T-butyl Peroxy Radicals Research Articles

ChemInform Abstract: Pulse Radiolysis and Fourier Transform Infrared Study of Neopentyl Peroxy Radicals in the Gas Phase at 297 K

The ultraviolet absorption spectrum of the neopentyl peroxy radical (CH3)3CCH2O2, and the kinetics and products of its self reaction have been studied in the gas phase at 298 K. Absorption cross sections were quantified over the wavelength range 230–290 nm. The measured cross section at 250 nm was; Errors represent statistical (2σ) together with our estimate of potential systematic errors(15%). The kinetics of the decay of the UV absorption following the generation of the neopentyl peroxy radicals was complicated by the rapid decomposition of the (CH3)3CCH2O radicals formed in channel (4a). By measuring the yield of t-butyl peroxy radicals, the branching ratio k4a/(k4a + k4b) was determined to be 0.39 ± 0.03. The rate constant for the self reaction of neopentyl peroxy radicals was k4 = (1.07 ± 0.22) × 10−12 cm3 molecule−1 s−1. Quoted errors represent 2σ. These results are discussed with respect to the available literature data. © John Wiley & Sons, Inc.

Heats of Formation of t-Butyl Peroxy Radical and t-Butyl Diazyl Ion: RRKM vs SSACM Rate Theories in Systems with Kinetic and Competitive Shifts

The dissociations of energy-selected di-t-butyl peroxide and di-t-butyl diazene ions have been studied by threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy. Di-t-butyl peroxide ions dissociate via two parallel channels: (1) methyl loss at a 0 K onset (E0) of 9.58 +/- 0.04 eV followed by a sequential dissociation of the daughter ion to produce C4H9O+ and acetone; and (2) the dominant dissociation channel, producing t-butyl ion and t-butyl peroxy radical at an E0 of 9.758 +/- 0.020 eV. Di-t-butyl diazene ions dissociate through three parallel channels: (1) a rearrangement to form isobutene ion; (2) C-N bond cleavage with the charge staying on the t-butyl diazyl species (E0 = 8.069 +/- 0.050 eV); and (3) C-N bond cleavage with the charge instead on the t-butyl (E0 = 8.122 +/- 0.050 eV); the coproduct for this latter channel is a weakly, or possibly unbound, N2...t-butyl structure. Both the peroxide and diazene ion dissociations produce metastable daughters, and the dissociation rates are modeled with two rate theories: the Rice-Ramsperger-Kassel-Marcus (RRKM) theory and a simplified version of the statistical adiabatic channel model (SSACM). Due to a large kinetic shift, RRKM incorrectly models the peroxide ion rate curve. Using SSACM, the heat of formation of t-butyl peroxy radical is determined to be DeltaH0Kdegrees = - 81.1 +/- 3.9 kJ mol-1, and, using B3LYP/6-311++G(d,p) thermal energy, DeltaH298Kdegrees = - 109.7 +/- 3.9 kJ mol-1. Due to a competitive shift of the higher energy channel onsets, RRKM also incorrectly models the diazene rate curves. The 298 K heat of formation of the t-butyl diazyl ion, which is bound by 14 kJ mol-1, is determined to be 701.2 +/- 5.9 kJ mol-1.

Fullerene peroxides

t-Butylperoxy radical adds to fullerenes to form a number of stable fullerene mixed peroxides. The reaction follows the stepwise radical addition mechanism, and usually gives a mixture of products with different numbers of t-butylperoxo groups attached on the fullerene cage. Sequential para-addition on the hexagons is the dominant addition pattern. The peroxo groups and fullerene carbons around them exhibit facile reactivity towards various Lewis acids and nucleophiles. The fullerene peroxides can be easily separated by normal column chromatography. Single-crystal X-ray analysis confirmed the presence of peroxo groups. To cite this article L. Gan C.R. Chimie 9 (2006).

Self- and cross-reactions of β-hydroxyperoxy radicals of relevance to tropospheric monoterpene oxidation: structure–activity relationships for rate coefficients

