Products from the gas-phase reaction of some unsaturated alcohols with nitrate radicals

Abstract

Five structurally similar unsaturated alcohols, 2-propene-1-ol (allyl alcohol), 3-butene-2-ol, 2-methyl-3-butene-2-ol (MBO232), 2-butene-1-ol (crotyl alcohol) and 3-methyl-2-butene-1-ol (MBO321), were examined to clarify their atmospheric degradation pathways via oxidation initiated by NO₃ radicals. The reactions were investigated using a 0.153 m³ static glass reactor equipped with long-path FTIR spectroscopy. The experiments were performed at a pressure of 1020±5 mbar and at a temperature of 297±2 K in air or nitrogen as the bath gas. The identified and quantified gas phase products were small carbonyl compounds such as acetone, formaldehyde, acetaldehyde, glycolaldehyde and 2-nitrooxy acetaldehyde. The specific products and their yields varied for the five studied alcohols as follows: formaldehyde 37(±1)% and 2-nitrooxy acetaldehyde 41(±7)% from allyl alcohol; acetaldehyde 28(±6)%, formaldehyde 2(±1)% and 2-nitrooxy acetaldehyde 33(±4)% from 3-butene-2-ol; acetone 63(±6)% and 2-nitrooxy acetaldehyde 67(±8)% from MBO232; acetaldehyde 12(±2)%, formaldehyde 10(±3)% and glycolaldehyde 7(±2)% from 2-butene-1-ol; acetone 21(±6)%, formaldehyde 11(±3)% and glycolaldehyde 29(±10)% from MBO321. In addition, yields were estimated for total organic nitrates using an average integrated absorption cross section of unspecified organic nitrates. Tentative reaction schemes were proposed from the yielded products. The distribution between bond breakage and other processes such as abstraction of a hydrogen atom from the alkoxy radical, formed in the degradation process, was estimated. The small carbonyl compounds were produced by the bond breakage mechanisms. Large multi-functional organic compounds e.g. 1-hydroxy-3-nitrooxy-3-methyl-2-butanone from MBO321 were proposed to be formed by hydrogen abstraction. From the product distribution, the contribution of the number of methyl group substituents at the α and γ carbon atoms, influencing the bond breakage pattern, is discussed. The observed bond cleavage trends are correlated to a substitution pattern where electron donating methyl substituents increase the stability of the leaving radical groups.