Um aparato para monitorar reações rápidas: um "stopped-flow" artesanal de baixo custo

Batistela, Vagner Roberto; Tessaro, André Luiz; Soares, Rafael R. da S.; Souza, Vagner Roberto de; Hioka, Noboru

doi:10.1590/s0100-40422006000500034

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“…The NaOH final concentration was 9.4 × 10 −3 mol L −1 . For higher acetonitrile percentage, the kinetics was followed in a homemade stopped‐flow apparatus having a mixture time around 0.3 s .…”

Section: Methodsmentioning

confidence: 99%

Microenvironment Effects on the Kinetics of the Alkaline Hydrolysis of Bispyridinium Conformers

Vanzin

Fiori

Biondo

et al. 2013

Int J of Chemical Kinetics

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The alkaline hydrolysis of a series of homologous reactants constituted by two reactive centers bridged by a methylene spacers chain, the 1,n-bis(2-azidepyridinium)alkanes (n = 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), n-(2-pyridone)alkanes (monocationic compounds), were isolated and characterized as well the reaction end products 1,n-(2-pyridone)alkanes (noncharged compounds). The kinetic analysis fitted to a two-step consecutive reaction, where the k 1 /k 2 values demonstrate the larger reactivity of the first step over the second one, especially for shorter bridged reactants. The OH − 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 k 1 values substantially decrease as KCl is added especially for shorter homologous whereas the effect on k 2 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 k 1 for the shorter derivatives, the existence of an equilibrium involving a conformer, a "sandwich-type" complex with the OH − between the two pyridinium rings, with an "open-stretched" conformer is proposed. For short-bridged reactants, the complex conformer

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Section: Methodsmentioning

confidence: 99%