Flow tube kinetics of benzocyclobutenedione thermolysis by an automated instrument with integrated GC-ion trap MS analysis

Abstract

An instrument has been designed for the automatic recording of flow tube kinetics. It consists of a precision oven for the preparation of a well defined initial concentration of reactant in an inert carrier gas stream. The flow tube reactor can be operated between 200 and 1100°C and residence times are between 0.4 and 2 s. Stable reaction products are sampled by adsorption to GC-Tenax. The reaction products are then analyzed by flash desorption into a gas chromatograph with an ion trap detector (GC-ITD). The instrument can be programmed to automatically record complete kinetic curves by stepwise heating of the pyrolysis tube. At least 30 decomposition analyses obtained at ten representative reactor settings have been measured for a single decomposition curve. Thermolytic reactions of interest in synthetic organic chemistry can be optimized by this method. A demonstration is given by the thermochemistry of benzocyclobutenedione (BCBD). This diketone can be obtained in thermolytic reactions from various derivatives of phthalic acid. BCBD in turn is thermolabile. It decomposes through loss of carbon monoxide in a first order reaction. A fit to the Arrhenius equation gives a value of 237.6 kJ mol −1 for the enthalpy of activation ( E a) and 15.54 for the logarithm of the frequency factor (log A). In addition to biphenylene, various trace products including naphthalene, biphenyl, phenol, ethylbenzene, phthalic anhydride, dibenzofuran, fluorenone, phenanthrene, fluoranthene and pyrene have been identified. The formation of diphenylene, the dimer of benzyne, is strongly dependent on the partial pressure of the reacting diketone. The formation of trace compounds, however, does not depend on the reactant pressure.