HYDROXYL-RADICAL-INITIATED OXIDATION OF ISOBUTYL ISOPROPYL ETHER UNDER LABORATORY CONDITIONS RELATED TO THE TROPOSPHERE - PRODUCT STUDIES AND PROPOSED MECHANISM

The products formed by the hydroxyl-radical-initiated oxidation of the model ether, isobutyl isopropyl ether [(CH3)(2)CHCH2OCH(CH3)(2)], hav e been investigated by irradiating synthetic air mixtures containing t he substrate, methyl nitrite, and nitric oxide at ppm levels in a Tefl on bag reactor at room temperature. The decay of reactant and formatio n of products were monitored by gas chromatography, mass spectrometry and by HPLC. The molar yields of the major products (mel of product fo rmed/mel of isobutyl isopropyl ether consumed) were as follows: aceton e, 0.56 +/- 0.04; isopropyl formate, 0.48 +/- 0.03; isobutyl acetate, 0.28 +/- 0.02; 2-hydroxy-2-methylpropyl acetate [CH3C(O)OCH2C(OH)(CH3) (2)], 0.25 +/- 0.1. The molar yields of the minor products were as fol lows: isobutyraldehyde, 0.06 +/- 0.05; isopropyl nitrate, 0.09 +/- 0.0 6; 1,1,4-trimethyl3-oxapentyl nitrate [(CH3)(2)CHOCH2C(CH3)(2)(ONO2)], 0.07 +/- 0.02; isopropyl isobutyrate [(CH3)(2)CHC(O)OCH(CH3)(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 -CH2- grou ps in the alkyl side chains of the ether followed by the subsequent re actions of the resulting carboncentred, organic peroxy, and organic ox y radicals. The observed products, in conjunction with the proposed re action pathways, account for a total yield of about 1.15, indicating t hat all the main routes are accounted for in the degradation of this e ther. The major reaction pathways of the three principal organic oxy r adicals are summarised as follows (percentage of overall reaction in b rackets): (CH3)(2)C(O)OCH2CH(CH3)(2) --> CH3C(O)OCH2CH(CH3)(2) + CH3 ( 28%) (CH3)(2)CHOCH(O)CH(CH3)(2) --> (CH3)(2)CHOC(O)H + CH(CH3)(2) (les s than or equal to 48) (CH3)(2)CHOCH2C(O)(CH3)(2) --> (CH3)(2)COCH2C(O H)(CH3)(2) (25%) This study supports the finding that organic oxy radi cals generated from ethers and containing the structure RCH(O .)OR und ergo mainly decomposition by C-C bond cleavage, whereas those oxy radi cals with the structure RCH(O .)CH2OR 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 th e relative rate technique: isobutyl isopropyl ether, 19.5 +/- 0.4; iso butyl 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-trimet hyl-3-oxapentyl nitrate, 16.5 +/- 0.7; and 2-hydroxy-2-methylpropyl ac etate, 9.5 +/- 1.6.