oxonium

Oxonium is a term for positively charged oxygen-containing ions. The most familiar example is the hydronium ion, H3O+, formed when a proton is transferred to a water molecule. In aqueous solutions, hydronium is the principal carrier of acidity: water acts as a base by accepting a proton, while acids donate protons to produce H3O+. The autoprotolysis of water is represented by 2 H2O ⇌ H3O+ + OH−, a key process in determining pH.

An oxonium ion refers to any cation in which oxygen bears a positive charge. The simplest case is H3O+, but protonation of other oxygen-containing molecules yields ions such as R3O+ or R2OH+. Common protonated species include protonated alcohols (R–OH2+) and protonated ethers (R2OH+). In general, oxonium ions are highly reactive and are typically short-lived in solution, though they can be stabilized in highly acidic media or as salts with non-nucleophilic counterions.

Oxonium ions play important roles as electrophiles in acid-catalyzed reactions. They act as reactive intermediates in solvolysis, dehydration, and glycoside hydrolysis, among other processes. The term highlights the oxygen-centered positive charge, as opposed to carbon-centered cations. In water, hydronium is the dominant oxonium species; in nonaqueous or superacidic environments, a wider range of oxonium ions can form and participate in chemical transformations.