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Beta-Bond Scission and the Yields of H and CH3 in the Decomposition of i-Butyl Radicals

Published

Author(s)

Laura A. Mertens, Jeffrey A. Manion

Abstract

The relative rates of C-C and C-H β-scission reactions of i-butyl radicals were investigated with shock tube experiments at temperatures of (950 to 1250) K and pressures of (200 to 400) kPa. We produce i-butyl radicals from the decomposition of dilute mixtures of i- pentylbenzene and have observed the stable decomposition products propene and i-butene. These alkenes are characteristic of C-C and C-H bond scission, respectively. Propene was the main product, approximately 30 times more abundant than i-butene, indicating that C-C β- scission is the primary pathway. Uncertainty in our measured ratio is mainly due to a small amount of side chemistry, which we account for using a kinetics model based on JetSurF 2.01. Our data are well-described after adding chemistry specific to our system and adjusting some rate constants. With the kinetics model, we place an upper limit of 3.1 % on the branching fraction for C-H β-scission in the i-butyl radical. While this data agrees with previous high quality experimental results, many combustion kinetics models assume C-H branching values well above this upper limit, possibly leading to large systematic inaccuracies in model predictions. Some kinetics models additionally assume contributions from 1,2-H shift reactions – which for i-butyl would produce the same products as C-H β-scission – and our upper limit includes possible involvement of such reactions. Kinetics models should be updated to better reflect current experimental measurements.
Citation
Journal of Physical Chemistry A

Keywords

kinetics, pyrolysis, combustion, modeling, shock tube
Created May 8, 2018, Updated September 21, 2018