Left panel: chemical model using the best performing model for c-FA and t-FA in the gas. Horizontal bands represent the range of observational abundances with respect to H₂ of both isomers in the three clouds where they have been detected, the Barnard-5 molecular cloud (Taquet et al. 2017), L483 (Agúndez et al. 2019) and TMC-1 (This work). Middle panel: equivalent to above but for ice abundances (comparing with H₂O) and comparing with the upper and lower bounds provided in Boogert et al. (2015) and with the well constrained narrow band of Rocha et al. (2024). We note that HCOOH and HCOO⁻ are summed in establishing the observational abundances. Right panel: t-FA/c-FA gas phase ratios for the best performing model in linear scale. The gray area represents an approximated characteristic timescale of TMC-1 (Pratap et al. 1997).