The effect of alkali metal ions on nucleophilic substitution reactions of p- and m-nitrophenyl 2-thiophenenates with alkali metal ethoxides in absolute ethanol

Abstract

Rate constants have been measured spectrophotometrically for the reactions of p- and m-nitrophenyl 2-thiophenecarboxylate (5a and 5b, respectively) with alkali metal ethoxides (EtO-M+) in absolute ethanol at 25.0±0.1°C. The reactivity of EtO-M+ exhibits dependence on the size of alkali metal ions, i.e. the reactivity of EtO-M+ toward 5a decreases in the order EtO-K+≥EtO-Na+>EtO -Li+>EtO-, while the one toward 5b does in the order EtO-Na+≥EtO-K+ >EtO-Li+>EtO-. This result indicates that ion paired EtO-M+ is more reactive than dissociated EtO-, and alkali metal ions form complexes with the substrate more strongly at the transition state than at the ground state. The catalytic effect shown by alkali metal ions appears to be less significant in the reaction of 5 than in the corresponding reaction of 4, indicating that complexation of alkali metal ions with 5 is not as strong as the one with 4.