{ "id": "ep02e796fn", "slug": "electrophilic-substitution", "title": "Electrophilic substitution", "updated": "2026-10-15", "topic_path": [ { "id": "physical", "label": "Physical world and mathematics", "api_url": "https://www.edgepedia.org/api/v1/topics/physical" }, { "id": "physical.chemistry", "label": "Chemistry", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.chemistry" }, { "id": "physical.chemistry.methods", "label": "Chemical principles and methods", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.chemistry.methods" }, { "id": "physical.chemistry.methods.chemical-synthesis", "label": "Chemical synthesis", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.chemistry.methods.chemical-synthesis" } ], "geo": [ { "id": "geo.nongeo.t1800.physical.chemistry.methods.chemical-synthesis", "label": "Non-geographic · 1800 to 1945: Chemical synthesis", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800.physical.chemistry.methods.chemical-synthesis", "path": [ { "id": "geo.nongeo", "label": "Non-geographic", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo" }, { "id": "geo.nongeo.t1800", "label": "Non-geographic · 1800 to 1945", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800" }, { "id": "geo.nongeo.t1800.physical", "label": "Physical world and mathematics", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800.physical" }, { "id": "geo.nongeo.t1800.physical.chemistry", "label": "Chemistry", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800.physical.chemistry" }, { "id": "geo.nongeo.t1800.physical.chemistry.methods", "label": "Chemical principles and methods", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800.physical.chemistry.methods" }, { "id": "geo.nongeo.t1800.physical.chemistry.methods.chemical-synthesis", "label": "Chemical synthesis", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800.physical.chemistry.methods.chemical-synthesis" } ] } ], "excerpt": "Electrophilic substitution is an organic reaction in which an electrophile replaces a hydrogen on an aromatic ring, installing halogen, nitro, sulfonic acid, alkyl, or acyl groups.", "snippet": "Electrophilic substitution is an organic reaction in which an electrophile replaces a hydrogen on an aromatic ring, installing halogen, nitro, sulfonic acid, alkyl, or acyl groups.", "node": "physical.chemistry.methods.chemical-synthesis", "markdown": "# Electrophilic substitution\n\nElectrophilic substitution is a class of organic reactions in which an electrophile replaces an atom, usually hydrogen, attached to an aromatic or other electron-rich molecule. In the aromatic version, electrophilic aromatic substitution (EAS), the electrophile adds to a ring position occupied by hydrogen to form a cationic \\( \\sigma_{\\mathrm{H}} \\) adduct, and a fast proton departure restores aromaticity; ipso substitution, where the electrophile replaces an existing substituent at the same carbon, is much less frequent.[1](https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202003770) EAS is the most characteristic reaction of aromatic compounds, installing halogen, nitro, sulfonic acid, alkyl, or acyl groups on the ring.[2](https://ocw.uci.edu/upload/files/51b_chapter18_f2014.pdf) Because it is often limited to electron-rich arenes, nucleophilic aromatic substitution serves as its complementary process for electron-deficient rings.