Carbonium Ion
Definition and meaning of Carbonium Ion in chemistry.
A carbonium ion is a positively charged carbon species in which the carbon atom is bonded to five (or more) atoms or groups, exceeding carbon's normal tetravalency through a three-center two-electron bond. It is a "nonclassical," pentacoordinate carbocation, distinct from the more common trivalent carbenium ion.
In more detail
Carbonium ions form when a strong superacid protonates a saturated C–H or C–C bond of an alkane, adding a fifth bonding partner to carbon; the extra electron pair is delocalized over three centers (C and two H, or C and an H plus a C) rather than localized in a normal two-electron bond. George Olah's superacid studies (using media such as SbF5/FSO3H, "magic acid") first demonstrated these species and earned him the 1994 Nobel Prize in Chemistry. IUPAC nomenclature now reserves "carbonium ion" for these pentacoordinate species, separating it from "carbenium ion," the trivalent sp2 carbocation once also called a carbonium ion in older, looser usage.
Key facts
| Formula (parent example) | CH5+ |
|---|---|
| Carbon coordination | Five bonds (pentacoordinate) |
| Key bonding feature | Three-center two-electron bond |
| Field | Organic Chemistry |
Protonation of methane by a superacid produces CH5+ (protonated methane), a carbonium ion in which carbon is bonded to five hydrogens via a three-center two-electron bond involving two of them.
Frequently asked questions
How does a carbonium ion differ from a carbenium ion?
A carbenium ion (e.g., CH3+, (CH3)3C+) is a classical, trivalent, sp2-hybridized carbocation with six valence electrons on carbon. A carbonium ion (e.g., CH5+) is a nonclassical, pentacoordinate cation in which carbon forms five bonds using a three-center two-electron bond.
Why are carbonium ions significant?
They explain how superacids activate normally unreactive alkane C–H and C–C bonds, underpinning processes like isomerization and cracking in petroleum chemistry, and were central to George Olah's Nobel Prize-winning work on carbocation chemistry.