The laser flash photolysis technique has been used to measure rate coefficients for the self- and cross-reactions of four peroxy radicals, which have been selected as surrogate of those peroxy radicals resulting from the OH addition to monoterpenes: the 2-hydroxycyclohexylperoxy c-C6H10(OH)O2 (I), 2-hydroxy-1,2-dimethylcyclohexylperoxy c-C6H8(CH3)2(OH)O2 (II), 2-hydroxy-2-methylcyclohexylperoxy c-C6H9(OH)CH3O2 (III) and 2-hydroxy-1-methylcyclohexylperoxy c-C6H9CH3(OH)O2 (IV) radicals. Those radicals were obtained by addition of OH and O2 to cyclohexene and 1,2-dimethylcyclohexene for I and II, respectively, and to 1-methylcyclohexene for (III) and (IV). Rate constants for self-reactions of radicals I and II are: k8=(1.60±0.10)×10−12 and k12=(2.00±0.05)×10−14 (units of cm3 molecule−1 s−1, errors 2σ) at 298°K and in 1 atm air. Assuming that both the secondary radicals I and III and both the tertiary radicals II and IV have similar self-reaction rate constants, the rate constant for cross-reaction (13) between III and IV has been determined: k13=(6.15±0.33)×10−13 cm3 molecule−1 s−1. The self-reaction rate constant of the peroxy radical c-C6H8(OH)O2, formed by OH and O2 addition to 1,4-cyclohexadiene, was also estimated and found to be of same order of magnitude as k8, thus showing that the non-conjugated double bond in the radical has no significant effect on the rate constant. Structure–activity relationships are discussed, particularly for self- and cross-reactions involving secondary and tertiary peroxy radicals, in view of their relevance to tropospheric oxidation modelling of alkenes and monoterpenes. In particular, emphasis is given to the large increase in reactivity resulting from the β-OH substitution, compared to non-substituted species. As part of this work, a rate constant of (1.5±0.5)×10−14 cm3 molecule−1 s−1 was measured for the self-reaction of the β-OH substituted t-butylperoxy radical (CH3)2C(O2)CH2OH.

Fluorescence characteristics of peroxidation products in porcine intestinal brush-border membranes

Treatment of the porcine intestinal brush-border membranes with 100 μ m ascorbic acid and 10 μ m Fe 2+ in the presence of various concentrations of tert-butyl hydroperoxide (t-BuOOH) resulted in a marked fluorescence development at 430 nm, depending on the hydroperoxide concentration. This fluorescence formation was closely related to lipid peroxidation of the membranes as assessed by formation of conjugated diene. However there is no linear relation between thiobarbituric acid-reactive substances (TBARS) and fluorescence formation. On the other hand, fluorescence formation in the membranes by treatment with ascorbic acid/Fe 2+ or t-BuOOH alone was negligible. The results with antioxidants and radical scavengers suggest that ascorbic acid/Fe 2+/t-BuOOH-induced lipid peroxidation of the membranes is mainly due to t-butoxyl and/or t-butyl peroxy radicals. Most TBARS produced during the peroxidation reaction were released from the membranes, but fluorescent products remained in the membrane components. The fluorescence properties of products formed by lipid peroxidation of the membranes were compared with those of products derived from the interaction of malondialdehyde (MDA) or acetaldehyde with the membranes. The fluorescence products in the acetaldehyde-modified membranes also exhibited the emission maximum at 430 nm, while the emission maximum of MDA-modified membranes was 470 nm. The fluorescence intensity of MDA-modified membranes was markedly decreased by treatment with 10 m m NaBH 4 but that of the peroxidized or acetaldehyde-modified membranes was enhanced by about twofold with the treatment. In addition, a pH dependence profile revealed that the fluorescence intensity of the peroxidized or acetaldehyde-modifled membranes decreases with increasing pH of the medium, whereas that of MDA-modified ones did not change over the pH range from 5.4 to 8.0. On the basis of these results, the fluorescence properties of products formed in the intestinal brush-border membranes by lipid peroxidation are discussed.

Flash photolysis study of the spectra and self-reactions of neopentylperoxy and t-butylperoxy radicals

