Gas phase addition of HI to ketene and the kinetics of decomposition of the acetyl radical

Abstract

The addition of HI to ketene results in the rapid formation of acetyl iodide at temperatures 498–525 K. The subsequent slower reaction between HI and acetyl iodide, which produces methane and acetaldehyde has been investigated at low conversions (<10 %), at various initial pressures at 507 K. Most experiments were done in the presence of the inert additives N₂ or cyclo-C₄F₈. The results are shown to be consistent with the mechanism (M)+ CH₃CO→CH₃+ CO +(M)(5), CH₃CO + HI→CH₃CHO + I (3), CH₃+ HI→CH₄+ I. (6)

The data at 507 K yield k^∞₅/k₃=(2.4 ± 0.3)× 10^–5 mol dm^–3 and I/S= 65 ± 8 dm³ mol^–1, where I and S refer to the intercept and slope of a Lindemann plot for the unimolecular step (5). An independent estimate of k₃ is used in conjunction with theoretical calculations on the acetyl decomposition reaction to show that, log(k^∞₅/s^–1)=(13.3 ± 0.5)–(91.2 ± 7.5 kJ mol^–1)/2.303 RT. The limits cover all reasonable uncertainties, both experimental and theoretical. This result disposes of previous reports of a “low”A factor for this reaction.