Introduction
Previous research carried out in our laboratory on aromatic nucleophile substitutions (S
NAr) of 2,4-dinitrochlorobenzene in aprotic solvent [
1,
2] have shown formation of aniline dimers acting as nucleophile, formation of molecular complexes substrate-nucleophile and substrate-product reaction, formation of mixed aggregates aniline-HBA additive and specific effects solvent on S
NAr mechanism.
In order to investigate the relevance of nucleophile structure in defining the mechanism of SNAr with amines in aprotic solvent, kinetic studies of the reaction of DNClB with aniline, p-anisidine, p-toluidine, 2,4-dimethylaniline and N-methylaniline in toluene at 40°C have been carried out.
Experimental
Aniline, p-toluidine, 2,4-dimethylaniline and N-methylaniline were distilled over zinc powder and then over sodium under nitrogen at reduced pressure; p-anisidine was purified to constant melting point by recrystallization with toluene; DNClB was crystallized twice from absolute ethanol. Toluene was kept over sodium wire for several days and distilled twice over sodium. The reaction products were prepared and purified following the procedure previously reported [
3].
Results and discussion
The second order rate coefficients, k
A, were found to increase rapidly with amine concentration, [B], the plots of k
A vs [B] show a quadratic dependence for the reaction of the primary anilines. The present results can be interpreted in terms of “dimer nucleophile” mechanism in which a dimeric aggregate of the amine (B:B) is considered to attack the substrate in the first step [
1].
In order to evaluate the magnitude of the curvature obtained, the experimental values were fitted to a second-degree polynomial function. The quadratic coefficients, except for N-methylaniline, are significantly different from zero.
When 2,4-dimethylaniline is used, the second order rate coefficients are considerably smaller than for aniline at any concentration and the quadratic coefficient is also much less relevant than for the other anilines. These results are consistent with the “dimer nucleophile” mechanism and can be easily explained by decreasing dimerization due to the steric hindrance produced by the methyl group at the ortho position.
For the reaction of DNClB with N-methylaniline in toluene, the plot of kA vs [B] shows a clear linear dependence with a zero intercept; where the spontaneous decomposition of the zwitterionic intermediate is negligible. This kinetic behaviour is consistent with what could be expected for the N-alkylanilines, considering that they undergo less-association than aniline. For this reason the attack by the “dimer nucleophile” is not relevant.