The kinetics of the formation of nitrile compounds in the atmospheres of Titan and Neptune

The nitrile compounds HCN and C2HCN (cyanoacetylene) were detected on Titan by Voyager 1. The ethylene‐analog nitrile to cyanoacetylene C2H3CN (acrylonitrile) remains undetected. More recently, ground‐based observers have detected HCN on Neptune and placed an upper limit on C2HCN. Kinetic aspects of the formation of acrylonitrile have been studied in a discharge‐flow mass spectrometric system at T = 298 K. The fractional yield for the formation of acrylonitrile from the fast gas phase reaction CN + C2H4 → C2H3CN + H has been determined to be (0.2 ± 0.1). The other possible gas phase reaction leading to the formation of acrylonitrile, HCN + C2H3 → C2H3CN + H, has been determined to be slow under planetary conditions at k(T = 298 K) = 2–7 × 10−14 cm3 molecule−1 s−1, but remains an important chemical sink for HCN. The planetary implications of the results are discussed in terms of the ratio of acrylonitrile to cyanoacetylene on Titan and Neptune.