Kinetic and Theoretical Studies on Pyridinolysis of 2,4-Dinitrophenyl X-Substituted Benzoates: Effect of Substituent X on Reactivity and Mechanism

Bulletin of the Korean chemical society

Li Ra Im¹  Ik Hwan Um¹  Eun Hee Kim¹  Masaaki Mishima¹ 

关键词: Pyridinolysis;    Br©ªnsted-type plot;    Intermediate;    Rate-determining step;    Reaction mechani;   

DOI  :  

学科分类：化学（综合）

来源: Korean Chemical Society

【 摘 要 】

Second-order rate constants (k_N) have been measured spectrophotometrically for reactions of 2,4-dinitrophenyl X-substituted benzoates (X = 4-MeO, H and 4-NO₂) with a series of Z-substituted pyridines in 80 mol % H₂O/20 mol % DMSO at 25.0 �? 0.1 ^oC. The Br┆nsted-type plots exhibit downward curvature (e.g., �?2 = 0.89 ~ 0.96 when pK_a < 9.5 while �?1 = 0.38 ~ 0.46 when pK_a > 9.5), indicating that the reaction proceeds through a stepwise mechanism with a change in rate-determining step (RDS). The pK_a^o, defined as the pK_a at the center of Br┆nsted curvature, has been analyzed to be 9.5 regardless of the electronic nature of the substituent X in the benzoyl moiety. Dissection of kN into the microscopic rate constants k1 and k₂/k_-1 ratio has revealed that k₁ is governed by the electronic nature of the substituent X but the k₂/k_-1 ratio is not. Theoretical calculations also support the argument that the electronic nature of the substituent X in the benzoyl moiety does not influence the k₂/k_-1 ratio.

【 授权许可】

Unknown