Model predictions of acetaldehyde and formamide after a shock passage. Upper panel: CH₃CHO/NH₂CHO calculated abundance ratio as a function of time from the passage of the shock. Solid lines refer to a model in which acetaldehyde and formamide are both synthesised in the gas phase, whereas dashed lines refer to predictions assuming that they are injected into the gas phase directly from the grain mantles. The vertical ranges of the three coloured boxes represent the measured ranges, including the uncertainties of the CH₃CHO/NH₂CHO abundance ratio towards the three zones identified in Fig. 2: SHOCK 1 (ratio >8), SHOCK 2 (ratio 2–8), and SHOCK 3 (ratio ≤2). When not detected, we derived the upper limits on CH₃CHO and NH₂CHO with the 3σ value. The two dotted vertical lines define the time when the formamide and acetaldehyde abundance ratios (as derived by the gas-phase model) fall below the minimum measured values. Lower panel: acetaldehyde (CH₃CHO, blue) and formamide (NH₂CHO, red) abundances, with respect to H-nuclei, as a function of time from the passage of the shock. The dashed blue and red regions show the maximum and minimum CH₃CHO and NH₂CHO measured abundances.