Empirical rate rules for hydroxyl radical reactions with alkenes

Abstract

Alkenes are not only constituents of practical fuels but are also key intermediates of the oxidation and pyrolysis of larger hydrocarbons. The interactions between alkenes and hydroxyl (OH) radicals play a pivotal role in the depletion of alkenes. Literature measurements of OH + alkene reactions have been limited to small molecules containing fewer than seven carbon atoms. Moreover, the competition between various channels in these reactions remains poorly understood. Here, we studied channel-specific rate coefficients of propene + OH and combined it with literature measurements to derive rate rules for alkene + OH reactions. This work presents the first direct measurement of the channel-specific rate coefficients (k_1a) for the reaction of OH + propene → allyl radical + H₂O. Using a sensitive UV absorption diagnostic scheme at 220 nm, we tracked the time-resolved formation of the product allyl radical. Our determined rate coefficients are described by the following Arrhenius expression (unit: cm³molecule^-1s^-1):$k_{1a}=1.38\times {10}^{-10}{e}^{\left(\frac{-3128}{T}\right)}$ (900–1200 K)

Between 900 and 1200 K, the H abstraction from allylic CH bonds of propene accounted for 55 - 65 % of the overall reactivity and exhibited a gentle positive temperature dependence.

Our investigation of hydroxyl reaction with propene serves as a prototype reaction of a molecule containing allylic CH bonds. In conjunction with literature-reported rate coefficients of OH + C₄ – C₆ alkenes, we propose a set of rate rules encompassing vinylic, alkylic, and allylic CH bonds. These rate rules could be used to predict the behavior of large alkene reactions with OH when direct measurements and calculations are not available. Notably, our rate rules revealed that the primary allylic CH bonds in propene and iso-butene react with about a 40 % slower rate with OH than the primary allylic CH bonds in 2-alkenes, cautioning against direct analogy between the rate coefficients of these CH bonds. Additionally, the secondary allylic CH bonds in a trans-2-alkene molecules are 33 % more efficient in consuming OH radicals than those in the cis-2-alkenes.

These rate rules are incorporated in literature models of alkenes and biofuels containing similar CH bonds, thus enabling improved accuracy of model predictions. Our work provides new insights into the channel-specific competition of OH + alkene reactions, and benefits automated modeling of alkene molecules and double-bond containing biofuels.