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

Abstract

The products formed by the hydroxyl-radical-initiated oxidation of the model ether, isobutyl isopropyl ether [(CH 3 ) 2 CHCH 2 OCH(CH 3 ) 2 ], 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 formation of products were monitored by gas chromatography, mass spectrometry and by HPLC. The molar yields of the major products (mol of product formed/mol of isobutyl isopropyl ether consumed) were as follows: acetone, 0.56 ± 0.04; isopropyl formate, 0.48 ± 0.03; isobutyl acetate, 0.28 ± 0.02; 2-hydroxy-2-methylpropyl acetate [CH 3 C(O)OCH 2 C(OH)(CH 3 ) 2 ], 0.25 ± 0.1. The molar yields of the minor products were as follows: isobutyraldehyde, 0.06 ± 0.05; isopropyl nitrate, 0.09 ± 0.06; 1,1,4-trimethyl-3-oxapentyl nitrate [(CH 3 ) 2 CHOCH 2 C(CH 3 ) 2 (ONO 2 )], 0.07 ± 0.02; isopropyl isobutyrate [(CH 3 ) 2 CHC(O)OCH(CH 3 ) 2 ] ca. 0.01; and isobutyl formate, ca. 0.01. The major products are explained by a mechanism involving initial OH attack at the –CH– and –CH 2 – groups in the alkyl side chains of the ether followed by the subsequent reactions of the resulting carbon-centred, organic peroxy, and organic oxy radicals. The observed products, in conjunction with the proposed reaction pathways, account for a total yield of about 1.15, indicating that all the main routes are accounted for in the degradation of this ether. The major reaction pathways of the three principal organic oxy radicals are summarised as follows (percentage of overall reaction in brackets):(CH3)2C(O˙)OCH2CH(CH3)2 → CH3C(O)OCH2CH(CH3)2]C˙H3 (28%)(CH3)2CHOCH(O˙)CH(CH3)2 → (CH3)2CHOC(O)H]C˙H(CH3)2 (⩽48%)(CH3)2CHOCH2C(O˙)(CH3)2 → (CH3)2C˙OCH2C(OH)(CH3)2 (25%)This study supports the finding that organic oxy radicals generated from ethers and containing the structure RCH(O˙)OR undergo mainly decomposition by C–C bond cleavage, whereas those oxy radicals with the structure RCH(O˙)CH 2 OR undergo preferential 1,5-H-atom transfer isomerisation reactions. The following rate coefficients (10 −12 cm 3 molecule −1 s −1 ) at room temperature for the reactions of OH radicals with the reactant and products have been determined by the relative rate technique: isobutyl isopropyl ether, 19.5 ± 0.4; isobutyl acetate, 6.0 ± 0.5; isobutyraldehyde, 25.8 ± 0.7; isopropyl formate, 2.1 ± 0.1; isopropyl isobutyrate, 6.5 ± 0.4; 1,1,4-trimethyl-3-oxapentyl nitrate, 16.5 ± 0.7; and 2-hydroxy-2-methylpropyl acetate, 9.5 ± 1.6.