karbokationen

A carbocation is a positively charged, carbon-centered ion that is a key reactive intermediate in many organic reactions. In classical models, the carbon bearing the positive charge is sp2-hybridized, giving a planar, trigonal arrangement around the cation. Carbocations are highly electron-deficient and typically short-lived, but their stability can be enhanced by substitution, resonance, and hyperconjugation.

Stability and stabilization factors: The general order of stability follows substitution: tertiary carbocations are more stable than secondary, which are more stable than primary. If the positive charge is on a benzylic or allylic carbon, resonance delocalization with adjacent π systems provides substantial stabilization. Hyperconjugation from neighboring C–H bonds also contributes to stability. Non-classical carbocations and rearrangements can further complicate stability in certain systems.

Formation and reactions: Carbocations commonly form via unimolecular processes such as SN1 or E1 mechanisms when a leaving group departs. Once formed, they may rearrange through hydride or methyl shifts (Wagner-Meerwein rearrangements) to yield more stable cations. They react with nucleophiles to give substitution products or with bases to form alkenes. Solvent polarity and stabilization by surrounding groups or solvent molecules influence their lifetimes and reaction pathways.

Examples: well-known carbocations include the tert-butyl carbocation, the benzyl carbocation, and the allyl carbocation. While many carbocations are transient, some are sufficiently stabilized under reaction conditions to direct product distributions in synthesis.

In summary, carbocations are fundamental, reactive intermediates that govern a wide range of substitution, elimination, and rearrangement processes in organic chemistry.