Illustrated Glossary of Organic Chemistry

Tetrahedral intermediate: A molecule formed by nucleophilic addition at the carbon atom of a carbonyl group. The name is derived from the change in this carbon from sp2 hybridization (trigonal planar geometry) to sp3 hybridization (tetrahedral geometry). Commonly encountered in carbonyl addition reactions and carbonyl substitution reactions. When the structure is the end product of a carbonyl addition reaction the term tetrahedral adduct may be more appropriate.

The reduction of acetophenone with sodium borohydride in ethanol (the solvent) begins with nucleophilic addition to the carbonyl carbon via hydride transfer. This results in an alkoxide ion tetrahedral intermediate (shown in red). This tetrahedral intermediate is then protonated by ethanol (a protic solvent) to give a secondary alcohol product.