Illustrated Glossary of Organic Chemistry

Suprafacial: A sigmatropic reaction or mechanism step in which the sigma bond changes occur on the same face of the molecule.

This suprafacial 1,3-H shift is thermally forbidden by the Woodward-Hoffmann rules. The migrating hydrogen atom (shown in red) started on the top face of the molecule, and is on the same face in the product.

This antarafacial 1,3-H shift is thermally allowed by the Woodward-Hoffmann rules, but prevented by excessive strain in the transition state. The migrating hydrogen atom (shown in red) started on the top face of the molecule and is on the bottom face in the product.