H-atom abstraction in reactions of cyclic polyalkenes with OH

A site-specific Structure-Activity Relationship for the addition of OH to (poly)alkenes was developed earlier and validated for non-cyclic (poly)alkenes with respect to the total rate constants kOH and the detailed product distributions. Also for reactions of cyclic (poly)alkenes with OH, the SAR-predicted rate constant is in general within 15% of the experimental value, except for a few (bi)cyclic biogenic compounds; for both a-phellandrene and α-terpinene, klit ≈ 1.7 × kpred. For these two terpenes and for the structurally similar 1,3-cyclohexadiene we have measured the rate constants relative to the isoprene+OH reaction: kα-terpinene+OH = (3.4±0.4)×10−10 cm3molec.−1s−1; kα-phellandrene+OH = (3.3±0.4)×10−10 cm3molec.−1 s−1; and k1,3cyclohexadiene+OH = (1.5±0.2)×10−10 cm3molec.−1s−1. The discrepancy between the SAR-predictions for OH-addition and the experimental k-values for the first two compounds —given their specific structure— suggests an important H-abstraction channel in their reactions with OH. We determined the importance of the H-abstraction path by directly measuring the amount of H2O formed in the reaction with OH in Discharge-Flow/Molecular Beam Mass Spectrometry experiments. Thus, an H-abstraction contribution of 30 ± 7 % was found for α-terpinene and 27 ± 10 % for a-phellandrene, which explains most of the deviation of the total kOH with respect to the SAR-prediction for OH-addition.