Chemical pathways to create C₂H₂ and other hydrocarbon molecules without mixing (left) and with α_mix = 10⁻³ (right) at one selected point in the disc at r = 0.15 au (see text). It is important to note the vastly different concentrations of all the hydrocarbon molecules. As in Fig. 7, the green numbers represent the particle concentrations and the blue numbers represent the rates [cm⁻³ s⁻¹] in the time-independent case. The big red ‘IN’ and ‘OUT’ arrows denote the in-mixing and out-mixing rates. Wiggled arrows indicate photo-dissociation. The light green wiggled arrows are X-ray induced processes. Three reactions appear twice with an added dashed line, when a reaction is important for two reactants or products (C₂H₂ + C → C₃ + H₂, C₂H₂ + C → C₃H + H and C₃ + hv → C₂ + C). On the left side, the reaction paths for C₂O, C⁺⁺, CH₅⁺ and H₃CS⁺ are not followed up.