Karbokationi

Karbokationi, known in English as a carbocation, refers to a positively charged carbon-centered ion. In most cases the carbon bears three substituents and carries the formal charge, giving a generic formula R3C+. The electronic structure is best described as sp2-hybridized with an empty p orbital, yielding a planar geometry and high electrophilicity.

Stability of karbokationet depends on substitution and resonance. Tertiary carbocations are typically more stable than secondary or primary ones due to hyperconjugation and inductive effects. If the positive charge is adjacent to a π-system, such as in allylic or benzylic carbocations, resonance delocalizes the charge and increases stability. Non-classical carbocations with bridged frameworks have been proposed and debated in the literature.

Formation and reactivity: Carbocation intermediates arise in many reactions, often under strongly acidic conditions. They are key intermediates in SN1 and E1 processes, such as the ionization or dehydration of tertiary alkyl halides and alcohols. They also participate in electrophilic additions to alkenes and in Friedel–Crafts alkylations. Because karbokationet are highly reactive, they frequently undergo rapid rearrangements to more stable carbocations, via hydride or alkyl shifts (Wagner-Meerwein rearrangements).

Examples: The tert-butyl cation is a classic, illustrating a relatively stable case among carbocations, while primary carbocations are typically short-lived and prone to rapid reaction or rearrangement. Allyl and benzyl carbocations are notable for stabilization by resonance with adjacent double bonds or aromatic rings.

Note: In solution, free carbocations are transient species, often observable only through their reaction products or with specialized spectroscopic methods. They remain a central concept in reaction mechanism and physical organic chemistry.