Pyrolysis of styrene: kinetics and mechanism of the equilibrium styrene ↔ benzene + acetylene

Abstract

The thermal unimolecular decomposition of styrene into benzene and acetylene, C_6H_CH=CH2 → C_6H_6 + HCCH (l), was investigated in a low pressure (≈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 (k_(1∞), s^(-1)) = 14.38 - 17 076/T, which was derived by detailed balance from high-pressure (≈50 Torr) low-temperature (878-973 K) kinetic data for the reverse reaction. This value of E(1∞) = 77.9 kcal/mol allows for the generation of vinylidene, H_2C=C, the carbene isomer of acetylene, as a primary product of the title reaction. A non-radical process involving the rate-determining extrusion of H_2C=C: from a [4.1.0]-7-methylene cyclohepta-2,4-diene intermediate in equilibrium with styrene is consistent with kinetic and thermochemical considerations.

Additional details