PYROLYSIS OF STYRENE - KINETICS AND MECHANISM OF THE EQUILIBRIUM STYRENE [--]BENZENE + ACETYLENE

The thermal unimolecular decomposition of styrene into benzene and acetylene, C6H5CH=CH2 --> CH6 + HCCH (1), was investigated in a low pressure (almost-equal-to 10 mTorr) flow reactor by on-line mass spectrometry between 1180 and 1350 K. Measured rates can be calculated, via RRKM extrapolation, from the expression log (k1infinity, s-1) = 14.38 - 17 076/T, which was derived by detailed balance from high-pressure (almost-equal-to 50 Torr) low-temperature (878-973 K) kinetic data for the reverse reaction. This value of E1infinity = 77.9 kcal/mol allows for the generation of vinylidene, H2C=C:, the carbene isomer of acetylene, as a primary product of the title reaction. A non-radical process involving the rate-determining extrusion of H2C=C: from a [4.1.0]-7-methylene cyclohepta-2,4-diene intermediate in equilibrium with styrene is consistent with kinetic and thermochemical considerations.