The self-reaction of neopentylperoxy radicals, neo-C5H11O2(NPTO2): NPTO2+ NPTO2→ 2t-C4H9CH2O + O2(1a), → t-C4H9CHO + t-C4H9CH2OH + O2(1b), → t-C4H9CH2OOCH2C4H9-t + O2(1c), has been studied from 248 to 373 K and from 50 to 760 Torr total pressure. The neopentylperoxy radicals fromed via channel (1a) react, under most experimental conditions, by unimolecular decomposition: t-C4H9CH2O + M → t-C4H9+ HCHO + M. (2). The t-butyl radicals so formed are rapidly converted into t-butylperoxy radicals under the conditions employed in this work; these radicals are unreactive on the timescale of the NPTO2 decay and enable the branching ratio for reaction (1) to be determined via their UV absorption. The overall rate constant for reaction (1) displays a strong negative temperature dependence, being well described by k1/cm3 molecule–1 s–1= 3.02 × 10–19(T/298)9.46 exp(4260/T) over the temperature range studied here. The non-terminating channel (1a) becomes increasingly important with increasing temperature, with β=(197 ± 67)exp[-(1658 ± 98)/T], where β is the ratio of those radicals which react via the non-terminating channel (1a) to those which react via the terminating channels (1b) and (1c). By measuring the reduction in the fraction of NPYO2 radicals converted to t-butylperoxy radicals with increasing oxygen concentration, rate constants for reaction (2) were determined, giving E2/kJ mol–1= 42.7 ± 2.1. The UV spectra of NPTO2 and t-C4H9O2 have been determined relative to that of CH3O2; both are similar in shape and magnitude to those of other alkylperoxy radicals, displaying maxima around 240 nm, with σ240 nm(NPTO2)/ cm2 molecule–1=(6.2 ± 1.1)× 10–18 and σ240 nm(t-C4H9O2)/cm2 molecule–1=(4.7 ± 0.8)× 10–18. The self-reaction of t-butylperoxy radicals: 2t-C4H9O2→ 2t-C4H9O + O2(3), was also briefly studied, resulting in k3/cm3 molecule–1 s–1≈ 1.0 × 10–11 exp(–3894/T). Errors are 1σ.

Selective oxidation of 2,6-dialkylphenols by t-butyle hydroperoxide in zeolite CoX

On a Co(II)-exchanged zeolite X t-butyl hydroperoxide (t-BuOOH) decomposes by a radical oxidation—reduction mechanism into t-butanol and dioxygen. The influence of the solvent and the activation temperature of the zeolite on this reaction has been studied. The t-butoxy and t-butyl peroxy radicals, generated during decomposition of t-BuOOH in CoX with benzene as the solvent, served to oxidize 2,6-dialkylphenols to the corresponding 2,6-dialkyl- p-benzoquinones and 3,3′,5,5′-tetraalkyl-4,4′-diphenoquinones. In contrast to the homogeneously catalyzed oxidation the CoX/t-BuOOH system showed high selectivity towards 2,6-dialkyl- p-benzoquinone formation. The results on other supported Co-catalysts show that selectivity and activity strongly depend on the support applied and on the size of the alkyl groups.

Recombination of tertiary butyl peroxy radicals. Part 1.—Product yields between 298 and 373 K

Overall product distributions resulting from the recombination of t-butyl peroxy radicals have been studied over the temperature range 298–373 K. The results indicate that over this range there is a switch from the terminating channels (forming alcohol and aldehyde/ketone) towards non-terminating channels (forming two alkoxy radicals) for the two further recombination processes that follow the initial combination of t-butyl peroxy radicals: CH3O2˙+ t-BuO2˙→ products, 2CH3O2˙→ products.There is also direct evidence for the presence of a terminating channel to form di-t-butyl peroxide. This reaction proceeds at a rate of ca. 0.14 of the non-terminating recombination rate at 298 K, but this fraction falls to 0.025 at 333 K and the reaction is not evident at 373 K.Our results demonstrate the importance of abstraction reactions involving alkoxy radicals (t-butoxy and methoxy) and one of the principal recombination products, t-butyl hydroperoxide. Rate constant ratios involving these processes have been derived from the product distributions and from additional studies in which t-butyl hydroperoxide was added. Rate constants of ca. 10–13 cm3 molecule–1 s–1 for these abstraction processes are consistent with our results.

Infrared absorption spectrum of t-butylperoxy radicals in the gas phase

Two transient absorptions have been detected in the infrared region during the gas-phase photolysis of azoisobutane/O 2 mixtures at 293 K. These transitions centred at 693.7 ± 0.5 cm −1 and 760 ± 2 cm −1 are assigned to the t-butylperoxy radical. A simple flow system at atmospheric pressure was used together with a multiple pass cell.

The roles of alkylperoxy and alkoxy radicals in alkyl radical oxidation at room temperature

