antarafacially

Antarafacially is an adverb used in chemistry to describe a type of chemical reaction or interaction that occurs across two different faces or surfaces of a molecule. This term is particularly relevant in the context of pericyclic reactions, such as Diels-Alder reactions, cycloadditions, and sigmatropic rearrangements. In these reactions, bonds are formed or broken in a concerted manner, meaning all the electron movements happen simultaneously.

The term "antarafacial" specifically denotes the mode of orbital overlap in such reactions. When a reaction proceeds antarafacially, the two components involved (for example, a diene and a dienophile in a Diels-Alder reaction) approach each other such that the interacting orbitals overlap on opposite sides of the pi system of one of the reactants. This is in contrast to a suprafacial interaction, where the overlap occurs on the same side.

The stereochemical outcome of a reaction can be significantly influenced by whether it proceeds in a suprafacial or antarafacial manner. The Woodward-Hoffmann rules, which predict the feasibility and stereochemistry of pericyclic reactions based on the symmetry of the participating molecular orbitals, often differentiate between these two modes of interaction. Antarafacial pathways are generally less common than suprafacial ones but can be favored under certain conditions or for specific types of reactions, particularly when steric factors are involved or when the orbitals involved have the correct symmetry. Understanding the antarafacial mode is crucial for predicting and controlling the products of complex organic transformations.