A MECHANISTIC STUDY ON REACTIONS OF ARYL BENZOATES WITH ETHOXIDE, ARYLOXIDES AND ACETOPHENONE OXIMATES IN ABSOLUTE ETHANOL

Second-order rate constants have been measured spectrophotometrically for the reactions of aryl benzoates (X-C6H4CO2 C6H4-Y) with EtO(-), Z- C6H4O- and Z-C6H4C(Me)=NO- in absolute ethanol at 25.0 degrees C. All the reactions have been performed in the presence of excess 18-crown-6 ether in order to eliminate the catalytic effect shown by alkali meta l ion. A good Hammett correlation has been obtained with a large rho(- ) value (-1.96) when sigma(-) (Z) constant was used for the reaction o f P-nitrophenyl benzoate (PNPB) with Z-C6H4O-. Surprisingly, the one f or the reaction of PNPB with Z-C6H4C(Me)=NO- gives a small but definit ely positive rho(-) value (+0.09). However, for reactions of C6H5CO2C6 H4-Y with EtO(-), correlation of log k with sigma(-) (Y) constant give s very poor Hammett correlation. A significantly improved linearity ha s been obtained when sigma '' (Y) constant was used, indicating that t he leaving group departure is little advanced at the TS of the RDS. Fo r reactions of X-C6H4CO2C6H4-4-NO2 with EtO(-), C6H5O- and C6H5C(Me)=N O-, correlations of log k with sigma (X) constants for all the three n ucleophile systems give good linearity with large positive rho values, e.g. 2.95, 2.81 and 3.06 for EtO(-), C6H5O- and C6H5C(Me)=NO-, respec tively. The large rho values clearly suggest that the present reaction proceeds via a stepwise mechanism in which the formation of the addit ion intermediate is the RDS.