Mechanism of the uncatalysed path of aromatic nucleophilic substitution in dipolar aprotic solvents when primary and secodary amines are the nucleophiles; a search for electrophilic catalysis of these reactions

Abstract

The kinetics of the reactions of 1-chloro- and 1-fluoro-2,4-dinitrobenzene with morpholine have been studied in dimethyl sulphoxide, dimethylformamide, and nitromethane. The results confirm that the decomposition of the intermediate to products by the uncatalysed path takes place by a unimolecular mechanism. The kinetics of the reactions of 2,4-dinitroanisole with n-butylamine and piperidine in dipolar aprotic solvents have been determined. They show that when primary amines are the nucleophiles, reaction by the uncatalysed path does not occur by the unimolecular mechanism, but by the specific base-general acid (SB–GA) route. The reactions in dimethyl sulphoxide give another example of secondary amines reacting by a base catalysis mechanism whereas the corresponding reaction of a primary amine of the same basicity is not base-catalysed.

In a search for electrophilic catalysis the effects of lithium, trialkylammonium, and tetraalkylammonium ions on the reactions of piperdine with 2,4-dinitroanisole and of morpholine with 2,4-dinitrophenyl phenyl ether in dimethyl sulphoxide were investigated. No catalysis was found and a tentative reason is given. When aniline reacts with 2,4,6-trinitrophenyl methyl ether in dimethyl sulphoxide, 82% of the reaction occurs at the methyl carbon atom, and when the substrate is 2,4,6-trinitrophenyl phenyl ether the reaction is not base-catalysed. The first two observations of the change from base-catalysed to uncatalysed aromatic nucleophilic substitution reactions brought about by a change from protic to dipolar aprotic solvent are recorded.