Rate constants for the reactions of OH radicals with a series of 1,4-hydroxyketones

A series of C5–C8 1,4-hydroxyketones were generated in situ from the OH radical-initiated reactions of their n-alkane precursors. By investigating the time-concentration behavior of these hydroxyketones during the reactions, rate constants for the reactions of OH radicals with the hydroxyketones at 298 ± 2 K were determined relative to the rate constant for the reaction of OH radicals with the parent n-alkane. The hydroxyketones were monitored by Solid Phase MicroExtraction, using fibers pre-coated with O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine for on-fiber derivatization of carbonyl compounds, with analysis of their oxime derivatives by gas chromatography with flame ionization detection. Initial experiments at ≤5% relative humidity showed that 5-hydroxy-2-hexanone and 6-hydroxy-3-hexanone (formed from n-hexane) disappeared more rapidly than could be accounted for by reaction with OH radicals, suggesting that these hydroxyketones were also undergoing cyclization with loss of water to form alkyl-substituted 4,5-dihdyrofurans. Accordingly, all further experiments were carried out at 50 ± 5% relative humidity, and the enhanced disappearance of hydroxyketones was markedly reduced (but was still evident for the hydroxyoctanones formed from n-octane). The rate constants derived for the 1,4-hydroxyketones formed from n-pentane through n-octane were in the range (1.5–2.4) × 10−11 cm3 molecule−1 s−1, consistent with rate constants estimated using the structure–reactivity method of Kwok and Atkinson [E.S.C. Kwok, R. Atkinson, Atmos. Environ. 29 (1995) 1685–1695].