Karbokatjonen

Karbokatjonen, or carbocation in English, is a positively charged carbon species. In most cases the carbon center bearing the positive charge is sp2-hybridized, leaving an empty p orbital and a planar arrangement. Carbocations are highly electrophilic and act as key intermediates in many organic reactions. They are typically short-lived unless stabilized by neighboring substituents or by resonance.

Stability and stabilization mechanisms vary. A common rule of thumb ranks alkyl-substituted carbocations as tertiary > secondary > primary > methyl, reflecting hyperconjugation and inductive effects from surrounding alkyl groups. Resonance stabilization can override simple substitution patterns: allylic and benzylic carbocations are particularly stabilized by delocalization of the positive charge over adjacent π systems. In some cases, carbocations may adopt non-classical, bridged structures that spread the positive charge over more than one atom.

Formation and reactions. Carbocations arise in many ways, most notably in acid-catalyzed reactions such as the dehydration of alcohols or the electrophilic addition to alkenes (SN1 and E1 pathways). They can also form briefly during solvolysis or rearrangement processes. Once formed, carbocations rapidly react with nucleophiles or undergo rearrangements, such as hydride or alkyl shifts, to form more stable cations or neutral products. They underpin a wide range of transformations, including protection-group removals, rearrangements, and polymerization initiation.

Occurrence and significance. Although rarely isolated due to their high reactivity, carbocations are fundamental as reaction intermediates in organic synthesis and catalysis. Their behavior helps explain rate laws, product distributions, and the mechanisms of many classic reactions.