Gas-phase reactions of hydroxyl radicals with aldehydes in flowing H2O2+ NO2+ CO mixtures

Abstract

The chain reaction in gaseous H₂O₂+ NO₂+ CO mixtures, previously used as a thermal source of OH radicals in static systems, has been adapted for use in a flow system operating at total pressures of 39.9 kPa and 298.2 K. The reaction mechanism is H₂O₂+ NO₂→ OH + HNO₃(1), OH + CO → CO₂+ H (2), H + NO₂→ OH + NO (3), OH + NO₂(+M)→ HNO₃(+M). (4), Addition of a substrate S reactive towards OH induces the competitive reaction OH + S → products. (S) From the variation in the yield of CO₂(monitored by gas chromatography) as a function of [S] the rate constant ratio k_S/k₂ may be derived. It is shown that such ratios can be successfully measured for systems (in this case a series of aldehydes) which would be difficult to study in the corresponding static system.

The aldehyde rate constants, k_S, are expressed relative to k_S(acetaldehyde), and taking k_S(acetaldehyde)= 0.94 × 10¹⁰ dm³ mol^–1 s^–1 values of 10^–10k_S/dm³ mol^–1 s^–1 of 1.08 ± 0.14, 1.52 ± 0.19, 1.05 ± 0.12, 0.83 ± 0.10, 1.11 ± 0.12 and 0.51 ± 0.05 are obtained for, respectively, propionaldehyde, n-butyraldehyde, i-butyraldehyde, n-valeraldehyde, i-valeraldehyde and trimethylacetaldehyde.

Comparison with other measurements is made and the variations in these values are discussed.