Evolution with time of the molecular fractional abundances predicted by our chemical kinetics model for a rectangular slab of gas (plane-parallel geometry) with physical properties similar to those of the walls of the hollow lobes of the molecular outflow of OH 231.8+4.2 (model input b, Sect. 5). The model presented is for a representative gas cell in the middle of the lobe walls (A_V = 1 mag). The species represented are as in Figs. 11 and 12, except for HC₃N, which has a predicted maximum abundance (≈10^-19) well below the lower limit of the y-axis.