Isomerization and Decomposition of 3,5-Dimethylisoxazole. Studies with a Single Pulse Shock Tube

Article abstract of DOI:10.1021/j100029a022

The isomerization and decomposition of 3,5-dimethylisoxazole were studied behind reflected shocks in a pressurized driver single pulse shock tube over the temperature range 880-1050 K and overall densities of ca. 2.5 * 10^-5 mol/cm³.The main thermal reaction of 3,5-dimethylisoxazole is an isomerization to 2-methyl-3-oxobutyronitrile (CH3COCH(CH3)CN).Acetonitrile, which is the main decomposition product in the system, is obtained from 3,5-dimethylisoxazole by a unimolecular process: 3,5-dimethylisoxazole -> CH3CN + CH3CHCO.The unimolecular decomposition of the thermally excited isomer, (CH3COCH(CH3)CN)^<*> -> CH2CH2CN + CH3CO, is assumed to be the source for a large concentration of free radicals and for carbon monoxide.Thus, ethane, methane, and carbon monoxide are the major products among the species without nitrogen.It is believed that hydrogen cyanide, acetaldehyde, and 2-buten-3-one, which are found in the postshock mixtures, are obtained by a four-center unimolecular elimination from 2-methyl-3-oxobutyronitrile from both the thermally excited state (low temperatures) and a state of thermal equilibrium (high temperatures).The isomerization reaction, which involves cleavage of the N-O bond and migration of a methyl group from position 3 to 4 in the ring, is a first-order process with a rate constant given by k_1st = 10^15.90 exp(-61.5 * 10³/RT) s^-1 where R is expressed in units of cal/(K mol).The overall decomposition of 3,5-dimethylisoxazole in terms of a first-order rate constant is given by k_total = 10^15.57 exp(-59.5 * 10³/RT) s^-1.A reaction scheme containing 26 species and 32 elementary reactions was constructed to describe the overall mechanism of 3,5-dimethylisoxazole decomposition and isomerization.