OH RADICAL-INITIATED OXIDATION OF 2-BUTOXYETHANOL UNDER LABORATORY CONDITIONS RELATED TO THE TROPOSPHERE - PRODUCT STUDIES AND PROPOSED MECHANISM

This type of study provides information on the reaction mechanism of t he conversion of a substrate molecule, in this case a glycol ether, in to 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 inpu t data for computer modeling studies used to derive ozone-creating pot entials of volatile organic compounds released into the atmosphere. Th e products formed by the hydroxyl radical-initiated oxidation of 2-but oxyethanol (C4H9OCH2CH2OH) have been investigated by irradiating synth etic 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 (mel of product formed/mol of 2-butoxyethanol consumed) were as follows: butyl formate (HC(O)OCH2CH2CH2CH3), 0.35 +/- 0.11; ethyle ne glycol monoformate (HC(O)OCH2CH2OH), 0.39 +/- 0.18; butoxyacetaldeh yde (CH3CH2CH2CH2OCH2C(O)H), 0.12 +/- 0.09; 3-hydroxybutyl formate (HC (O)OCH2CH2CHOHCH3), similar to 0.20; and propionaldehyde, similar to 0 .2-0.3. The yields of minor products were as follows: 2-propyl-1,3-dio xolane (CH3CH2CH2CHOCH2CH2O), 0.025 +/- 0.005; ethylene glycol monobut yrate (CH3CH2CH2C(O)OCH2CH2OH), similar to 0.02-0.03; 2-hydroxybutyl f ormate (HC(O)OCH2CHOHCH2CH3), similar to 0.05; acetaldehyde, <0.05; pr opyl nitrate, 0.038 +/- 0.018; and butyraldehyde, <0.011. The product distribution is explained by a mechanism involving initial OH attack a t 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. Envir on. Sci. Technol. 1996, 30, 3385-3391), in which the alkoxy radicals p redominantly undergo decomposition reactions, but also includes isomer ization reactions for the alkoxy radicals that are derived from the bu tyl side chain in 2-butoxyethanol, The observed products, in conjuncti on with the proposed mechanism, account for a total molar yield of abo ut 1.1, indicating that all the main routes are accounted for in the d egradation of this hydroxy ether. Rate coefficients at room temperatur e for the reactions of OH radicals with butoxyacetaldehyde and 2-propy l-1,3-dioxolane have been determined to be 20.6 x 10(-12) and 10.8 x 1 0(-12) cm(3) molecule(-1) s(-1), respectively, with error limits of ab out +/-40%, in each case.