Second-order rate constants (k$_{Nu?}$) have been measured for nucleophilic substitution reactions of Y-substituted phenyl benzoates (1a-i) with butane-2,3-dione monoximate ($Ox^-;an;alpha$-nucleophile) and Z-substituted phenoxides in 80 mol% H$_2$O/20 mol% DMSO at 25.0${pm}$0.1$^{circ}C$. Hammett plots correlated with ${sigma}^o$ and ${sigma}^-$ constants for reactions of 1a-h with Ox$^?$ exhibit many scattered points. In contrast, the Yukawa-Tsuno plot results in a good linear correlation with ${ ho}_Y$ = 2.20 and r = 0.45, indicating that expulsion of the leaving group occurs in the rate-determining step (RDS). A stepwise mechanism with expulsion of the leaving-group being the RDS has been excluded, since Y-substituted phenoxides are less basic and better nucleofuges than Ox$^?$. Thus, the reactions have been concluded to proceed through a concerted mechanism. Ox$^?$ is over 10$^2$ times more reactive than its reference nucleophile, 4-chlorophenoxide (4-ClPhO$^?$). One might suggest that stabilization of the transition-state (TS) through intramolecular general acid/base catalysis is responsible for the ${alpha}$-effect since such general acid/base catalysis is not possible for the corresponding reactions with 4-ClPhO$^?$. However, destabilization of the ground-state (GS) of Ox$^?$ has been concluded to be mainly responsible for the ${alpha}$-effect found in this study on the basis of the fact that the magnitude of the ${alpha}$-effect is independent of the nature of the substituent Y.