tetrahedral intermediate

https://doi.org/10.1351/goldbook.T06289
A reaction @I03096@ in which the bond arrangement around an initially double-bonded carbon atom (typically a carbonyl carbon atom) has been transformed from trigonal to tetrahedral. For example, aldol in the @C01238@ of acetaldehyde (but most tetrahedral intermediates have a more fleeting existence).
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1173 [Terms] [Paper]