On the reaction of OH radicals with C2 hydrocarbons

The reaction of hydroxyl radicals with fuel components and combustion intermediates is one of the most important steps for fuel oxidation. These reactions constitute the primary consumption pathways for hy- drocarbons at atmospheric and combustion conditions. Depending on the chemical structure and thermo- dynamic conditions, different chemical pathways are available for the reaction of OH with hydrocarbons. Primarily, OH may abstract an H atom directly or may undergo addition reaction forming a complex which may produce various bimolecular products. The knowledge of the branching fractions and competition of these channels is crucial to understand the combustion behavior of practical fuels. In this work, we report experimental study on the reaction of two C2 hydrocarbons, ethylene and acetylene, with OH radicals and combine it with our previous work on ethane to draw conclusions on the effect of C �C bond type on the competition between association and abstraction/bimolecular channels over a wide range of thermodynamic conditions . Experiments were carried out behind reflected shock waves over 800�1300 K and the reaction progress was monitored by probing OH radicals using UV laser absorption near 306 nm. To discern association channel from C �H bond breaking channels (direct H-abstraction and bimolecular channels), reaction of OH radicals was studied with ethylene, deuterated ethylene, acetylene and deuterated acetylene. We pre- viously showed that ethane + OH reaction expectedly follows solely direct H-abstraction pathway. Here, we found that ethylene + OH reaction presents a competition between association, bimolecular channels and direct H-abstraction of the vinylic H atoms, where association pathway becomes negligible for T > 700 K. On the other hand, acetylene is found to react with OH mainly through the association channel which dominates till temperatures as high as 1050 K.