.     

Collision between polymeric radicals for termination rate constants

The motion of each polymeric radical during a collision between the polymeric radicals with the same radius is treated as completely random motion. The result obtained is: k_t = 0.250k_s (where k_t is the chain-termination rate constant and k_s is the reaction rate constant between radical chain ends). On taking the motion of the primary radical during a collision between a primary radical and a large polymeric radical to be completely random, the result obtained is: k_ti = 0.250k_si (where k_ti is the primary radical termination rate constant and k_si is the reaction rate constant between primary radical and radical chain end). On substituting k_s for k_si in the second equation, the rate constant obtained becomes the chain termination rate constant between the very small polymeric radical and the very large polymeric radical, and identical to the former equation. This identity indicates that the effect of the difference of the size of the polymeric radicals on the collision process relating to the chain termination rate constant should not be large.     

Methyl-and tert-butyl-substituted hydroquinones and semiquinone radicals: Bond strength estimates, enthalpy of formation, and the rate constants of their reactions with peroxy radicals

The strength of the O-H bonds (D) in hydroquinone (HQH) and its alkyl derivatives has been estimated by the intersecting parabolas method using rate constants known for the reactions of these compounds with the styrene peroxy radical. For unsubstituted HQH, D = 352.6 kJ/mol; for substituted HQH derivatives, D = 349.9 (Me), 346.9 (2,5-Me₂), 343.0 (Me₃), 347.6 (CMe₃), and 340.2 (2,5-(CMe₃)₂) kJ/mol. The enthalpies of formation of these HQH derivatives have been calculated. The O-H bond strengths in the semiquinone radicals (HQ^.) resulting from the above HQH derivatives have been calculated using a thermochemical equation to be $D_{HQ^. } $ = 236.7, 237.4, 239.8, 244.7, 240.1, and 247.5 kJ/mol, respectively. Rate constants have been determined for the reactions of the hydroquinones with tertiary and secondary peroxy radicals and HOO^. at 323 K. The rate constants of the reactions between HOO^. and benzoquinones and the relative reactivities of the HQ^. radicals in their reactions with ROO^. have been estimated.     

Determination of the rate constants of recombination of macroradicals and stable radicals via the competitive-inhibition method

The rate constants of recombination, k _X, of propagating radicals with nitroxides in pseudoliving radical polymerization are determined via the competitive-inhibition method with the use of ESR spectroscopy. This method is applicable to determination of k _X in the reactions of propagating radicals of styrene, acrylic acid, and methyl methacrylate with two stable radicals, the nitroxide diethylphosphono-2,2-dimethylpropyl nitroxide and the phenoxide galvinoxyl. The values of k _X determined at 50°C increase in the following sequence: diethylphosphono-2,2-dimethylpropyl nitroxide-TEMPO-galvinoxyl. The selectivity of the low-activity propagating radicals of styrene in reactions with stable radicals is shown.     

Spectra of propylperoxy radicals and rate constants for mutual interaction

The flash photolyses of azo-n-propane and azoisopropane in the presence of oxygen have been studied by kinetic spectroscopy. The transient absorption spectra observed in the region of 210–290 nm are assigned to the n-propylperoxy and isopropylperoxy radicals. For the n-propylperoxy radical, ε_max = 1148 ± 29 L/mol cm at 242.5 nm and for the isopropylperoxy radical, ε_max = 1273 ± 75 L/mol cm at 240 nm. The rate constants for the mutual reactions (7) 2RO₂· → products were measured to be k₇ = (2.0 ± 0.2) X 10⁸ L/mol s for the n-propylperoxy radical and k₇ = (7.8 ± 2.2) X 10⁵ L/mol s for the isopropylperoxy radical.     

