Second-order rate constants have been measured for reactions of Y-substituted phenyl diphenylphosphinates (1aâ h) with EtO^âK⁺ in anhydrous ethanol. A linear Brønsted-type plot is obtained with βLg = â 0.54, a typical βLg value for reactions which proceed through a concerted mechanism. The Hammett plots correlated with Ï ^o and Ï ^â constants are linear but exhibit many scattered points, while the corresponding Yukawaâ Tsuno plot results in excellent linear correlation with r = 0.41. The r value of 0.41 indicates that the leaving group departs at the rate-determining step (RDS) whether the reactions proceed through either a concerted or a stepwise mechanism. However, a stepwise mechanism in which departure of the leaving group occurs at the RDS is excluded since the incoming EtO^â ion is much more basic and a poorer leaving group than the leaving aryloxide. The Î H^{â ¡} values determined in the current reactions are strongly dependent on the nature of the substituent Y, while the Î S^{â ¡} values remain constant on changing the substituent Y in the leaving group, i.e., from Y = H to Y = 4-NO2 and Y = 3,4-(NO2)2. These Î H^{â ¡} and Î S^{â ¡} trends also support a concerted mechanism.