Krikor Sahetchian

ISOMERIZATION REACTIONS OF THE N-C4H9O AND N-OOC4H8OH RADICALS IN OXYGEN

Reactions of n-C4H9O radicals have been investigated in the temperature range 343–503 K in mixtures of O2/N2 at atmospheric pressure. Flow and static experiments have been performed in quartz and Pyrex vessels of different diameters, walls passivated or not towards reactions of radicals, and products were analyzed by GC/MS. The main products formed are butyraldehyde, hydroperoxide C4H8O3 of MW 104, 1-butanol, butyrolactone, and n-propyl hydroperoxide. It is shown that transformation of these RO radicals occurs through two reaction pathways, H shift isomerization (forming C4H8OH radicals) and decomposition. A difference of activation energies ΔE = (7.7 ± 0.1 (σ)) kcal/mol between these reactions and in favor of the H-shift is found, leading to an isomerization rate constant kisom (n-C4H9O) = 1.3 × 1012 exp(− 9,700/RT). Oxidation, producing butyraldehyde, is proposed to occur after isomerization, in parallel with an association reaction of C4H8OH radicals with O2 producing OOC4H8OH radicals which, after further isomerization lead to an hydroperoxide of molecular weight 104 as a main product. Butyraldehyde is mainly formed from the isomerized radical HOCCCC˙ + O2 ··· O (DOUBLE BOND) CCCC + HO2, since (i) the ratio butyraldehyde/(butyraldehyde + isomerization products) = 0.290 ± 0.035 (σ) is independent of oxygen concentration from 448 to 496 K, and (ii) the addition of small quantities of NO has no influence on butyraldehyde formation, but decreases concentration of the hydroperoxides (that of MW 104 and n-propyl hydroperoxide). By measuring the decay of [MW 104] in function of [NO] added (0–22.5 ppm) at 487 K, an estimation of the isomerization rate constant OOC4H8OH HOOC4H7OH, κ5 ≅ 1011exp(−17,600/RT) is made. Implications of these results for atmospheric chemistry and combustion are discussed.

Determination of the isomerization rate constant HOCH2CH2CH2CH(OO·)CH3 →HOC·HCH2CH2CH(OOH)CH3. Importance of intramolecular hydroperoxy isomerization in tropospheric chemistry

The rate constant of the title reaction is determined during thermal decomposition of di-n-pentyl peroxide C5H11O()OC5H11 in oxygen over the temperature range 463–523 K. The pyrolysis of di-n-pentyl peroxide in O2/N2 mixtures is studied at atmospheric pressure in passivated quartz vessels. The reaction products are sampled through a micro-probe, collected on a liquid-nitrogen trap and solubilized in liquid acetonitrile. Analysis of the main compound, peroxide C5H10O3, was carried out by GC/MS, GC/MS/MS [electron impact EI and NH3 chemical ionization CI conditions]. After micro-preparative GC separation of this peroxide, the structure of two cyclic isomers (3S*,6S*)3α-hydroxy-6-methyl-1,2-dioxane and (3R*,6S*)3α-hydroxy-6-methyl-1,2-dioxane was determined from 1H NMR spectra. The hydroperoxy-pentanal OHC()(CH2)2()CH(OOH)()CH3 is formed in the gas phase and is in equilibrium with these two cyclic epimers, which are predominant in the liquid phase at room temperature. This peroxide is produced by successive reactions of the n-pentoxy radical: a first one generates the CH3C·H(CH2)3OH radical which reacts with O2 to form CH3CH(OO·)(CH2)3OH; this hydroxyperoxy radical isomerizes and forms the hydroperoxy HOC·H(CH2)2CH(OOH)CH3 radical. This last species leads to the pentanal-hydroperoxide (also called oxo-hydroperoxide, or carbonyl-hydroperoxide, or hydroperoxypentanal), by the reaction HOC·H(CH2)2CH(OOH)CH3+O2O()CH(CH2)2CH(OOH)CH3+HO2. The isomerization rate constant HOCH2CH2CH2CH(OO·)CH3HOC·HCH2CH2CH(OOH)CH3 (k3) has been determined by comparison to the competing well-known reaction RO2+NORO+NO2 (k7). By adding small amounts of NO (0–1.6×1015 molecules cm−3) to the di-n-pentyl peroxide/O2/N2 mixtures, the pentanal-hydroperoxide concentration was decreased, due to the consumption of RO2 radicals by reaction (7). The pentanal-hydroperoxide concentration was measured vs. NO concentration at ten temperatures (463–523 K). The isomerization rate constant involving the H atoms of the CH2()OH group was deduced: or per H atom: The comparison of this rate constant to thermokinetics estimations leads to the conclusion that the strain energy barrier of a seven-member ring transition state is low and near that of a six-member ring. Intramolecular hydroperoxy isomerization reactions produce carbonyl-hydroperoxides which (through atmospheric decomposition) increase concentration of radicals and consequently increase atmospheric pollution, especially tropospheric ozone, during summer anticyclonic periods. Therefore, hydrocarbons used in summer should contain only short chains (

