Oxonium ion

This article is about the group of ions. For the ion with the systematic name "oxonium", see hydronium.

In chemistry, an oxonium ion is a type of cation (chemical compound with positive electric charge) that contains an oxygen atom with three chemical bonds.^[1] Hydronium is the simplest oxonium ion, where oxygen is bonded to three hydrogen atoms.

Types

Oxonium ions are divided up by degree of substitution. Starting from hydronium, replacing one hydrogen atom gives a primary oxonium, replacing two gives secondary oxonium, and replacing all three gives tertiary oxonium.^[2] This is similar to the naming of amines, which are isoelectronic to oxonium ions.

Primary and secondary oxonium compounds are important reaction intermediates in elimination reactions. Protonation of alcohols and ethers gives primary and secondary oxonium ions, which break apart more easily than the original chemicals. The dehydration of alcohols to alkenes and the breaking up of ethers both have oxonium intermediates.^[3][4]

Tertiary oxonium compounds are strong ingredients for alkylation reactions (adding alkyl groups like −CH₃ to molecules). They are usually produced as tetrafluoroborate salts.^[5]

Structure

Most oxonium ions have a pyramidal molecular geometry.