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Molecules 2014, 19(1), 459-481; doi:10.3390/molecules19010459
Article

NMR Structural Study of the Prototropic Equilibrium in Solution of Schiff Bases as Model Compounds

1
, 2
, 3
, 3
, 2
, 1
 and 1,*
1 Laboratorio de Posgrado, Facultad de Ciencias Químicas, Universidad de Colima, Km 9 Carretera Colima-Coquimatlán, Colima 28400, Mexico 2 Departamento de Química, Centro de Investigación y Estudios Avanzados del Instituto Politécnico Nacional, Av. IPN 2508, San Pedro Zacatenco 07360, México D. F., Mexico 3 Departamento de Ciencias Básicas, Unidad Profesional Interdisciplinaria de Biotecnología del Instituto Politécnico Nacional, Av. Acueducto s/n Barrio la Laguna Ticomán, México D. F. 07340, Mexico
* Author to whom correspondence should be addressed.
Received: 28 October 2013 / Revised: 12 December 2013 / Accepted: 13 December 2013 / Published: 31 December 2013

Abstract

An NMR titration method has been used to simultaneously measure the acid dissociation constant (pKa) and the intramolecular NHO prototropic constant ΔKNHO on a set of Schiff bases. The model compounds were synthesized from benzylamine and substituted ortho-hydroxyaldehydes, appropriately substituted with electron-donating and electron-withdrawing groups to modulate the acidity of the intramolecular NHO hydrogen bond. The structure in solution was established by 1H-, 13C- and 15N-NMR spectroscopy. The physicochemical parameters of the intramolecular NHO hydrogen bond (pKa, ΔKNHO and ΔΔG°) were obtained from 1H-NMR titration data and pH measurements. The Henderson–Hasselbalch data analysis indicated that the systems are weakly acidic, and the predominant NHO equilibrium was established using Polster–Lachmann δ-diagram analysis and Perrin model data linearization.
Keywords: Schiff bases; NHO prototropic tautomerism; NMR titration; δ-diagram Schiff bases; NHO prototropic tautomerism; NMR titration; δ-diagram
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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MDPI and ACS Style

Ortegón-Reyna, D.; Garcías-Morales, C.; Padilla-Martínez, I.; García-Báez, E.; Aríza-Castolo, A.; Peraza-Campos, A.; Martínez-Martínez, F. NMR Structural Study of the Prototropic Equilibrium in Solution of Schiff Bases as Model Compounds. Molecules 2014, 19, 459-481.

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