Dominant reactions controlling the abundance of C₂H₂ and related species at the position of peak fractional abundance in the disk atmospheres at a radius of 1 AU. The solid lines indicate gas-phase two-body reactions and the dotted lines indicate UV or X-ray-induced reactions. The width of the arrows for the gas-phase reactions shows the magnitude of the activation barrier. The left-hand side, highlighted in red, is the chemistry most dominant in UV-dominated gas with a higher gas temperature (e.g., in Herbig Ae disks), whereas that highlighted in blue on the right-hand side is that which dominates in X-ray irradiated gas with a lower gas temperature (e.g., in X-ray irradiated M dwarf disks).