Carbokations

Carbokations, or carbocations, are positively charged carbon-centered species. In a typical carbokation the carbon carries a formal positive charge and is sp2-hybridized, with a vacant p orbital that accepts electron density. The positive charge may be localized on a single carbon or delocalized by adjacent pi systems or by hyperconjugation from neighboring C-H bonds.

Substitution pattern determines stability: primary, secondary, and tertiary carbocations show increasing stability in that order, due largely to hyperconjugation and inductive effects from alkyl substituents. Additional stabilization arises in resonance-stabilized carbocations, such as allylic and benzylic cations. Some systems display charge delocalization across a framework, giving nonclassical structures that have been debated historically.

Formation and reactions: Carbokations are typically generated in solution by ionization of suitable precursors, dehydration, or electrophilic addition to alkenes. They are highly reactive and commonly undergo rapid rearrangements, such as hydride or alkyl shifts, to form more stable cations. Depending on conditions, they are captured by nucleophiles, solvated, or eliminated.

Examples and significance: The tert-butyl cation illustrates a highly stabilized tertiary carbocation; the cyclopropylcarbinyl and norbornyl cations illustrate unusual stabilization and nonclassical behavior. Understanding carbokations is central to mechanisms of SN1 and E1 reactions and to catalytic processes in organic chemistry.