Disclosed is a process for the manufacture of a compound of formula (I) wherein Hal represents fluoro or chloro, and R1 and R2 represent, independently from one another, hydrogen or Hal; in which process a compound of formula (II) is converted to a corresponding alkyl, fluoroalkyl or aryl sulfonic acid ester, which is then reacted with an alkali metal nitrite in the presence of a suitable crown ether in a polar non-nucleophilic solvent at a temperature of −10 to 50° C. to give the compound of formula (I).