The invention relates to a process for the preparation of a compound of formula I (I), wherein R1 is hydrogen, fluoro or chloro; which process comprises a) reacting a compound of formula II (II), wherein R1 is hydrogen, fluoro or chloro; with a nitration agent to the compound of formula (III), wherein R1 is hydrogen, fluoro or chloro; and b) reacting the compound of formula III with chlorine gas at temperature from 180° C. to 250° C. to the compound of formula I.