The Thermal Decomposition of the Benzyl Radical in a Heated Micro-Reactor. II. Pyrolysis of the Tropyl Radical

Cycloheptatrienyl (tropyl) radical, C₇H₇, was cleanly produced in the gas-phase, entrained in He or Ne carrier gas, and subjected to a set of flash-pyrolysis micro-reactors. The pyrolysis products resulting from C₇H₇ were detected and identified by vacuum ultraviolet photoionization mass spectrometry. Complementary product identification was provided by infrared absorption spectroscopy. Pyrolysis pressures in the micro-reactor were roughly 200 Torr and residence times were approximately 100 μs. Thermal cracking of tropyl radical begins at 1100 K and the products from pyrolysis of C₇H₇ are only acetylene and cyclopentadienyl radicals. Tropyl radicals do not isomerize to benzyl radicals at reactor temperatures up to 1600 K. Heating samples of either cycloheptatriene or norbornadiene never produced tropyl (C₇H₇) radicals but rather only benzyl (C₆H₅CH₂). The thermal decomposition of benzyl radicals has been reconsidered without participation of tropyl radicals. There are at least three distinct pathways for pyrolysis of benzyl radical: the Benson fragmentation, the methyl-phenyl radical, and the bridgehead norbornadienyl radical. These three pathways account for the majority of the products detected following pyrolysis of all of the isotopomers: C₆H₅CH₂, C₆H₅CD₂, C₆D₅CH₂, and C₆H₅¹³CH₂. Analysis of the temperature dependence for the pyrolysis of the isotopic species (C₆H₅CD₂, C₆D₅CH₂, and C₆H₅¹³CH₂) suggests the Benson fragmentation and the norbornadienyl pathways open at reactor temperatures of 1300 K while the methyl-phenyl radical channel becomes active at slightly higher temperatures (1500 K).