Thermal Decomposition of Acetylene in Shock Waves

Using a shock tube and an improved T.O.F. mass spectrometer displaying spectra every 20 μsec, the pyrolysis of acetylene was shown to proceed through the sequence $${\mathrm{C}}_2{\mathrm{H}}_2\to {\mathrm{C}}_4{\mathrm{H}}_3\to {\mathrm{C}}_4{\mathrm{H}}_2\to {\mathrm{C}}_6{\mathrm{H}}_2\to {\mathrm{C}}_8{\mathrm{H}}_2\mathrm{\to ···}$$ in the temperature range 1600°—2400°K. All these intermediates reach steady‐state concentrations, that of the radical C₄H₃ being as large as that of diacetylene under favorable conditions. The primary bimolecular reaction of acetylene in which the radical C₄H₃ is formed must also yield hydrogen atoms. These catalyze an isotopic exchange in mixtures of C₂H₂+C₂D₂ that is about three orders of magnitude faster than the formation of C₄H₂. The initial yield of acetylene‐d₁ is given by the expression $$[{\mathrm{C}}_2\mathrm{HD}{]}_t\mathrm{=k[}{\mathrm{C}}_2{\mathrm{H}}_2+{\mathrm{C}}_2{\mathrm{D}}_2{]}_0^3{t}^2,$$ which is consistent with the proposed partial reaction mechanism. The mechanism of conversion of C₄H₃ into C₄H₂, etc., is uncertain but undoubtedly involves free radicals.