Carbokation

Carbokation, more commonly called a carbocation in English, refers to a positively charged carbon atom that typically bears only six electrons in its valence shell. The simplest representation is R3C+, a cationic center in which the carbon lacks a full octet. Carbokations are highly reactive intermediates that appear in many organic reactions, often governing the course of substitutions, eliminations, and rearrangements.

Carbokations are typically classified by the degree of substitution: primary, secondary, or tertiary. Stability generally increases with more alkyl substitution due to hyperconjugation and inductive donation from adjacent carbon groups, making tertiary carbocations among the most stable simple alkyl cations. Stability is further enhanced by resonance when the positive charge can be delocalized into adjacent pi systems, giving allylic or benzylic carbocations. Non-classical carbocations, where charge is distributed over a larger framework or bridged, are another important type in some systems.

Formation of carbocations occurs in reactions that generate a cationic center, notably SN1 and E1 mechanisms, where a leaving group departs to yield the carbocation, which may then be trapped by a nucleophile or lose a proton to form an alkene. Carbocations can undergo rapid rearrangements, such as hydride or methyl shifts, to reach more stable cations and thus influence product distributions.

Because carbocations are typically short-lived, their existence is inferred from reaction kinetics, product evidence, and trapping experiments, or from indirect spectroscopic observations under specialized conditions. They remain central to understanding reaction mechanisms in organic chemistry.