  We investigate the origin of the small, chemically rich molecular clumps
observed along the main axis of chemically rich outflows such as CB3 and L1157.
We develop a chemical model where we explore the chemical evolution of these
clumps, assuming they are partially pre-existing to the outflow, or
alternatively newly formed by the impact of the outflow on the surrounding
medium. The effects of the impact of the outflow are reproduced by density and
temperature changes in the clump. We find that the observed abundances of
CH3OH, SO and SO2 are best reproduced by assuming a scenario where the dense
molecular gas observed is probably pre-existing in the interstellar medium
before the formation of their exciting (proto)stars and that the clumpiness and
the rich chemistry of the clumps are a consequence of a pre-existing density
enhancement and of its interaction with the outflow.