Using a molecular modulation spectrometer, the ultraviolet absorption spectrum of t-butylperoxy radicals has been found on photolysing azoisobutane/oxygen mixtures and shown to be virtually identical to those of methyl- and ethyl-peroxy radicals: it is a broad continuum extending from 210 to 290 nm with a maximum near 240 nm, where the absorption crosssection is 2.2±0.4×10−18 cm2 mole−1. The radical combination process is very slow and is controlled by the following reactions: 2t-BuO2→2t-BuO+O2 k13=3.6×10−18 cm3 molecule−1 sec−1 t-BuO→CH3+(CH3)2CO CH3+O2→CH3O2 CH3O2+t-BuO2→CH3O+t-BuO+O2 k18a=5×10−14 cm3 molecule−1 sec−1 →CH2O+t-BuOH+O2 k18b=5×10−14 cm3 molecule−1 sec−1 CH3O+t-BuO2→CH2O+t-BuO2H The production of this slowly reacting t-butylperoxy radical has been demonstrated in mixtures of azomethane, oxygen and isobutane and the yield has been used to measure the amount of non-termination in the combination of methylperoxy radicals with themselves. Isobutane scavenges the methoxy products leading to t-butyl and then l-butylperoxy radicals. At room temperature the fraction is measured to be 0.33±0.05. The overall rate constant for methylperoxy radical combination, k1=3.9×10−13 cm3 mol−1 sec−1. 2CH3O2→2CH3O+O2 k1a=1.45×10−13 cm3 molecule−1 sec−1 →CH3OH+CH2O+O2 k1b=2.45×10−13 cm3 molecule−1 sec−1.

Metal Complexes as Antioxidants. I. The Reaction of Zinc Dialkyldithiophosphates and Related Compounds with Peroxy Radicals

The kinetics of the inhibition of the autoxidation of several hydrocarbons by a number of zinc dialkyldithiophosphates and by zinc isopropylxanthate and zinc diethyldithiocarbamate have been studied at 30c and at 50c. The oxidations were generally auto-retarding but initial rate measurements showed that these compounds trapped peroxy radicals and allowed rate constants for this process to be calculated. Rate constants for the reaction of t-butylperoxy radicals with these compounds have been measured by a kinetic e.p.r. method in the temperature range 0c to −90c. Extrapolation of the e.p.r. data to the temperatures of the inhibition studies showed that the various experimental procedures yielded results in satisfactory agreement with one another.It is suggested that the reaction of peroxy radicals with zinc complexes involves reaction at the metal center either by an electron transfer or an SH2 process.

A kinetic ESR study of the reaction of t-butylperoxy radicals with hexaphenylditin and hexaphenyldilead

Absolute rate constants for the reaction of t-butylperoxy radicals with hexaphenylditin and hexaphenyldilead have been found to fit the equations log ( k p / M −1·s −1) = (5.5±0.15) − (3800±150)/Θ and log ( k p / M −1·s −1) (9.5±0.3) − (6000±250)/Θ, where Θ = 2.303 RT cal mol −1, respectively.

Absolute Rate Constants for Hydrocarbon Autoxidation. XXI. Activation Energies for Propagation and the Correlation of Propagation Rate Constants with Carbon–Hydrogen Bond Strengths

The rate constants, kp, for the abstraction of a hydrogen atom from a wide variety of organic substrates at 30° have been correlated with the strengths of the C—H bond that are broken, D[R—H] (in units of kcal/mol). Two empirical relations have been derived, one for tertiary peroxy radicals,[Formula: see text]and one for secondary peroxy radicals,[Formula: see text]These equations hold extremely well for hydrocarbons and reasonably well for substrates containing heteroatoms. The activation energies and pre-exponential factors for the oxidation of some hydrocarbons by t-butylperoxy radicals have been measured and the empirical relation[Formula: see text]derived. If D[t-ROO—H] is taken as 88 kcal/mol, this yields the Polanyi equation,[Formula: see text]where ΔH is the heat of the hydrogen abstraction reaction. These empirical equations permit the calculation of the rate constants for chain propagation in the autoxidation of many organic substrates at 30° and also at other temperatures.

Absolute Rate Constants for Hydrocarbon Autoxidation. XXII. The Autoxidation of some Vinyl Compounds

Absolute propagation and termination rate constants have been determined for the autoxidation of some vinyl compounds at 30°. Rates of propagation depend on the structure of both the peroxy radical and the vinyl compound. The reactivity of peroxy radicals towards addition increases as the electron-withdrawing capacity of the α-substituent increases. Rate constants for addition of t-butylperoxy radicals to vinyl compounds, [Formula: see text] fit the equation[Formula: see text]where Es is the estimated stabilization energy of the β-peroxyalkyl radical (in kcal/mol) formed in the addition reaction.

Absolute Rate Constants for Hydrocarbon Autoxidation. Part XX. Oxidation of Some Organic Sulfides

Absolute rate constants for the liquid phase autoxidation of some organic sulfides at 30 °C have been measured. The reactivities of organic sulfides towards t-butylperoxy radicals are equal to or somewhat less than the reactivities of structurally analogous ethers. The α-alkylthiylalkylperoxy radicals appear to be about 3–5 times more reactive in hydrogen atom abstraction than the α-alkoxyalkylperoxy radicals.

