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

Rate constants have been measured spectrophotometrically for the react ions of alkali metal ethoxides (EtOM) with S-p-nitrophenyl 2-thiofuroa te (Ib) and 2-thiophenethiocarboxylate (2b) in absolute ethanol at 25. 0+/-0.1 degrees C. Ib is observed to be more reactive than 2b toward a ll 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. Equil ibrium association constants of alkali metal ions with the transition state (K-a(TS)) have been calculated from the known equilibrium associ ation constants of alkali metal ion with ethoxide ion (K-a) and the ra te constants for the reactions of EtOM with Ib and 2b. The catalytic e ffect (K-a(TS)/K-a) is larger for the reaction of Ib than 2b, and decr eases 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.