Metal Ion Catalysis in Nucleophilic Substitution Reaction of 4-Nitrophenyl Picolinate with Alkali Metal Ethoxides in Anhydrous Ethanol

• Published in: Bulletin of the Korean Chemical Society Vol. 31; no. 9; pp. 2483 - 2487
• Main Authors: Hong, Yeon-Ju, Kim, Song-I, Um, Ik-Hwan
• Format: Journal Article
• Language: English
• Published: 대한화학회 20.09.2010
• Subjects: 화학
• ISSN: 0253-2964; 1229-5949
• DOI: 10.5012/bkcs.2010.31.9.2483

Pseudo-first-order rate constants (kobsd) were measured spectrophotometrically for nucleophilic substitution reactions of 4-nitrophenyl picolinate (6) with alkali metal ethoxides (EtOM, M+ = K+, Na+ and Li+) in anhydrous ethanol at 25.0 ±0.1 oC. The plot of kobsd vs. [EtOM] exhibits upward curvature regardless of the nature of M+ ions. However, the plot for the reaction of 6 with EtOK is linear with significantly decreased kobsd values when 18-crown-6-ether (18C6, a complexing agent for K+ ion) is added in the reaction medium. Dissection of kobsd into kEtO‒ and kEtOM (i.e., the second-order rate constant for the reaction with dissociated EtO‒ and ion-paired EtOM, respectively) has revealed that ion-paired EtOM is 3 ~ 17 times more reactive than dissociated EtO‒. The reaction has been proposed to proceed through a 5-membered cyclic transition state, in which M+ ion increases the electrophilicity of the reaction site. Interestingly, Na+ ion exhibits the largest catalytic effect. The presence of a nitrogen atom in the pyridine moiety of 6 has been suggested to be responsible for the high Na+ ion selectivity. KCI Citation Count: 8