Oxonium ion
From Wikipedia, the free encyclopedia
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:
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.
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oxonium ion |
trimethyloxonium cation |
of trimethyloxonium |
of trimethyloxonium |
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.
