The mechanism of activated nucleophilic substitution in the aromatic series (S. N. A. *) can be integrated in the general scheme of S. N. synchronous mechanisms. The relative order of mobilities of the halogens in S. N. A. * type reactions, wich varies with the nucleophilic activity of the reagent is in good agreement with a gradation of S . N. meehanisms in aromatic and aliphatic bimolecular nucleophilic substitutions. This phenomenon depends on the balance of bond-breaking and bondmaking which has occurred in the transition state; this is determined, to a large extent, by the nucleophilic power of the reagent and the nature of the repi:it.ed g ~o u p in the organic substrate. Experimental results show that S. N. A. *-type reactions are sensitive to steric as well as polar effects of substituents.
L, a discussion de nos rksultats expcrimentaux (l) ( 2 ) (3) prend appui sur le mkcanisnie synchrone propostl par et Hevan ( 5 ) , considere comme hypothese de travail.
Les donnkes du tableau I montrent que dans le cas de l'action de I'iodure de potassium sur les l-halogeno-2,4-dinitro-* Associk au Fonds National Belge de la Recherche Scientifique ( I ) P.