Volume 11 Issue 3
NMR Structural Study of the Prototropic Equilibrium in Solution of Schiff Bases as Model Compounds
David Ortegón-Reyna,Cesar Garcías-Morales,Itzia Padilla-Martínez,Efren García-Báez,Armando Aríza-Castolo,Ana Peraza-Campos andFrancisco Martínez-Martínez
1Laboratorio de Posgrado, Facultad de Ciencias Químicas, Universidad de Colima, Km 9 Carretera Colima-Coquimatlán, Colima 28400, Mexico
2Departamento de Química, Centro de Investigación y Estudios Avanzados del Instituto Politécnico Nacional, Av. IPN 2508, San Pedro Zacatenco 07360, México D. F., Mexico
3Departamento 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.
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