This article presents the results of the study of explanatory models produced by 81 students of first-year of medicine at the Universidad del Norte in Barranquilla-Colombia. The students face a case in the area of chemistry applied to medicine, through a contextualized problem situation (SPC), constructed from a perspective of teaching chemistry in context. The explanatory models generated by the students were analyzed qualitatively at four moments of the implementation of a teaching proposal. The results obtained at the beginning of the implementation of the proposal indicate that the students did not elaborate any type of explanatory model. However, these arised from the teaching proposal that involves the SPC in which the students begin to generate explanatory models supported by chemistry content and categorized mostly in the research as Descriptive and Interpretative, a situation fostered by the theoretical and methodological coherence of the SPC, which was designed from a perspective of teaching in context based on modeling and leveraged by scaffolding.

Las reacciones de formación de 2-propenal y 1-hidroxipropan-2-ona a partir de glicerol, vía deshidratación molecular en fase gaseosa, fueron examinadas teóricamente usando diferentes métodos de la teoría funcional de la densidad (B3LYP, B3PW91, MW1PW91 y PBEPBE), con dos bases, 6-31G(d,p) y 6-31++G(d,p), empleando el paquete computacional G09W. La formación de 2-propenal fue estudiada a través de una reacción de tres pasos; el primer paso involucra la eliminación de agua a partir del grupo hidroxilo secundario, seguido por una reacción de tautomerización y, finalmente, otra deshidratación. La formación de 1-hidroxipropan-2-ona fue estudiada a través de una reacción que involucra una primera deshidratación y, posteriormente, una tautomerización ceto-enol. Los resultados obtenidos demuestran que la primera deshidratación es el paso limitante en ambas reacciones. El paso determinante en la etapa limitante es la polarización del enlace C-OH, la cual es asistida por un átomo de hidrógeno en posición b. Los parámetros cinéticos y termodinámicos de activación, el análisis NBO y la distribución de cargas en los átomos involucrados en el estado de transición, demuestran que todos los grupos OH presentes en el glicerol son cinéticamente equivalentes; por lo tanto, la selectividad de la descomposición molecular de glicerol depende de la rapidez con que se alcanza el equilibrio ceto-enol en la segunda etapa de ambas reacciones.

The gas-phase dehydration reactions of glycerol to 2-propenal and 1-hydroxypropan-2-one, were examined by meaning of several methods: B3LYP, B3PW91, MW1PW91 y PBEPBE, combined with two basis-set: 6-31G(d,p) and 6-31++G(d,p) using density functional theory (DFT) and the package G09W. The formation of 2-propenal was studied via three stepway reaction: the first step involves a dehydration of secondary hydroxyl group, followed by keto-enol equilibrium reaction, and, finally, a second dehydration. Formation of 1-hidroxypropan-2-one was studied through a stepway reaction that involves a first dehydration of primary hydroxyl group and, finally, a keto-enol equilibrium reaction. Theoretical results suggest the first dehydration is the limiting-step in the two studied reactions. The important step in the limiting stage was the polarization of C-OH bond, which is assisted for b-Hydrogen atom. Thermodynamics and kinetics parameters, NBO calculations and charge distribution along the transition state, suggest that all hydroxyl groups occurring in the glycerol molecules are kinetically equivalent; thus, selectivity of molecular glycerol decomposition is depending of equilibrium keto-enol rate in the second step of both reactions.