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 (C4H9OCH2CH2OH) 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)OCH2CH2CH2CH3), 0.35 ± 0.11; ethylene glycol monoformate (HC(O)OCH2CH2OH), 0.39 ± 0.18; butoxyacetaldehyde (CH3CH2CH2CH2OCH2C(O)H), 0.12 ± 0.09; 3-hydroxybutyl formate (HC(O)OCH2CH2CHOHCH3), ∼0.20; and propionaldehyde, ∼0.2−0.3. The yields of minor products were as follows: 2-propyl-1,3-dioxolane (CH3CH2CH2CHOCH2CH2O), 0.025 ± 0.005; ethylene glycol monobutyrate (CH3CH2CH2C(O)OCH2CH2OH), ∼0.02−0.03; 2-hydroxybutyl formate (HC(O)OCH2CHOHCH2CH3), ∼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 -CH2- 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 cm3 molecule-1 s-1, respectively, with error limits of about ±40%, in each case.