Time-resolved mass spectrometric study of the reaction H + Trans-2-Butene

The title reaction was studied in a discharge flow system using mass flow and modulated molecular beam sampling with phase‐sensitive detection in order to obtain time‐resolved mass spectrometric analysis. At total conversion exceeding 30%, the major products are methane and ethane when initially hydrogen atoms are in excess; when butene is in excess, the major products are ethane and propylene. No hydrocarbons with more than 4 carbon atoms were detected in the products. The reaction is a complicated one since the simplest reaction scheme that successfully simulates the experimental results comprises 20 elementary reactions. The simulation, coupled with sensitivity analysis, shows that with hydrogen atoms in excess, significant amounts of propylene formed in the initial decomposition of the butyl radical react further with hydrogen atoms to form methane and ethane. When butene is in excess, approximately [C3H6] ≈ [CH4] + 1⁄2[C2H6] which means that this propylene does not react further and almost all methyl radicals end up as CH4 or C2H6. At small conversion, simulation shows that the major product by far is propylene regardless of the [H]/[butene] ratio. The absence of higher hydrocarbons in the products is at variance with earlier results of Rabinovitch and coworkers; however the present work leads to a comparable value for the average rate constant ka = ωD/S where D and S is the amount of products arising from the decomposition and stabilization, respectively, of the butyl radical and ω is the collision frequency.