tetraederintermediate

Tetraederintermediate, in English commonly called tetrahedral intermediate, is a transient, high-energy species that arises during many nucleophilic additions to carbonyl compounds. The term may appear as tetraederintermediate in German-language texts. The essential feature is a temporary conversion of the carbonyl carbon from sp2 to sp3 hybridization, with the carbon atom bearing four single bonds as a tetrahedral center.

Formation occurs when a nucleophile attacks the carbonyl carbon, pushing electrons onto the oxygen to form an anionic or protonated oxygen. The resulting species has a tetrahedral geometry around the former carbonyl carbon and is often referred to as a tetrahedral adduct. Stabilization may arise from resonance and, in solution, from proton transfers that distribute charge among substituents. This intermediate usually collapses to expel a leaving group, reforming a carbonyl and yielding the substitution product.

In practice, tetrahedral intermediates are central to nucleophilic acyl substitution reactions such as hydrolysis, alcoholysis, and amidation of carboxy derivatives. They also appear in enzymatic mechanisms, notably in serine proteases, where an acyl-enzyme intermediate is followed by formation of a tetrahedral transition state/adduct stabilized by the enzyme's oxyanion hole. The lifetime of the intermediate is typically brief, but in some systems it can be trapped or observed indirectly by spectroscopic methods or computational studies.

Usage notes: the preferred English term is tetrahedral intermediate; tetraederintermediate is a cognate spelling in some languages. The concept remains a cornerstone of reaction mechanism analyses in organic chemistry and biochemistry.