AbstractWith conventional phase‐transfer catalysis using quaternary ammonium salts, yields of 2, 4‐dinitrobenzenesulfonic acid (2) in the sulfodechlorination of 1‐chloro 2.4‐dinitrobenzene (1) with sulfite ions are no better than those obtained with the classical method in aqueous ethanol (80–82%). Yields up to 97% and a very pure product are obtained, however, by using protonated tertiary amines as catalysts. The optimum chain‐length of the amine is found with tributylamine. On mixing solutions of the reagents and catalysts a strong bluish‐red colour develops immediately, but disappears withinca.1 h. Comparison of the NMR. spectrum of this primary product with model adducts of sulfite ions and di‐ and trinitrobenzene derivatives demonstrates that the primary addition of the nucleophile to 1‐chloro‐2, 4‐dinitrobenzene does not take place at C (1), but at C(5). It is shown that the increments calculated for a C sp 3‐SO 3−group and for the 1, 3‐dinitro‐ and 1, 3, 5‐trinitropentadienyl ring moieties can be employed for the approximate calculation of1H‐chemical shifts usingClerc&Pretsc