Time-averaged chemical evolution of the studied species across all second stage models, as defined in Table 2. Line styles and shading follow those in Fig. 1. Only gas-phase abundances above the detectable threshold (X > 10⁻¹⁴) are considered. Changes in abundance, including both enhancement and depletion, generally follow the timescales seen in the average temperature profiles (see Fig. 1). In models of protostellar objects, the species that most frequently dominates over the others is HCN, whereas in shocks CH₃OH is the most dominant during the shock phase, while SiO takes over in the post-shock phase.