Impurity propagation was done for a single step, i.e. step impurities, arising from side reactions and otherwise, were propagated to the next step only, as per the default parameters. In the case of paracetamol synthesis, we only observed the propagation of impurities from the first step, R1, under the reaction conditions of R2 (Figure ). It can be observed that the step impurities have a number of combinatorial choices as they move through the H 2 reduction, followed by dimerization, thereby resulting in a large number of dimers in the propagated impurities.