Oxonium ion

The oxonium ion in chemistry is any positive oxygen cation, which has three bonds. The simplest oxonium ion is the hydronium ion H₃O⁺. Another oxonium ion frequently encountered in organic chemistry is obtained by protonation or alkylation of a carbonyl group e.g. R-C=O⁺-R' which forms a resonance structure with the fully fledged carbocation R-C⁺-O-R' and is therefore especially stable.

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Stable alkyloxonium salts exist; they are extensively used as alkylating agents. For example, triethyloxonium tetrafluoroborate (Et₃O⁺)(BF₄^–) is a white crystalline solid. It is a powerful ethylating agent. It can be used, for example, to produce ethyl esters when the conditions of traditional Fischer esterification are unsuitable.

Other hydrocarbon oxonium ions are formed by protonation or alkylation of alcohols e.g. R-C-O⁺-R¹R². In acidic media, the oxonium functional group produced by protonating an alcohol can be a leaving group in the E2 elimination reaction, because when it receives an electron, it becomes a water molecule. The product is an alkene. Extreme acidity, heat and dehydrating conditions are usually required.

See also

• Onium compounds