Illustrated Glossary of Organic Chemistry

Oxonium ion fates: In a mechanism step where one of the reactants is an oxonium ion, that oxonium ion generally reacts in only one of four ways.

Oxonium ion fate #1: Ionization to form a carbocation. Water and alcohols are moderate leaving groups. Ease of this ionization depends strongly on the stability of the carbocation product.


Oxonium ion fate #2: Deprotonation. An oxonium ion O-H bond is strongly acidic (R₂OH⁺ pK_a ~ -2).


Oxonium ion fate #3: Elimination to form a pi bond. Water and alcohols are moderate leaving groups.


Oxonium ion fate #4a: Nucleophilic attack (at sp³ carbon). When the oxonium ion has an sp³ carbon with low steric hindrance, it can undergo an S_N2 nucleophilic substitution.


Oxonium ion fate #4b: Nucleophilic attack (at a carbonyl carbon). When the oxonium ion is a protonated carbonyl group, the carbonyl can accept a nucleophile at the carbon atom, producing a tetrahedral adduct. This is one of the three common carbonyl fates.