A mechanistic study on reactions of aryl benzoates with ethoxide, aryloxides and acetophenone oximates in absolute ethanol

Abstract

Second-order rate constants have been measured spectrophotometrically for the reactions of aryl benzoates (X-C 6H 4CO 2 C 6H 4-Y) with EtO -, Z-C 6H 4O - and Z-C 6H 4C(Me)=NO - in absolute ethanol at 25.0 °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 metal ion. A good Hammett correlation has been obtained with a large ρ - value (-1.96) when σ - (Z) constant was used for the reaction of p-nitrophenyl benzoate (PNPB) with Z-C 6H 4O -. Surprisingly, the one for the reaction of PNPB with Z-C 6H 4C(Me)=NO - gives a small but definitely positive ρ value (+0.09). However, for reactions of C 6H 5CO 2C 6H 4-Y with EtO -, correlation of log k with σ - (Y) constant gives very poor Hammett correlation. A significantly improved linearity has been obtained when σ° (Y) constant was used, indicating that the leaving group departure is little advanced at the TS of the RDS. For reactions of X-C 6H 4CO 2C 6H 4-4-NO 2 with EtO -, C 6H 5O - and C 6H 5C(Me)=NO -, correlations of log k with σ (X) constants for all the three nucleophile systems give good linearity with large positive ρ values, e.g. 2.95, 2.81 and 3.06 for EtO -, C 6H 5O - and C 6H 5C(Me)=NO -, respectively. The large ρ values clearly suggest that the present reaction proceeds via a stepwise mechanism in which the formation of the addition intermediate is the RDS.