Analysis of Linear Free‐Energy Relationships Combined with Activation Parameters Assigns a Concerted Mechanism to Alkaline Hydrolysis of X‐Substituted Phenyl Diphenylphosphinates

A kinetic study is reported for alkaline hydrolysis of X‐substituted phenyl diphenylphosphinates ( 1 a – i ). The Brønsted‐type plot for the reactions of 1 a – i is linear over 4.5 p K a units with β lg =−0.49, 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 ρ =1.42 and r =0.35. The r value of 0.35 implies that leaving‐group departure is partially advanced at the rate‐determining step (RDS). A stepwise mechanism, in which departure of the leaving group from an addition intermediate occurs in the RDS, is excluded since the incoming HO − ion is much more basic and a poorer nucleofuge than the leaving aryloxide. A dissociative (D N + A N ) mechanism is also ruled out on the basis of the small β lg value. As the substituent X in the leaving group changes from H to 4‐NO 2 and 3,4‐(NO 2 ) 2 , Δ H   ≠ decreases from 11.3 kcal mol −1 to 9.7 and 8.7 kcal mol −1 , respectively, while Δ S   ≠ varies from −22.6 cal mol −1  K −1 to −21.4 and −20.2 cal mol −1  K −1 , respectively. Analysis of LFERs combined with the activation parameters assigns a concerted mechanism to the current alkaline hydrolysis of 1 a – i .