The Pyrolysis of 3-Picoline: Ab Initio Quantum Chemical and Experimental (Shock Tube) Kinetic Studies

The pyrolysis of 3-picoline dilute in argon was investigated using a single-pulse shock tube over the temperature range of 1400–1650 K and total pressures of 12–13 atm. The principal products observed were HCN, acetylene, benzene, cyanoacetylene, methane, and pyridine. Assuming that 3-picoline decomposes according to first-order kinetics, the rate constant for its overall disappearance was determined to be k_dis = 10^{16.9 (±0.8)} exp[–99 (±6) kcal mol⁻¹/RT] s⁻¹. The principal initial decomposition routes were found to be via the formation of the 3-picolyl and m-pyridyl radicals whose subsequent ring-opening led to the observed products. A 68-step kinetic model was developed that successfully fits the experimental data. The dominant reactions, i.e., the formation of picolyl and pyridyl radicals and their subsequent chain-opening reactions, were studied using ab initio quantum chemical techniques. The ab initio data were also incorporated into the kinetic model in the form of energies and A-factors for reactions for which no kinetic or thermochemical data were previously available. Optimization of the kinetic model yields a value of 64 ± (3) kcal mol⁻¹ for the heat of formation of 3-picolyl, a value lower than that for 2-picolyl, suggesting that the decomposition of 3-picoline more closely resembles that of toluene, rather than its isomer 2-picoline.