First rate constants for reactions of OH radicals with amides

Rate constants have been measured for the reaction of OH radicals with four amides, R₁N(CH₃)—C(O)R₂ (R₁ = H or Methyl, R₂ = Methyl or Ethyl), at 300 and 384 K using flash photolysis/resonance fluorescence. Reactants are introduced under slow flow conditions and are controlled by two independent methods, gas saturation and continuous injection. It turns out that the reactivities of the amides are considerably lower than those of the corresponding amines. The pattern of rate constants obtained at 300 K: 14, 21, 5.2, and 7.6 · 10⁻¹² cm³/s for N,N-Dimethylacetamide (dmaa), N,N-Dimethylpropionamide (dmpa), N-Methylacetamide (maa), and N-Methylpropionamide (mpa), respectively, indicates a single, dominating reaction center and strong electronic effects of the substituents at both sides of the amide function. Correspondingly, the observed negative temperature dependence (E/R = − 400 to − 600 K) excludes a direct abstraction mechanism. © 1997 John Wiley & Sons, Inc.     

Chemiluminescence simulation method for the evaluation of the termination rate constant for 2-cyanopropyl peroxy and α-phenyl-ethyl peroxy radicals

An iterative method has been devised for the simulation of chemiluminescence data during the oxidative decomposition of αα′ azobisisobutyronitrile in the presence of ethylbenzene. From this simulation the cross termination rate constant of the two types of peroxy radicals present has been estimated.     

Mechanism and rate constants for the decomposition of 1-pentenyl radicals

This paper is concerned with the mechanisms and rate constants for the decomposition of 1-penten-3-yl, 1-penten-4-yl, and 1-penten-5-yl radicals. They are formed from radical attack on 1-pentene, which is an important decomposition product of normal alkyl radicals with more than 6 carbon atoms in combustion systems. This work is based on related data in the literature. These involve rate constants for the reverse radical addition process under high-pressure conditions, chemical activation experiments, and more recent direct studies. The high-pressure rate constants are based on detailed balance. The energy transfer effects and the pressure dependences of the rate constants are determined through the solution of the master equation and are projected to cover combustion conditions. The low barriers to these reactions make it necessary to treat these thermal reactions as open systems, as in chemical activation studies. The multiple reaction channels make the nature of the pressure effects different from those usually described in standard texts. The order of stability is 1-penten-3-yl approximately 1-penten-4-yl > 1-penten-5-yl and straddles those for the n-alkyl radicals. A key feature in these reactions is the effects traceable to allylic resonance. However, the 50 kJ/mol allylic resonance energy is not fully manifested. The important unsaturated products are 1,3-butadiene, the pentadienes, allyl radicals, and vinyl radicals. The results are compared with the recommendations in the literature, and significant differences are noted. Extensions to larger radicals with similar structures are discussed.     

Direct measurements of the addition and recombination of acrylate radicals: Access to propagation and termination rate constants?

Acrylate radicals produced by the addition of an aminoalkyl radical to five acrylate monomers were directly observed by transient absorption spectroscopy, which allowed us to easily follow their chemical reactivity. It was possible (1) to characterize their absorption in the visible part of the spectrum, (2) to calculate their absorption properties, (3) to determine the energy barriers of the addition through quantum mechanical calculations, (4) to monitor the kinetics of the subsequent addition to another monomer unit, and (5) to follow the recombination of two acrylate radicals. These two latter points could mimic the propagation and termination reactions of polymerization‐propagating acrylate radicals. Methacrylate and acrylonitrile radicals were also studied. The obtained results were in good agreement with the propagation rate constants determined by the well‐established pulsed laser polymerization techniques. Our method could likely provide rapid access to both the propagation and termination rate constants in suitable systems and appears to be powerful and promising for studying and comparing the reactivities of different acrylate monomer structures. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3577–3587, 2006     

Tertiary aminomethylphenols and methylene bisphenols with isobornyl substituents in the reaction with diphenylpicrylhydrazyl and peroxy radicals in ethylbenzene

