The invention provides 5-membered heteroaromatic compounds of formula I ArN(Y)COCH.sub.2 Cl I wherein Ar is a 5-membered heteroaromatic group comprising 1 or 2 heteroatoms selected from O, S and N and linked by a ring C-atom to the N-atom of the N(Y)COCH.sub.2 Cl group to which it is bound, whereby where Ar is pyrazolyl, said N(Y)COCH.sub.2 Cl group is in the 4-position, and Y is as specified in the description, the use of these compounds as herbicides, compositions for facilitating such use and the preparation of the chloroacetamides.