Combined dual substituent constant and activation parameter analysis assigns a concerted mechanism to alkaline ethanolysis at phosphorus of Y-substituted phenyl diphenylphosphinates

Abstract

Second-order rate constants have been measured for reactions of Y-substituted phenyl diphenylphosphinates (1a–h) with EtO⁻K⁺ in anhydrous ethanol. A linear Brønsted-type plot is obtained with β_Lg = −0.54, a typical β_Lg value for reactions which proceed through a concerted mechanism. The Hammett plots correlated with σ^o and σ⁻ constants are linear but exhibit many scattered points, while the corresponding Yukawa–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⁻ ion is much more basic and a poorer leaving group than the leaving aryloxide. The ΔH^‡ values determined in the current reactions are strongly dependent on the nature of the substituent Y, while the ΔS^‡ 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 ΔH^‡ and ΔS^‡ trends also support a concerted mechanism.