Enol formation in a low-pressure flame occurs in the preheat zone, and its precursors are likely to be fuel species or early products of fuel decomposition. The OH + ethylene reaction has been shown to dominate vinyl alcohol production in ethylene flames, although this reaction appears to be underpowered to describe vinyl alcohol formation in all hydrocarbon oxidation systems. In this work, the mole fraction distributions of vinyl alcohol in several representative low-pressure flames were correlated with those of possible precursor species as a means of judging possible formation pathways in the flames. These correlations and modeling suggest that the reaction of OH with ethylene is in fact the main source of vinyl alcohol in many hydrocarbon flames, and that addition-elimination reactions of OH with other alkenes may also be responsible for the formation of enols in flames. On this basis, enols are expected to be minor intermediates in most flames and should be most prevalent in olefinic flames, where the reaction of fuel with OH can directly produce enols.

Ethanol, 1-d-ethanol, O-d-ethanol and Z-2-d-ethanol were prepared by pyrolysis of the corresponding 5-norbornenol at 800°C/2 × 10-6 Torr. The most important fragments in the electron impact mass spectrum of vinyl alcohol are [C2H3O]+ and CHO+ and CH3˙. The hydrogen atoms removed from the molecular ion were mainly from hydroxyl groups (68%), with small amounts from C(1) (25%) and C(2) (7%). Prior to the loss of hydroxyl hydrogen, hydrogen atoms migrate from C(1) to C(2) in a rate-determining manner, generating CH3COH+˙ ions, which then decompose into CH3CO+ and H˙. The loss of deuterium in O-d-vinyl alcohol showed very little major isotope effect (kH/kD=1.07), whereas a significant effect was observed for the loss of hydrogen in 1-d-vinyl alcohol (kH/kD=1.28). The emergence energy of [C2H2DO]+ from 1-d-vinyl alcohol, AE=11.32 eV, gives the critical energy of hydrogen loss, E=203 kJ mol−1, which is 90 kJ mol−1 higher than the thermochemical threshold of CH3CO ++H˙. The emergence energy of CDO+ from 1-d-vinyl alcohol was measured at 12.96±0.07 eV, which sets the barrier for isomerization to CH3CDO+˙ at 1121 kJ mol−1. The ionization energy of ethanol was found to be 9.22 ± 0.03 eV.