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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 26, Number 12
BKCSDE 26(12)
December 20, 2005 

 
Title
Kinetics and Mechanism of Alkaline Hydrolysis of [(Methoxy)(p-substituted styryl)-carbene] Pentacarbonyl Chromium(0) Complexes in Aqueous Acetonitrile
Author
Gap-Cheol Shin, Jaeyoung Hwang, Kiyull Yang, In Sun Koo*, Ikchoon Lee
Keywords
Kinetic study, Hydrolysis, Activation parameters, Hammett equation, Yukawa-Tsuno equation
Abstract
Kinetic studies have been performed for alkaline hydrolysis of a series of [(methoxy)(p-substituted styryl)carbene]pentacarbonyl chromium(0) complexes ((CO)5Cr=C(OCH3)CH=CHC6H4X, X = p-OCH3, p-CH3, H, p-Cl, p-NO2). Second-order rate constants (kOH-) for the alkaline hydrolysis in 50% acetonitrile-water (v/v) were determined spectrophotometrically at various temperatures. At a low pH region (pH < 7.5), the observed rate constant (kobs) remained constant with a small value, while in a high pH region (pH > 9.5), kobs increases linearly with increasing the pH of the medium. The second-order rate constants (kOH-) increase as the substituent X changes from a strong electron donating group to a strong electron withdrawing group. The Hammett plot obtained for the alkaline hydrolysis is consisted of two intersecting straight lines. The nonlinear Hammett plot might be interpreted as a change in the rate-determining step. However, the fact that the corresponding Yukawa-Tsuno plot is linear with ρ and r values of 0.71 and 1.14, respectively indicates that the nonlinear Hammett plot is not due to a change in the rate-determing step but is due to ground-state stabilization through resonance interaction. The positive ρ value suggests that nucleophilic attack by OH- to form a tetrahedral addition intermediate is the rate-determining step. The large negative ΔS value determined in the present system is consistent with the proposed mechanism.
Page
1981 - 1985
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