onium

Onium is a term used in chemistry to denote a family of positively charged ions that arise when a neutral molecule is protonated or alkylated. The name uses the suffix -onium to indicate a cation, and such ions are typically isolated as salts with various counteranions. Onium ions can involve a range of elements, including nitrogen, oxygen, sulfur, and phosphorus.

Common examples include ammonium (NH4+), the protonated form of ammonia; oxonium or hydronium (H3O+), the protonated water molecule; phosphonium (PR4+); and sulfonium (R2S+R' or R3S+). For arenes, protonation produces arenium or benzenium ions, which are key transient intermediates in electrophilic aromatic substitution.

Formation and properties: Onium ions form in strongly acidic or alkylating environments, where a base accepts a proton or alkyl group. They are typically stabilized as salts, for example ammonium chloride or tetrafluoroborate salts of phosphonium or sulfonium cations. The stability of an onium ion depends on substituents and the surrounding medium.

Applications and significance: Onium salts are important in synthesis, catalysis, and materials science. Quaternary ammonium salts serve as phase-transfer catalysts; sulfonium and phosphonium salts are used as reagents and as precursors to ylides in organic synthesis. Arenium ions are transient intermediates in electrophilic aromatic substitution.

Nomenclature: The -onium suffix signals a cationic species derived from a neutral parent. Hydronium is a common oxonium example (H3O+), and many other onium ions are named by adding -onium to the name of the parent atom or fragment.