Homogeneous and heterogeneous reactions of the n-C5H11O, n-C5H10OH and OOC5H10OH radicals in oxygen. Analytical steady state solution by use of the Laplace transform

The different reaction routes of n-pentoxy radicals in O2/N2 mixtures were investigated in the temperature range 423–523 K under atmospheric pressure. Flow experiments were performed in several reactors, with wall efficiencies increasing from passivated quartz to Pyrex, and with a great variety of mol fractions of O2 (% of O2), which was varied from ∽0 to 100% O2; the products, a pentanal-hydroperoxide C5H10O3 of Mr 118, pentanal, a methylfuranone (γ-valerolactone), pentanol, and three methylfurans were analyzed and identified by GC-MS. Pentanal-hydroperoxide, also called 4-hydroperoxypentanal, OCH(CH2)2CH(OOH)CH3, is the major product in passivated quartz vessels. It is formed by two consecutive isomerizations: (i) a fast one, RO→R-HOH, via a six-membered ring transition state and (ii) a much slower one, involving an OOR-HOH radical, OOR-HOH→HOOR-2HOH, via a seven-membered ring intermediate: CCCCCO ——min CCCCCOH ——minCC(OO)CCCOH ——min CC(OOH)CCCOH ——min CC(OOH)CCCO+HO2. A chemical mechanism, taking into account all of the experimental results is proposed for reactions of the n-pentoxy radical in oxygen. An analytical steady state solution, based on the Laplace transform, has been helpful in rejecting or validating candidate models. Rate constants appearing in the proposed mechanism have been evaluated by the use of an optimization method. This analysis shows that (i) isomerization is the predomi-nant reaction route, accounting for the diversity of end products; (ii) homogeneous pentanal is formed by reaction with O2 of anisomerized pentoxy radical: CCCCCO ——min CCCCCOH ——min CCCCCOH ——min CCCCCO+HO2; (iii) methylfurans are also homogeneously produced through an analogous reaction; (iv) γ-valerolactone is formed by a heterogeneous reaction; pentanal and methylfurans are produced in significant quantities through heterogeneous reactions, especially at low concentrations of oxygen (<5% O2). The implications of heterogeneous reactions of RO radicals in atmospheric chemistry and in combustion are discussed for those reactions can correspond to a sink for radicals.

Identification of an unexpected peroxide formed by successive isomerization reactions of the n-butoxy radical in oxygen

A previously unreported peroxide, C4H8O3(5), has been identified and its mechanism of formation proposed. It is generated by two successive isomerization reactions of n-C4H9O radicals in O2. These radicals are produced by di-n-C4H9O—OC4H9 pyrolysis at 480 K in a wall-passivated quartz vessel. The peroxide is collected, among other end-products, on a liquid-nitrogen trap and recovered in liquid acetonitrile. Analysis was carried out by GC–MS, GC–MS–MS [electron impact (EI) and NH3(or ND3)–chemical ionization (CI) conditions] and GC–FTIR. After micropreparative GC separation of the titled peroxide, 1H NMR and high-resolution EIMS were also obtained. The compound was identified as 3α-hydroxy-1,2-dioxane. The hydroperoxybutyraldehyde OHC—(CH2)2—CH2O2H is proposed to be initially formed in the gas phase and to be in equilibrium with its cyclic form (six-membered ring peroxide), by far predominant in the liquid phase at room temperature. The implications of this hydroperoxybutyraldehyde in atmospheric pollution (due to the peroxide producing capability of radicals) and in combustion are discussed.

Homogeneous decomposition of dialkylperoxides in oxygen

The pyrolysis of n-dialkyl peroxides from 150 to 250 °C is a homogeneous reaction controlled by the intial O—O bond fission. When the reaction occurred in an excess of O2 and with a reactant concentration of 100 ppm the reaction sequence was as follows: RCH2O2CH2→ 2RCH2O (1), RCH2O + O2→ RCHO + HO2(2), 2HO2→ H2O2+ O2. (3) The yields of H2O2 and trapped HO2 radicals have been measured. Rate constant values are as follows: (i) for di-n-heptylperoxide log10k1/s–1=(15.00 ± 0.82)–(152.6 ± 9.2) kJ mol–1/2.303 RT for di-n-butyl peroxide log10k1/s–1=(16.02 ± 0.49)–(160.2 ± 10.8) kJ mol–1/2.303 RT. Such values for A and E are in line with those obtained by another method for dimethyl peroxide and di-t-butyl peroxide and are in agreement with empirical correlations. When di-t-butyl peroxide was studied under the same conditions the main radical observed was CH3O2; the main peroxides formed were CH3O2H and H2O2.

Reactions of n - and s -Butoxy Radicals in Oxygen. Importance of Isomerisation and Formation of Pollutants

Reactions of C4H9O radicals have been investigated in mixtures of O2/N2 under atmospheric pressure in the temperature range 313–503 K, for s-C4H9O, and 343–503 K, for n-C4H9O radicals. Flow and static experiments were performed in various vessels (quartz or Pyrex, different diameters, walls passivated or not towards reactions of radicals), and products were analysed by HPLC and GC/MS.