# Carbocation
A carbocation is a positively charged ion whose electron count is even and in which one or more carbon atoms bear much of the surplus positive charge. The term is general: it embraces carbenium ions, all types of carbonium ions, vinyl cations, and related species, and it says nothing about whether the charged carbon is three-coordinated or five-coordinated[1](https://goldbook.iupac.org/terms/view/C00817). Carbocations are the central reactive intermediates of organic chemistry, governing the rates and products of substitution, elimination, and electrophilic addition reactions.
| Key fact | Detail |
|---|---|
| IUPAC definition | A cation with an even number of electrons and significant positive charge on one or more carbon atoms; a general term covering carbenium ions, carbonium ions, and vinyl cations[1](https://goldbook.iupac.org/terms/view/C00817) |
| Geometry | sp² hybridized and trigonal planar at the charged carbon, with an empty p orbital above and below the plane[2](https://www.saskoer.ca/intro-organic-chemistry/chapter/11-3/) |
| Stability order | Tertiary > secondary > primary > methyl, from inductive electron donation and hyperconjugation[3](https://openstax.org/books/organic-chemistry/pages/7-9-carbocation-structure-and-stability) |
| Resonance-stabilized series | CH₃⁺ < CH₃CH₂⁺ < (CH₃)₂CH⁺ ≈ CH₂=CH–CH₂⁺ < C₆H₅CH₂⁺ ≈ (CH₃)₃C⁺[4](https://chem.libretexts.org/Courses/Chabot_College/Chem_12A%3A_Organic_Chemistry_Fall_2022/08%3A_Nucleophilic_Substitution_Reactions/8.07%3A_Characteristics_of_the_S1_Reaction) |
| Role in SN1 reactions | Formation of the carbocation is the rate-determining step; rate = k[substrate], independent of nucleophile strength[4](https://chem.libretexts.org/Courses/Chabot_College/Chem_12A%3A_Organic_Chemistry_Fall_2022/08%3A_Nucleophilic_Substitution_Reactions/8.07%3A_Characteristics_of_the_S1_Reaction) |
| Rearrangements | 1,2-hydride and 1,2-alkyl shifts convert less stable cations to more stable ones[5](https://openstax.org/books/organic-chemistry/pages/7-11-evidence-for-the-mechanism-of-electrophilic-additions-carbocation-rearrangements) |
| Aromatic cations | The tropylium ion (C₇H₇⁺) carries 6 π electrons over seven carbons and is among the most stable carbocations[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC10224505/) |
## Definition and nomenclature
IUPAC divides the field into two named subfamilies. A carbenium ion is a carbocation with at least one important contributing structure containing a tervalent carbon atom with a vacant p orbital, the name implying a protonated carbene or a substitution derivative of one[7](https://goldbook.iupac.org/terms/view/C00812). The term was proposed, and rejected, as a replacement for the traditional name carbonium ion, and IUPAC advises against using it as the root for systematic nomenclature because of a historical ambiguity: "ethylcarbonium ion" has at times been used for either the ethyl cation CH₃CH₂⁺ or, correctly, the propyl cation CH₃CH₂CH₂⁺[7](https://goldbook.iupac.org/terms/view/C00812). A simple way to name any carbocation is to add the word "cation" to the name of the corresponding radical, as in the ethyl cation[1](https://goldbook.iupac.org/terms/view/C00817).
## Structure and geometry
The charged carbon of a simple carbenium ion is sp² hybridized and trigonal planar. The unhybridized p orbital is empty and stands perpendicular to the three σ bonds, and this vacant orbital, extending above and below the plane, is the electrophilic site that nucleophiles attack[2](https://www.saskoer.ca/intro-organic-chemistry/chapter/11-3/). Because the center is planar, a carbon that was a stereocenter before ionization ceases to be one, and a nucleophile can approach from either face; this is why SN1 reactions at a single stereocenter give racemized or partially racemized products[2](https://www.saskoer.ca/intro-organic-chemistry/chapter/11-3/).
## Stability
**Alkyl substitution stabilizes cations in two ways.** Inductive effects shift electron density in σ bonds toward the positive charge: electrons from a larger, more polarizable alkyl group shift toward a neighboring positive charge more easily than the electron from hydrogen, so the more alkyl groups bonded to the charged carbon, the more the charge is relieved[3](https://openstax.org/books/organic-chemistry/pages/7-9-carbocation-structure-and-stability). Hyperconjugation adds a second channel: neighboring C–H σ bonds that are roughly parallel to the vacant p orbital interact with it and lower the cation's energy, and more alkyl groups mean more properly oriented bonds. Only the roughly parallel C–H bonds participate; the bond perpendicular to the p orbital does not[3](https://openstax.org/books/organic-chemistry/pages/7-9-carbocation-structure-and-stability).
