Reactions of Atomic Oxygen with Acetylene. I. Kinetics and Mechanisms

The room‐temperature reaction of acetylene with atomic oxygen has been studied in a moderately fast flow system. The course of the reaction was followed using both the Bendix T.O.F. mass spectrometer and a photometric method. The major products of the reaction were carbon monoxide and hydrogen. Large amounts of atomic hydrogen were also observed. The following reaction scheme is consistent with our results: $$\text{(1)\hspace{0.17em}\hspace{0.17em}}{\mathrm{C}}_2{\mathrm{H}}_2+\mathrm{O}\mathrm{\to [}{\mathrm{C}}_2{\mathrm{H}}_2{\mathrm{O}}^{*}\mathrm{]\to }{\mathrm{CH}}_2+\mathrm{CO},$$ $$\text{(2)\hspace{0.17em}\hspace{0.17em}}{\mathrm{CH}}_2+\mathrm{O}\to \mathrm{HCO}+\mathrm{H},$$ $$\text{(3)\hspace{0.17em}}{\mathrm{CH}}_2+\mathrm{O}\to {\mathrm{H}}_2+\mathrm{CO},$$ $$\text{(4)\hspace{0.17em}\hspace{0.17em}}\mathrm{HCO}+\mathrm{H}\to {\mathrm{H}}_2+\mathrm{CO},$$ $$\text{(5)\hspace{0.17em}\hspace{0.17em}}\mathrm{HCO}+\mathrm{O}\to \mathrm{OH}+\mathrm{CO},$$ $$\text{(6)\hspace{0.17em}\hspace{0.17em}}\mathrm{HCO}+\mathrm{O}\to \mathrm{H}+{\mathrm{CO}}_2,$$ $$\text{(7)\hspace{0.17em}\hspace{0.17em}}\mathrm{OH}+\mathrm{O}\to \mathrm{H}+{\mathrm{O}}_2.$$ The rate of the over‐all reaction is determined by the first initial step in which atomic oxygen adds to acetylene to yield an excited complex which then undergoes unimolecular decomposition to give triplet methylene. The rate constant for this initial reaction was measured, at room temperature, as (8.9±2.8)×10⁻¹⁴ cc molecule⁻¹·sec⁻¹. The activation energy was less than 1 kcal mole⁻¹. Experiments with acetylene‐d₂ have established that Reactions (2) and (4) constitute the major pathway by which triplet methylene, produced in Reaction (1), is consumed.