suprafacially

Suprafacially is an adverb used primarily in organic chemistry to describe a chemical reaction or process that occurs at or involves the surface of a molecule or a species. It is often employed in the context of pericyclic reactions, such as cycloadditions and sigmatropic rearrangements, to denote the specific pathway of bond formation or breaking. In these reactions, electron delocalization across a conjugated system allows for concerted transformations where new sigma bonds are formed simultaneously with the rearrangement of pi bonds. The term "suprafacial" indicates that the bonds are formed on the same face of the pi system involved.

For instance, in a Diels-Alder reaction, a conjugated diene reacts with a dienophile to form a six-membered ring. If both the diene and the dienophile react in a suprafacial manner, it means that the new sigma bonds are formed on the same side of the diene and the same side of the dienophile. This contrasts with an antarafacial approach, where the reaction occurs on opposite faces. The stereochemistry of the product is determined by whether the reaction proceeds suprafacially or antarafacially. Understanding the facial selectivity of these reactions is crucial for predicting the outcome and designing synthetic strategies in organic chemistry. The term is also applicable to other types of concerted reactions where the relative orientation of the reacting fragments is important.