Why do prima facie intuitive theories work in organic chemistry?

Abstract

In modern German ‘Anschauung’ is translated as intuition. But in Kant’s technical philosophical context, it means an intuition derived from previous visualizations of physical processes in the world of perceptions. The nineteenth century chemists’ predilection for Kantian Anschauung led them to develop an intuitive representation of what exists beyond the bounds of the senses. Molecular structure is one of the illuminating outcomes. (Ochiai 2021, pp. 1–51) This mental habit seems to be dominant among chemists even in the twentieth century, as is illustrated by the electronic theory of organic chemistry and the frontier orbital theory as well. The former assumes that (1) bonds are paired electrons shared by bonded atoms—in fact, electrons in molecules are not localized in bonds; (2) the difference of electronegativities between bonded atoms causes electron drifts—expressed by the curly arrow—that result in bond formation or bond cleavage. The latter focuses on the orbitals that make the greatest contribution to the energy of a system undergoing electron delocalization, while the LCAO method says, as is suggested by the word Linear Combination of Atomic Orbitals, molecular orbitals should be constructed from all of the atomic orbitals that have the appropriate symmetry. In other words, every molecular orbital contributes to some extent to the electronic state of a molecule. The curly arrow in the electronic theory and the orbital lobe in the frontier orbital theory illustrate an intuitive character of these theories. Although both theories rely on such simple and qualitative models rather than mathematically rigid quantum mechanical calculations, they are successful in explaining, predicting, and designing chemical reactions. What makes these prima facie intuitive theories so successful? In this study we address this problem from a historical and philosophical as well as scientific point of view. The key to solve this problem is that they are concerned with only bond formation or bond cleavage, in which the localized-bond principle holds.