The Effect of Alkali Metal Ions on Nucleophilic Substitution Reactions of Alkali Metal Ethoxides with S-p-nitrophenyl 2-thiofuroate and 2-Thiophenethiocarboxylate in Absolute Ethanol

摘要

Rate constants have been measured spectrophotometrically for the reactions of alkali metal ethoxides (EtOM) with S-p-nitrophenyl 2-thiofuroate (1b) and 2-thiophenethiocarboxylate (2b) in absolute ethanol at 25.0【0.1 ∩. 1b is observed to be more reactive than 2b toward all the EtOM studied. The reactivity of EtOM is in the order EtOK > EtONa > EtO- > EtOLi for both substrates, indicating that K+ and Na+ behave as a catalyst while Li+ acts as an inhibitor in the present system. Equilibrium association constants of alkali metal ions with the transition state (KaTS) have been calculated from the known equilibrium association constants of alkali metal ion with ethoxide ion (Ka) and the rate constants for the reactions of EtOM with 1b and 2b. The catalytic effect (KaTS/Ka) is larger for the reaction of 1b than 2b, and decreases with decreasing the size of the alkali metal ions. Formation of 5-membered chelation at the transition state appears to be responsible for the catalytic effect.