Analysis of linear free-energy relationships combined with activation parameters assigns a concerted mechanism to alkaline hydrolysis of x-substituted phenyl diphenylphosphinates.

Abstract

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 pK(a) units with beta(lg)=-0.49, a typical beta(lg) value for reactions which proceed through a concerted mechanism. The Hammett plots correlated with sigma(o) and sigma(-) constants are linear but exhibit many scattered points, while the corresponding Yukawa-Tsuno plot results in excellent linear correlation with rho=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 beta(lg) value. As the substituent X in the leaving group changes from H to 4-NO(2) and 3,4-(NO(2))(2), DeltaH++ decreases from 11.3 kcal mol(-1) to 9.7 and 8.7 kcal mol(-1), respectively, while DeltaS++ 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.