Absolute rate constants for hydrocarbon autoxidation. XIX. Oxidation of some α-substituted toluenes

Absolute propagation and termination rate constants have been measured for the liquid phase autoxidation of some α-substituted toluenes at 30 °C. Rate constants for reaction of α-substituted toluenes with their own peroxy radical and with the t-butylperoxy radical have been compared. It would appear that the reactivity of an α-substituted benzylperoxy radical depends on the nature of the α-substituent.Reactivities of α-substituted toluenes towards the t-butylperoxy radical, the bromine atom, and the phenyl radical have been compared.

Absolute rate constants for hydrocarbon autoxidation. XVIII. Oxidation of some acyclic ethers

Propagation and termination rate constants have been measured for autoxidation of benzyl phenyl ether, benzyl-t-butyl ether, isopropyl ether, and benzyl ether. In the case of isopropyl ether and benzyl ether, estimates have been made of inter- and intramolecular propagation rate constants. Reactivities of acyclic ethers towards the t-butylperoxy radical have been determined. Rate constants for autoxidation of cyclic and acylic ethers have been summarized and compared.

Reactions of alkyl free radicals with 4-vinylcyclohexene

Tertiary alkyl and cyanoalkyl free radicals prepared from t-butyl peroxypivalate and azobisisobutyronitrile were reacted with 4-vinylcyclohexene (1) for comparison with results previously obtained in the reaction of certain alkyl, alkoxy, peroxy, and thiyl radicals with this model olefin. The (CH3)3C• and (CH3)2(CN)C• radicals (like thiyl but in contrast to t-butoxy and t-butyl peroxy radicals) added to the vinyl double bond with subsequent abstraction of hydrogen by the resulting secondary alkyl radicals to give RH adducts along with higher molecular weight material.Vinylcyclohexenyl radicals formed by hydrogen abstraction from 1 reacted mainly by coupling to form dehydro dimers. They also formed substituted products by coupling with cyanoisopropyl radicals, but not with t-butyl radicals. The presence of both stabilized and unstabilized alkyl radicals of both secondary and tertiary types in these reactions with 1 permitted the observation of differences in the relative tendencies of these types of radicals to participate in addition, hydrogen abstraction, disproportionation, and coupling reactions. The observed effect of temperature (60 and 90°) on product distribution, together with estimated bond dissociation energies, suggest that the addition of cyanoisopropyl radicals to 1 is a reversible process.

Studies by electron spin resonance of radicals produced in the photolysis of alkyl peroxides and hydroperoxides

Ultra-violet photolysis of degassed, purified t-butyl peroxide or of its solutions in hydrocarbons, in no case yielded concentrations of radicals detectable by electron spin resonance. In commercial samples of t-butyl peroxide or in t-butyl peroxide containing t-butyl hydroperoxide, large radical concentrations were obtained by photolysis, while in solutions of t-butyl peroxide in pentane, iso-pentane, hexane, heptane or methyl cyclohexane, containing dissolved oxygen or air, radicals were readily detected below 253°K. In aromatic solvents, except in cumene, only low radical concentrations were observed in any circumstances. Photolysis of t-butyl hydroperoxide yielded high radical concentration as did photolysis of its solutions in aliphatic solvents either in the presence or absence of oxygen. In aromatic solvents no radicals were observed unless oxygen was present. These results can be interpreted on the assumption that the radicals responsible for the e.s.r. signals are either t-butyl peroxy radicals or, for some oxygen-saturated solutions a peroxy radical, derived from the solvent. Attempts to obtain the t-butyl-oxy radical by photolysis of t-butyl peroxide adsorbed on silica gel at low temperatures yielded only methyl radicals and a radical believed to be CH3˙ CO ˙ CH2·.

THE OXIDATION OF PHENOLS: II. THE OXIDATION OF 2,4-DI-t-BUTYLPHENOL WITH PEROXY RADICALS

Ten products from the oxidation of 2,4-di-t-butylpheuol with t-butyl peroxy radicals have been identified. They can be subdivided into three groups, depending on the type of radical–radical coupling reaction by which they are produced or in which their precursors are formed: (i) phenoxy–peroxy coupling, (ii) phenoxy–phenoxy coupling at the 6 positions, and (iii) phenoxy–phenoxy coupling through the oxygen atom of one radical and the 6 position of the second radical. Products resulting from still other phenoxy–phenoxy coupling reactions are undoubtedly formed in minor amounts.