Microenvironment Effects on the Kinetics of the Alkaline Hydrolysis of Bispyridinium Conformers. Issue 11 (21st September 2013)

• Record Type: Journal Article
• Authors: Vanzin, Douglas; Fiori, Simone; França Biondo, Polyana B.; Feltrim, Gisele; Gracetto, Augusto C.; Tessaro, André L.; Politi, Mario José; Caetano, Wilker; Hioka, Noboru
• Is Part Of: International journal of chemical kinetics. Volume 45:Issue 11(2013:Nov.)
• Page Start: 703
• Page End: 711
• Publication Date: 2013-09-21
• Subjects: Chemical kinetics -- Periodicals; 541.394
• DOI: 10.1002/kin.20803
• Languages: English
• ISSNs: 0538-8066

Abstract

<abstract abstract-type="main"> <title>ABSTRACT</title> <p>The alkaline hydrolysis of a series of homologous reactants constituted by two reactive centers bridged by a methylene spacers chain, the 1, <italic>n</italic>‐bis(2‐azidepyridinium)alkanes (<italic>n</italic> = 3, 4, 5, 6, and 8), is investigated. The reaction under pseudo–first‐order condition was followed by ultraviolet–visible spectrophotometry. The presence of clear isosbestic point suggests the absence of stable intermediates. However, the intermediates 1‐(2‐azidepyridinium), <italic>n</italic>‐(2‐pyridone)alkanes (monocationic compounds), were isolated and characterized as well the reaction end products 1, <italic>n</italic>‐(2‐pyridone)alkanes (noncharged compounds). The kinetic analysis fitted to a two‐step consecutive reaction, where the <italic>k</italic><sub>1</sub>/<italic>k</italic><sub>2</sub> values demonstrate the larger reactivity of the first step over the second one, especially for shorter bridged reactants. The OH<sup>−</sup> reaction order is one for each step. Although Debye–Hückel law was obeyed, the experimental point at ionic strength zero is much higher than the extrapolated one. In addition, the <italic>k</italic><sub>1</sub> values substantially decrease as KCl is added especially for shorter homologous whereas the effect on <italic>k</italic><sub>2</sub> is almost negligible. Simple charge density effects as a function of the spacer's length do not explain the observations. On the other hand, from the pronounced anion selectivity inhibition effects on <italic>k</italic><sub>1</sub> for the shorter derivatives, the existence of an equilibrium involving a conformer, a "sandwich‐type" complex with the OH<sup>−</sup> between the two pyridinium rings, with an "open‐stretched" conformer is proposed. For short‐bridged reactants, the complex conformer prevails whereas for long‐bridged compounds the stretched structure prevails. The results in acetonitrile/water seem to reflect changes in the equilibrium. The reaction is controlled by the balance between enthalpy and entropy. The contribution of the activation entropy in a short‐bridged reactant is higher than the long‐bridged one, which suggests an activated complex with high structural organization in the first reactants.</p> </abstract>