The resulting order, tertiary > secondary > primary > methyl, is the standard reference scale[3](https://openstax.org/books/organic-chemistry/pages/7-9-carbocation-structure-and-stability). Resonance delocalization stabilizes a cation further and is described as the most important single stabilizing factor[2](https://www.saskoer.ca/intro-organic-chemistry/chapter/11-3/). A commonly cited series places the allylic cation near the secondary cation and the benzylic and tertiary cations at the top: CH₃⁺ < CH₃CH₂⁺ < (CH₃)₂CH⁺ ≈ CH₂=CH–CH₂⁺ < C₆H₅CH₂⁺ ≈ (CH₃)₃C⁺[4](https://chem.libretexts.org/Courses/Chabot_College/Chem_12A%3A_Organic_Chemistry_Fall_2022/08%3A_Nucleophilic_Substitution_Reactions/8.07%3A_Characteristics_of_the_S1_Reaction).
Aromaticity produces an extreme case. The tropylium ion (cycloheptatrienyl cation, C₇H₇⁺) has a planar cyclic structure with (4n + 2) π electrons, n = 1, spread over seven carbon atoms, and is one of the most stable carbocations known[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC10224505/) • [8](https://icho.csj.jp/37/pre/organic.pdf). Its bromide salt is a stable isolable compound[8](https://icho.csj.jp/37/pre/organic.pdf).
## Role as a reaction intermediate
**SN1 substitution.** In an [SN1 reaction](https://www.edgepedia.org/sn1-reaction) the leaving group departs first, in a slow unimolecular step, to form a carbocation; a nucleophile then attacks rapidly. The rate law is first order in substrate alone, rate = k[R–LG], and the strength of the nucleophile does not affect the rate because it takes no part in the rate-determining step[4](https://chem.libretexts.org/Courses/Chabot_College/Chem_12A%3A_Organic_Chemistry_Fall_2022/08%3A_Nucleophilic_Substitution_Reactions/8.07%3A_Characteristics_of_the_S1_Reaction). By the Hammond postulate, any factor that stabilizes the high-energy carbocation intermediate also stabilizes the transition state leading to it, so the more stable the carbocation, the faster the reaction; this is why tertiary, allylic, and benzylic halides are the best SN1 substrates[9](https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_%28OpenStax%29/11%3A_Reactions_of_Alkyl_Halides-_Nucleophilic_Substitutions_and_Eliminations/11.05%3A_Characteristics_of_the_SN1_Reaction).
**Solvent.** Polar solvents stabilize the carbocation by solvation, with the electron-rich ends of solvent dipoles oriented toward the positive charge, and thereby increase the rate. For 2-chloro-2-methylpropane, a rate increase of 100,000 is observed on going from ethanol to water[9](https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_%28OpenStax%29/11%3A_Reactions_of_Alkyl_Halides-_Nucleophilic_Substitutions_and_Eliminations/11.05%3A_Characteristics_of_the_SN1_Reaction). Polar protic solvents are generally preferred; some polar aprotic solvents can themselves react with the cation and give side products[4](https://chem.libretexts.org/Courses/Chabot_College/Chem_12A%3A_Organic_Chemistry_Fall_2022/08%3A_Nucleophilic_Substitution_Reactions/8.07%3A_Characteristics_of_the_S1_Reaction).
**E1 elimination.** E1 eliminations share the same first step with SN1, formation of a carbocation, so the two pathways always compete; heat generally favors elimination over substitution[10](https://www.youtube.com/watch?v=-3EBmWHjMqE).
