OH Radical-Initiated Oxidation of 2-Butoxyethanol under Laboratory Conditions Related to the Troposphere:  Product Studies and Proposed Mechanism

Abstract

This type of study provides information on the reaction mechanism of the conversion of a substrate molecule, in this case a glycol ether, into its oxidation products under polluted tropospheric conditions. Such detailed pathways for the breakdown of the substrate molecule lead to the generation of photooxidants, mainly ozone, and are essential input data for computer modeling studies used to derive ozone-creating potentials of volatile organic compounds released into the atmosphere. The products formed by the hydroxyl radical-initiated oxidation of 2-butoxyethanol (C 4 H 9 -OCH 2 CH 2 OH) have been investigated by irradiating synthetic air mixtures containing the substrate, methyl nitrite, and nitric oxide at ppm levels in a Teflon bag reactor at room temperature. The decay of reactant and the formation of products were monitored by gas chromatography and by mass spectrometry. The molar yields of the major products (mol of product formed/mol of 2-butoxyethanol consumed) were as follows: butyl formate (HC(O)OCH 2 CH 2 CH 2 CH 3 ), 0.35 ( 0.11; ethylene glycol monoformate (HC(O)OCH 2 CH 2 -OH), 0.39 ( 0.18; butoxyacetaldehyde (CH 3 CH 2 CH 2 CH 2 -OCH 2 C(O)H), 0.12 ( 0.09; 3-hydroxybutyl formate (HC(O)OCH 2 -CH 2 CHOHCH 3 ), ∼0.20; and propionaldehyde, ∼0.2-0.3. The yields of minor products were as follows: 2-propyl-1,3dioxolane (CH 3 CH 2 CH 2 CHOCH 2 CH 2 O), 0.025 ( 0.005; ethylene glycol monobutyrate (CH 3 CH 2 CH 2 C(O)OCH 2 CH 2 OH), ∼0.02-0.03; 2-hydroxybutyl formate (HC(O)OCH 2 CHOHCH 2 -CH 3 ), ∼0.05; acetaldehyde, <0.05; propyl nitrate, 0.038 ( 0.018; and butyraldehyde, <0.011. The product distribution is explained by a mechanism involving initial OH attack at the five -CH 2 -groups in 2-butoxyethanol followed by the subsequent reactions of the resulting alkyl and alkoxy radicals. The mechanism is analogous to that proposed for 2-ethoxyethanol (Stemmler et al. Environ. Sci. Technol. 1996, 30, 3385-3391), in which the alkoxy radicals predominantly undergo decomposition reactions, but also includes isomerization reactions for the alkoxy radicals that are derived from the butyl side chain in 2-butoxyethanol. The observed products, in conjunction with the proposed mechanism, account for a total molar yield of about 1.1, indicating that all the main routes are accounted for in the degradation of this hydroxy ether. Rate coefficients at room temperature for the reactions of OH radicals with butoxyacetaldehyde and 2-propyl-1,3-dioxolane have been determined to be 20.6 × 10 -12 and 10.8 × 10 -12 cm 3 molecule -1 s -1 , respectively, with error limits of about (40%, in each case.

The hydroxyl radical reaction rate constant and atmospheric transformation products of 2-propoxyethanol

“…From the mechanism proposed, PF is a product of hydrogen abstraction from methylene IV in the following structure: Using structure reactivity [12], a 39% propyl formate yield is expected. PF observation is consistent with previous studies of the hydroxyl reaction products of 2-ethoxyethyl acetate and ethyl 3-ethoxypropionate: the methylenes next to the ether oxygen are activated [4,5,10,11,21]. The PF product yield is larger than the trend expected using the data for 2-ethoxyethanol and 2-butoxyethanol that was reported by Stemmler et al [1,4].…”

Products of the OH radical‐ and O3‐initiated reactions of myrcene and ocimene

“…[16] It is also possible that isomerization proceeds through five-or seven-membered transition states [Stemmler et al, 1997;Tuazon et al, 1998], as shown for the CH 2 = CHC(= CH 2 )CH 2 CH 2 CH(O . )C(OH)(CH 3 ) 2 radical formed from myrcene.…”

“…It is also possible that isomerization proceeds through five‐ or seven‐membered transition states [ Stemmler et al , 1997; Tuazon et al , 1998], as shown for the CH 2 = CHC(= CH 2 )CH 2 CH 2 CH(O • )C(OH)(CH 3 ) 2 radical formed from myrcene. Subsequent reactions of the alkoxy radicals CH 2 = CHC(= CH 2 )CH(O • )CH 2 CH(OH)C(OH)(CH 3 ) 2 (formed from myrcene) and CH 2 = CHC(CH 2 O • ) = CHCH 2 CH(OH)C(OH)(CH 3 ) 2 and CH 2 = CHC(CH 3 ) = CHCH(O • )CH(OH)C(OH)(CH 3 ) 2 (formed from ocimene) could possibly lead to the formation of acetone, plus multifunctional coproducts that are not amenable to gas chromatography.…”

Hydroxyl-radical-initiated oxidation of isobutyl isopropyl ether under laboratory conditions related to the troposphere Product studies and proposed mechanism

“…For hydrocarbon molecules such as alkanes and alkenes,2 there is now a sufficient body of laboratory kinetic and thermodynamic data to enable the product distributions to be estimated via sets of empirical rules for the reactions of the hydrocarbon alkoxy radicals.3 In the case of oxygencontaining organic compounds, the data base for the reactions of the corresponding oxygen-containing alkoxy radicals is still too limited to develop the empirical approach.3 To provide more laboratory data on the atmospheric degradations of oxygen-containing compounds, detailed product and mechanistic studies of the atmospheric degradations of ethers4 and glycol ethers have been carried out. 5,6 In the present study we have extended this approach by investigating the mechanism of the atmospheric oxidation of a model ether, isobutyl isopropyl ether. This molecule contains two readily transferable tertiary CwH bonds together with a sufficiently large carbon skeleton to enable the resulting oxy radicals to undergo rapid isomerisation reactions.…”