Abstract:

Simple, intuitive and easy to learn, the resonance model (RM) is still one of the most often employed tools for explaining mechanisms of chemical processes or predicting their products. Thus, chemistry students dedicate a significant amount of time to study this model, which is a topic included and worked with detail in Chemistry curricula. Nevertheless, several research groups have pointed out diverse evidences which seem to indicate the RM is not suitable for describing the evolution of the electron density in various simple chemical processes. Thus, reaction steps and even whole reaction mechanisms based upon it have be questioned. Moreover, some experimental facts like the evolution of pKas along certain series of organic compounds, cannot be explained by the RM. Conformational equilibria, protonations or hydride additions are examples of simple processes where the resonance model leads to explanations that contradict those obtained using modern quantum chemical methods for the electron density analysis. Among them, the discrepancies with the quantum theory of atoms in molecules (QTAIM) are particularly remarkable.

QTAIM can be considered as a very reliable method for electron density analysis, as it is based exclusively on the basic principles of Physics without introducing any other hypothesis. This work provides a quick overview on QTAIM and reviews part of contradictions between its results and RM interpretations, aiming to state simple messages to teach in the class room with clear examples based on firmly established results. To this end, several cases are selected in this work: rotational barrier of esters and amides, atomic charges of diazonium ions, distribution of electron density in flavylium cation, evolution of the electron density upon protonation, formation of hydrogen bonded adducts, and nucleophilic addition to carbonylic compounds.

Keywords:

Resonance Model, Misconceptions in Chemistry.