NUCLEOPHILIC REACTIVITY - NUCLEOPHILIC AROMATIC-SUBSTITUTION REACTIONS OF 2,4-DINITROCHLOROBENZENE AND PICRYL CHLORIDE IN AQUEOUS AND METHANOL SOLUTIONS

Rate constants for nucleophilic aromatic substitution reactions of picryl chloride, I, with acetate ion and a series of amine nucleophiles, including primary, secondary, tertiary (quinuclidines), and amines which show the alpha-effect, in aqueous and methanol solutions have been measured. Rate constants have also been measured, in water, for the reactions of 2,4-dinitrochlorobenzene, II, with trifluoroethylamine and semicarbazide. Reactions of picryl chloride with 3-quinuclidinol and DABCO yield picrate ion via general base catalyzed hydrolysis. On the other hand, reaction with quinuclidine proceeds at least in part via a mechanism in which N-picrylquinuclidinium ion is formed and then converted into a N-picrylpiperidine derivative. When the kinetic data are combined with data previously reported in the literature, the slopes of plots of log of the second-order rate constants against Ritchie's N+ parameter are 0.95 +/- 0.13 and 0.79 +/- 0.11 for the reactions of 2,4-dinitrochlorobenzene and picryl chloride, respectively (the reported error limits are the 95% confidence intervals). Steric effects may be important in these reactions, but these effects do not significantly change the slopes of the N+ correlation lines. The slope smaller than one for the reactions of picryl chloride is consistent with decreased selectivity towards nucleophilic attack, and with the modified form of Ritchie's equation, log k = S+N+ + log k0, in which relative nucleophilic reactivities are substrate dependent.