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Kinetic Study on Alkaline Hydrolysis of 2‐Pyridyl and 4‐Pyridyl X‐substituted‐Benzoates: Effects of Benzoyl Substituent X and Leaving‐Group Basicity on Reactivity and Reaction Mechanism
Author(s) -
Shin YoungHee,
Koh HanJoong,
Um IkHwan
Publication year - 2017
Publication title -
bulletin of the korean chemical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.237
H-Index - 59
ISSN - 1229-5949
DOI - 10.1002/bkcs.11233
Subject(s) - chemistry , benzoates , substituent , hammett equation , moiety , medicinal chemistry , leaving group , reactivity (psychology) , hydrolysis , reaction rate constant , alkaline hydrolysis , polar effect , stereochemistry , kinetics , organic chemistry , catalysis , medicine , physics , alternative medicine , pathology , quantum mechanics
Second‐order rate constants for alkaline hydrolysis of 2‐pyridyl X‐substituted‐benzoates ( 4a–4l ) and 4‐pyridyl X‐substituted‐benzoates ( 5a–5l ) are reported. The Hammett plots are nonlinear, e.g ., substrates possessing a weak electron‐withdrawing group ( EWG ) or an electron‐donating group ( EDG ) in the benzoyl moiety deviate negatively from the linear Hammett plots composed of substrates bearing a strong EWG . In contrast, the Yukawa–Tsuno plots exhibit excellent linearity with ρ X = 1.57, r = 0.43, and R 2 = 0.998 for the reactions of 4a–4l and ρ X = 1.47, r = 0.41, and R 2 = 0.999 for those of 5a–5l , indicating that the nonlinear Hammett plots are not due to a change in the rate‐determining step (RDS) but are caused by ground‐state (GS) stabilization through resonance interactions. The ρ X values found for the reactions of 4a–4l and 5a–5l are similar to those reported previously for reactions that proceed through a stepwise mechanism with formation of an intermediate being the RDS. Small β lg values ( e.g ., −0.2 ~ −0.3) calculated from the Brønsted‐type correlation of rate constants with leaving‐group basicity also indicate that the reactions proceed through a stepwise mechanism, in which expulsion of the leaving group occurs after the RDS.