The effect of the structure of aminomethylphenols and methylene bisphenols with isobornyl substituents on their reactivity in interactions with peroxy radicals in ethylbenzene and with 1,1-diphenyl-2-picrylhydrazyl (DPPH) is studied. Isobornylphenols with o-aminomethyl substituents, as opposed to p-aminomethyl derivatives, were found to possess rather low activity in the initiated oxidation of ethylbenzene, due to the formation of intramolecular hydrogen bonds between the hydrogen atom of the OH group and the nitrogen atom of the aminomethyl substituent. An increase in activity of o-aminomethyl-substituted phenols with increasing polarity of the medium is observed in the reaction with DPPH. The reaction rate constants for the interaction between two isomeric 2,2′- and 4,4′-methylene-bisphenols having isobornyl moieties and ethylbenzene peroxy radicals are measured. The ratio between activities of the first and second OH groups in 2,2-methylene-bisphenol is shown to be close to 50.     

Branching ratios for the reaction of selected carbonyl-containing peroxy radicals with hydroperoxy radicals

An important chemical sink for organic peroxy radicals (RO(2)) in the troposphere is reaction with hydroperoxy radicals (HO(2)). Although this reaction is typically assumed to form hydroperoxides as the major products (R1a), acetyl peroxy radicals and acetonyl peroxy radicals have been shown to undergo other reactions (R1b) and (R1c) with substantial branching ratios: RO(2) + HO(2) → ROOH + O(2) (R1a), RO(2) + HO(2) → ROH + O(3) (R1b), RO(2) + HO(2) → RO + OH + O(2) (R1c). Theoretical work suggests that reactions (R1b) and (R1c) may be a general feature of acyl peroxy and α-carbonyl peroxy radicals. In this work, branching ratios for R1a-R1c were derived for six carbonyl-containing peroxy radicals: C(2)H(5)C(O)O(2), C(3)H(7)C(O)O(2), CH(3)C(O)CH(2)O(2), CH(3)C(O)CH(O(2))CH(3), CH(2)ClCH(O(2))C(O)CH(3), and CH(2)ClC(CH(3))(O(2))CHO. Branching ratios for reactions of Cl-atoms with butanal, butanone, methacrolein, and methyl vinyl ketone were also measured as a part of this work. Product yields were determined using a combination of long path Fourier transform infrared spectroscopy, high performance liquid chromatography with fluorescence detection, gas chromatography with flame ionization detection, and gas chromatography-mass spectrometry. The following branching ratios were determined: C(2)H(5)C(O)O(2), Y(R1a) = 0.35 ± 0.1, Y(R1b) = 0.25 ± 0.1, and Y(R1c) = 0.4 ± 0.1; C(3)H(7)C(O)O(2), Y(R1a) = 0.24 ± 0.15, Y(R1b) = 0.29 ± 0.1, and Y(R1c) = 0.47 ± 0.15; CH(3)C(O)CH(2)O(2), Y(R1a) = 0.75 ± 0.13, Y(R1b) = 0, and Y(R1c) = 0.25 ± 0.13; CH(3)C(O)CH(O(2))CH(3), Y(R1a) = 0.42 ± 0.1, Y(R1b) = 0, and Y(R1c) = 0.58 ± 0.1; CH(2)ClC(CH(3))(O(2))CHO, Y(R1a) = 0.2 ± 0.2, Y(R1b) = 0, and Y(R1c) = 0.8 ± 0.2; and CH(2)ClCH(O(2))C(O)CH(3), Y(R1a) = 0.2 ± 0.1, Y(R1b) = 0, and Y(R1c) = 0.8 ± 0.2. The results give insights into possible mechanisms for cycling of OH radicals in the atmosphere.     

Use of stable radicals for the determination of rate constants of elementary processes

A direct method is proposed for determining the concentration of active centers by introducing the stable radical 2, 2, 6, 6-tetramethyl-4-hydroxypiperidine-1-oxyl at the commencement, and during the course of, the polymerization process of tetrafluoroethylene. On the basis of the data obtained, the rate constants of the elementary processes are calculated.