Reactions of Methyl Radicals Produced by the Pyrolysis of Azomethane or Ethane in Reflected Shock Waves

A shock tube coupled to a time‐of‐flight mass spectrometer has been used to study the reactions of methyl radicals at temperatures much higher than before. The radicals were formed by heating mixtures containing azomethane or dimethylmercury highly diluted in neon to between 1120 and 1400°K in reflected shock waves. Detailed measurements of methane, ethane, and ethylene production have been made over the 380 μsec of observation time. The effects of adding ethane and ethylene to the reaction mixture have also been investigated. Possible mechanisms for the hydrocarbon product formation are discussed. It is concluded that, despite the low concentrations and short observation time used, only a chain mechanism describes our observations. The main reactions are $${\mathrm{CH}}_3+{\mathrm{CH}}_3\to {\mathrm{C}}_2{\mathrm{H}}_6,$$ $${\mathrm{CH}}_3+{\mathrm{C}}_2{\mathrm{H}}_6\to {\mathrm{C}}_2{\mathrm{H}}_5+{\mathrm{CH}}_4,$$ $${\mathrm{C}}_2{\mathrm{H}}_5\to {\mathrm{C}}_2{\mathrm{H}}_4+\mathrm{H},$$ $$\mathrm{H}+{\mathrm{C}}_2{\mathrm{H}}_6\to {\mathrm{C}}_2{\mathrm{H}}_5+{\mathrm{H}}_2.$$ The temperature‐independent rate coefficient $k_1$ was found to be $k_1{\text{\hspace{0.17em}=\hspace{0.17em}1.4\hspace{0.17em}±\hspace{0.17em}0.6\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−11}}{\mathrm{cm}}^3/\mathrm{particle·sec}$ at the total density 9 × 10¹⁷ particle/cm³. Numerical integrations of this chain mechanism at 1350°K are consistent with the experimental data only when the initiation reaction (2) is assigned a rate coefficient 5.9 × 10⁻¹³ cm³/particle·sec which is close to 12 times greater than that expected from the rate data given in the literature. Other rate coefficients are within 50% of the values obtained using literature data. To test these conclusions, experiments have been made on the decomposition of ethane at 1485°K. The same chain mechanism described the product formation, and as above the numerical integration required the larger value of $k_2$ .