Gas–grain astrochemical model results for C₅H₆ in dark clouds as a function of cloud age. (a–c) Nominal network results (solid black line); nominal network but with only some specific reactions as a source of C₅H₆ (solid red line). (d) Nominal with H + C₄H₅ leading to C₄H₆ (solid black line), nominal with H + C₄H₅ leading to C₄H₆ with only H + C₅H₅ leading to C₅H₆ (solid red line), and nominal with H + C₄H₅ leading to C₄H₆ with only CH + C₄H₆ leading to C₅H₆ (solid blue line). The horizontal gray region represents the observed abundances in TMC-1 with an arbitrary error associated (±2). The estimated age of the cloud, marked by a vertical gray region, corresponds to the time at which our model predictions best match the observed abundances of 67 key species.