Chemical diagram describing the main reaction routes to form HCN, C₂H₂, CH₄ and NH₃ in warm gas in protoplanetary disks or hot cores depending on the radiation fields, temperature and different molecular abundances, adapted from Agúndez et al. (2008). At high temperatures of several hundred K the reaction routes (in red) starting with C reacting with H₂ to CH and NH to NH₂ dominate over lower temperature chemical routes (in blue) of C⁺ going to CH⁺ and NH and NH. The different activation energy (E_a) barriers for the chemical reactions are represented by different types of arrows; the thicker the arrow, the higher the activation energy.