Carbocation

A carbocation is a positively charged carbon species that typically contains an empty p orbital. In most cases the carbon center is sp2-hybridized and planar, with only six electrons in the valence shell. Carbocations are highly reactive intermediates found in many organic reactions, and they often form transiently during bond-breaking steps or protonation of alkenes.

Carbocations are commonly classified by the degree of substitution at the positively charged carbon: primary (one alkyl group), secondary (two alkyl groups), and tertiary (three alkyl groups). More substituted carbocations are generally more stable due to hyperconjugation and inductive effects. In addition, carbocations can be stabilized by resonance when the positive charge is adjacent to a pi system, leading to allylic or benzylic carbocations. Vinylic carbocations, where the charge resides on a carbon of a double bond, are typically very unstable and short-lived. Non-classical or bridged carbocations are a more complex subset in which the positive charge is delocalized over multiple carbon atoms.

Rearrangements are common as reactions proceed toward more stable carbocations. Hydride or alkyl shifts can occur to form tertiary or resonance-stabilized cations, influencing product distributions in reactions such as SN1 and E1. Carbocations also serve as key intermediates in electrophilic addition to alkenes and in certain rearrangement or dehydration processes.

Solvent and temperature strongly affect carbocations: polar, protic solvents stabilize them through solvation, while high temperatures can promote competing pathways. Understanding carbocations helps explain many aging and synthesis processes in organic chemistry.