## Rearrangements
Carbocations commonly rearrange to more stable isomers before being trapped. In the reaction of HCl with 3-methyl-1-butene, the secondary carbocation formed by protonation rearranges by a hydride shift, the migration of a hydrogen atom with its electron pair (:H⁻) to the adjacent positively charged carbon, producing a tertiary cation; the reaction yields roughly equal amounts of unrearranged 2-chloro-3-methylbutane and rearranged 2-chloro-2-methylbutane[5](https://openstax.org/books/organic-chemistry/pages/7-11-evidence-for-the-mechanism-of-electrophilic-additions-carbocation-rearrangements). Alkyl groups can shift the same way: 3,3-dimethyl-1-butene with HCl gives an equal mixture of unrearranged and methyl-shifted chlorides[5](https://openstax.org/books/organic-chemistry/pages/7-11-evidence-for-the-mechanism-of-electrophilic-additions-carbocation-rearrangements). Such rearrangements are a common feature of carbocation chemistry and also occur in biological pathways, for example in a hydride shift during the biosynthesis of cholesterol[5](https://openstax.org/books/organic-chemistry/pages/7-11-evidence-for-the-mechanism-of-electrophilic-additions-carbocation-rearrangements).
## Nonclassical ions
The most contested chapter in carbocation chemistry concerned the 2-norbornyl cation. The debate, centered almost exclusively on the behavior of 2-norbornyl esters, asked whether the cation was a pair of rapidly equilibrating classical carbenium ions or a single symmetrical species with a bridged, pentacoordinated carbon[11](https://onlinelibrary.wiley.com/doi/10.1002/poc.3846). The bridged, or "nonclassical", structure, featuring three-center, two-electron bonding, is now supported by [X-ray crystallography](https://www.edgepedia.org/x-ray-crystallography), NMR spectroscopy, calculations, thermochemical studies, and mechanistic evidence[12](https://onlinelibrary.wiley.com/doi/10.1002/poc.3290). One review notes that while the nonclassical structure of the free ion has been unequivocally established, possible steric contributions to the rate differences seen in solution solvolyses remained a matter of debate[11](https://onlinelibrary.wiley.com/doi/10.1002/poc.3846).
## References
1. [IUPAC Compendium of Chemical Terminology, carbocation (C00817)](https://goldbook.iupac.org/terms/view/C00817)
2. [Introduction to Organic Chemistry, 11.3: SN1 Reactions (S. Langdon, SaskOER)](https://www.saskoer.ca/intro-organic-chemistry/chapter/11-3/)
3. [OpenStax Organic Chemistry, 7.9 Carbocation Structure and Stability (J. McMurry)](https://openstax.org/books/organic-chemistry/pages/7-9-carbocation-structure-and-stability)
4. [Chemistry LibreTexts, 8.7: Characteristics of the SN1 Reaction (Chabot College)](https://chem.libretexts.org/Courses/Chabot_College/Chem_12A%3A_Organic_Chemistry_Fall_2022/08%3A_Nucleophilic_Substitution_Reactions/8.07%3A_Characteristics_of_the_S1_Reaction)
5. [OpenStax Organic Chemistry, 7.11 Carbocation Rearrangements (J. McMurry)](https://openstax.org/books/organic-chemistry/pages/7-11-evidence-for-the-mechanism-of-electrophilic-additions-carbocation-rearrangements)
6. [Tropylium Ion, an Intriguing Moiety in Organic Chemistry, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC10224505/)
7. [IUPAC Compendium of Chemical Terminology, carbenium ion (C00812)](https://goldbook.iupac.org/terms/view/C00812)
8. [IChO 2022 Preparatory Problem 13: Carbocations and Aromaticity](https://icho.csj.jp/37/pre/organic.pdf)
9. [Chemistry LibreTexts, 11.5: Characteristics of the SN1 Reaction (OpenStax)](https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_%28OpenStax%29/11%3A_Reactions_of_Alkyl_Halides-_Nucleophilic_Substitutions_and_Eliminations/11.05%3A_Characteristics_of_the_SN1_Reaction)
10. [Substitution & Elimination 5: E1 Elimination, Oxford College Chemistry (YouTube)](https://www.youtube.com/watch?v=-3EBmWHjMqE)
11. [Schneider (2018). The controversy about nonclassical ions: Finished too early? Journal of Physical Organic Chemistry](https://onlinelibrary.wiley.com/doi/10.1002/poc.3846)
12. [Moss (2014). The 2-norbornyl cation: a retrospective. Journal of Physical Organic Chemistry](https://onlinelibrary.wiley.com/doi/10.1002/poc.3290)
---
*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Organic substances › Organic reactions, structure and reference › Organic reactions and synthetic methods › Physical organic chemistry and reaction mechanisms › Reactive intermediates › Carbocations*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 15, 2026 · Edited: Sep 30, 2026; Oct 15, 2026 · Last review: Oct 15, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license