Isomerization reactions of then-C4H9O andn-OOC4H8OH radicals in oxygen

Abstract

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. © 1996 John Wiley & Sons, Inc.