Substitution Reactions in the Pyrolysis of Acetone Revealed through a Modeling, Experiment, Theory Paradigm

The development of high-fidelity
mechanisms for chemically reactive
systems is a challenging process that requires the compilation of
rate descriptions for a large and somewhat ill-defined set of reactions.
The present unified combination of modeling, experiment, and theory
provides a paradigm for improving such mechanism development efforts.
Here we combine broadband rotational spectroscopy with detailed chemical
modeling based on rate constants obtained from automated ab initio
transition state theory-based master equation calculations and high-level
thermochemical parametrizations. Broadband rotational spectroscopy
offers quantitative and isomer-specific detection by which branching
ratios of polar reaction products may be obtained. Using this technique,
we observe and characterize products arising from H atom substitution
reactions in the flash pyrolysis of acetone (CH3C­(O)­CH3) at a nominal temperature of 1800 K. The major product observed
is ketene (CH2CO). Minor products identified include acetaldehyde
(CH3CHO), propyne (CH3CCH), propene (CH2CHCH3), and water (HDO). Literature mechanisms
for the pyrolysis of acetone do not adequately describe the minor
products. The inclusion of a variety of substitution reactions, with
rate constants and thermochemistry obtained from automated ab initio
kinetics predictions and Active Thermochemical Tables analyses, demonstrates
an important role for such processes. The pathway to acetaldehyde
is shown to be a direct result of substitution of acetone’s
methyl group by a free H atom, while propene formation arises from
OH substitution in the enol form of acetone by a free H atom.