Karbocation

Karbocation (carbocation) is a positively charged carbon-centered ion. In most cases the charge resides on a carbon atom bearing three substituents, often written as R3C+. Related species include R2CH+ and RCH2+. Carbocations are common intermediates in organic reactions and exhibit substantial instability; their stability depends on substitution, resonance, and solvation.

Stability increases with substitution due to hyperconjugation: tertiary carbocations are generally more stable than secondary, which are more stable than primary. Delocalization of the positive charge by adjacent pi systems also stabilizes carbocations. Allylic and benzylic carbocations are particularly stabilized by resonance, allowing charge to be delocalized over an allyl or benzyl system.

Some carbocations are stabilized by non-classical bonding or structural features, such as cyclopropylcarbinyl cations, where charge is delocalized into a small ring. In contrast, vinyl and aryl carbocations are typically very unstable because the positive charge cannot be effectively delocalized by the adjacent sp2 framework.

Formation often occurs via heterolysis in ionization reactions (SN1, E1) or through rearrangements from less stable cations. They act as electrophiles that react with nucleophiles or undergo rearrangements such as 1,2-hydride or alkyl shifts to form more stable cations. Solvent polarity and solvation by counterions strongly influence their lifetimes.