Antarafacial

Antarafacial refers to a type of chemical reaction or molecular arrangement where a bond is formed or broken across a double or triple bond in a way that involves the two different faces of the pi system. In simpler terms, imagine a double bond as having an "up" side and a "down" side. An antarafacial process occurs when a new atom or group attaches to one side of the double bond and another atom or group attaches to the opposite side. This is in contrast to a suprafacial process, where both new atoms or groups would attach to the same side of the double bond.

This concept is particularly important in the study of pericyclic reactions, such as the Diels-Alder reaction, which is a classic example of a [4+2] cycloaddition. In a Diels-Alder reaction, a conjugated diene (which has two double bonds) reacts with a dienophile (which has one double bond). The antarafacial pathway describes how the electrons move and bonds are formed during this reaction. Understanding whether a reaction proceeds in an antarafacial or suprafacial manner is crucial for predicting the stereochemistry of the product, meaning the three-dimensional arrangement of atoms.

The antarafacial mode is often dictated by orbital symmetry rules, which state that for a reaction to occur efficiently, the symmetry of the molecular orbitals of the reactants must match. In many common pericyclic reactions, the suprafacial pathway is symmetry-allowed, while the antarafacial pathway is symmetry-forbidden, leading to the latter being less frequently observed under thermal conditions. However, under photochemical conditions, the symmetry rules can be reversed, making antarafacial pathways more plausible.