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)alken es with respect to the total rate constants k(OH) 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 alpha-phellandren e and alpha-terpinene, k(lit) approximate to 1.7 x k(pred). For these two t erpenes and for the structurally similar 1,3-cyclohexadiene we have measure d the rate constants relative to the isoprene+OH reaction: k(alpha-terpinen e+OH) = (3.4+/-0.4)x10(-10) cm(3)molec.(-1)s(-1): k(alpha-phellandrene+OH) = (3.3+/-0.4)x10(-10) cm(3)molec.(-1) s(-1): and k(1,3cyclohexadiene+OH) = (1.5+/-0.2)x10(-10) cm(3)molec.(-1)s(-1). The discrepancy between the SAR-p redictions for OH-addition and the experimental k-values for the first two compounds -given their specific structure suggests an important H-abstracti on 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 re action with OH in Discharge-Flow/Molecular Beam Mass Spectrometry experimen ts. Thus, an H-abstraction contribution of 30 +/- 7 % was found for alpha-t erpinene and 27 +/- 10 % for alpha-phellandrene, which explains most of the deviation of the total k(OH) with respect to the SAR-prediction for OH-add ition.