Effect of water vapor concentration on the conversion of a series of 1,4-hydroxycarbonyls to dihydrofurans

It has previously been shown that in dry air 5-hydroxy-2-pentanone cyclizes and dehydrates to form 4,5-dihydro-2-methylfuran. A series of C5-C8 1,4-hydroxycarbonyls were generated in situ from the OH radical-initiated reactions of their n-alkane precursors, and their dark decays in air investigated as a function of water vapor concentration. To remove any reactive dihydrofurans formed, in some experiments O3 was added after 120-240 min and the 1,4-hydroxycarbonyls monitored for a further time period. In general, at low water vapor concentrations the 1,4-hydroxycarbonyl decayed in the dark in the absence of added O3, with the concentration reaching a plateau indicating that an equilibrium between the 1,4-hydroxycarbonyl and the dihydrofuran had been attained. Addition of O3 led to further decay of the hydroxycarbonyl. At higher water vapor concentrations, no significant decay of the 1,4-hydroxcarbonyl was observed in the absence of added O3, but addition of O3 resulted in a measurable decay of the 1,4-hydroxycarbonyl. Finally, at yet higher water vapor concentrations, no decay of the 1,4-hydroxycarbonyl was observed in the absence or presence of O3. At >50% relative humidity at 296 K, the C5-C8 1,4-hydroxycarbonyls examined here were stable against cyclization and dehydration.