Illustrated Glossary of Organic Chemistry

Syn: Atoms, groups, or other moieties pointing in the same direction. Syn entities have a dihedral angle of less than 90o.

The two methyl groups are syn and eclipsed in the syn-periplanar conformation of butane. Verify the CH3/CH3 dihedral angle of 0o with a molecular model.

This catalytic hydrogenation reaction is a syn addition.