Kinetics and Reaction Mechanism for Alkaline Hydrolysis of Y-Substituted-Phenyl Diphenylphosphinates

The second-order rate constants (k(OH)-) for the reactions of Y-substituted-phenyl diphenylphosphinates (4a-4i) with OH- in H2O at 25.0 +/- 0.1 degrees C have been measured spectrophotometrically. Comparison of k(OH)- with k(EtO)- (the second-order rate constants for the corresponding reactions with EtO- in ethanol) has revealed that EtO- is less reactive than OH- although the former is ca. 3.4 pK(a) units more basic than the latter, indicating that the reactivity of these nucleophiles is not governed by their basicity alone. The Bronsted-type plot for the reactions of 4a-4i with OH- is linear with beta(lg) = -0.36. The Hammett plot correlated with sigma(-) constants results in a slightly better correlation than that correlated with sigma(o) constants but exhibits many scattered points. In contrast, the Yukawa-Tsuno plot for the same reactions exhibits an excellent linear correlation with rho = 0.95 and r = 0.55. The r value of 0.55 implies that a negative charge develops partially on the 0 atom of the leaving group. Thus, the reactions of 4a-4i with OH- have been concluded to proceed through a concerted mechanism.