Kinetics and Mechanism of Alkaline Hydrolysis of [(Methoxy)(p-substituted styryl)-carbene] Pentacarbonyl Chromium(0) Complexes in Aqueous Acetonitrile

Bulletin of the Korean chemical society

In Sun Koo¹  Ikchoon Lee¹  Gap-Cheol Shin¹  Kiyull Yang¹  Jaeyoung Hwang¹ 

关键词: Kinetic study;    Hydrolysis;    Activation parameters;    Hammett equation;    Yukawa-Tsuno equation;   

DOI  :  

学科分类：化学（综合）

来源: Korean Chemical Society

【 摘 要 】

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 (k_OH-) 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 (k_obs) remained constant with a small value, while in a high pH region (pH > 9.5), k_obs increases linearly with increasing the pH of the medium. The second-order rate constants (k_OH-) 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 �? 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 �? value suggests that nucleophilic attack by OH^- to form a tetrahedral addition intermediate is the rate-determining step. The large negative ツS^× value determined in the present system is consistent with the proposed mechanism.

【 授权许可】

Unknown