[1](https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202003770)\n\n| Key fact | Detail |\n|---|---|\n| Definition | An electrophile replaces a ring hydrogen (rarely, an existing substituent, by ipso substitution)[1](https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202003770) |\n| Mechanism | Two-step addition–deprotonation via a σ-complex; addition is usually rate-limiting (secondary KIE < 1)[1](https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202003770) • [3](https://www.ch.ic.ac.uk/local/organic/tutorial/EHS_1.pdf) |\n| Main electrophiles | NO2+ (nitration), SO3 or HSO3+ (sulfonation), X+ (halogenation), R+ and RCO+ (Friedel–Crafts)[4](https://www.chemistry-online.com/organic-chemistry/aromatic-substitution-reactions-in-benzene-and-derivatives/) |\n| Regioselectivity example | Nitration of toluene: 58.5% ortho, 37% para, 4.5% meta[5](https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map%3A_Organic_Chemistry_%28Smith%29/16%3A_Electrophilic_Aromatic_Substitution/16.13%3A_Electrophilic_Aromatic_Substitution_of_Substituted_Benzenes) |\n| Classic limitation | Friedel–Crafts alkylation gives polyalkylation and fails with vinyl or aryl halides[2](https://ocw.uci.edu/upload/files/51b_chapter18_f2014.pdf) • [4](https://www.chemistry-online.com/organic-chemistry/aromatic-substitution-reactions-in-benzene-and-derivatives/) |\n| Industrial use | Sodium alkylbenzenesulfonate detergents, ethylbenzene for styrene–polystyrene, sulfa drugs[6](https://chem.libretexts.org/Courses/SUNY_Oneonta/Chem_322_Lecture_Content/04%3A_Arenes_Electrophilic_Aromatic_Substitution/4.05%3A_Electrophilic_Aromatic_Substitution) • [7](https://ecampusontario.pressbooks.pub/orgbiochemsupplement/chapter/aromatic-reactions/) |\n\n## How it works\n\nThe reaction passes through an intermediate known variously as the σ-complex, Wheland intermediate, or arenium ion.[3](https://www.ch.ic.ac.uk/local/organic/tutorial/EHS_1.pdf) For benzene, this cation (a benzenium ion) has four π electrons delocalized over five carbons and one sp3-hybridized carbon bearing both the electrophile and the hydrogen; substitution is completed by proton loss, which regenerates aromaticity.[6](https://chem.libretexts.org/Courses/SUNY_Oneonta/Chem_322_Lecture_Content/04%3A_Arenes_Electrophilic_Aromatic_Substitution/4.05%3A_Electrophilic_Aromatic_Substitution)\n\nAddition of the electrophile is the rate-determining step, because the ring loses its aromatic stabilization in forming the σ-complex; the second step is fast since loss of the proton regenerates the aromatic system.[3](https://www.ch.ic.ac.uk/local/organic/tutorial/EHS_1.pdf) Consistently, electrophilic substitution shows a secondary kinetic isotope effect KIE < 1, whereas nucleophilic aromatic substitution can show KIE < 1 or KIE > 1 depending on which step is rate-limiting.[1](https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202003770)\n\nSubstituents control both rate and orientation. Alkyl and alkoxy groups such as methoxy and tert-butyl are ortho/para directors.[8](http://www.chem.ucla.edu/~harding/IGOC/E/electrophilic_aromatic_substitution.html) [Nitration](https://www.edgepedia.org/nitration) of anisole gives 30–40% ortho, 0–2% meta, and 60–70% para product, while nitrobenzene gives 5–8% ortho, 90–95% meta, and 0–5% para.[5](https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map%3A_Organic_Chemistry_%28Smith%29/16%3A_Electrophilic_Aromatic_Substitution/16.13%3A_Electrophilic_Aromatic_Substitution_of_Substituted_Benzenes) Activating substituents accelerate the ring; electrophilic chlorination of toluene occurs hundreds of times faster than chlorination of benzene, giving 60% ortho, 39% para, and 1% meta isomers.[5](https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map%3A_Organic_Chemistry_%28Smith%29/16%3A_Electrophilic_Aromatic_Substitution/16.13%3A_Electrophilic_Aromatic_Substitution_of_Substituted_Benzenes)\n\n## How it is done\n\nEach classic reaction uses a distinct electrophile generated in situ.[4](https://www.chemistry-online.com/organic-chemistry/aromatic-substitution-reactions-in-benzene-and-derivatives/)\n\n**Nitration** uses concentrated nitric and sulfuric acids; the nitronium ion forms by protonation of HNO3 and loss of water,[7](https://ecampusontario.pressbooks.pub/orgbiochemsupplement/chapter/aromatic-reactions/) according to \\( \\mathrm{HNO_3} + 2\\,\\mathrm{H_2SO_4} \\rightleftharpoons \\mathrm{NO_2^+} + \\mathrm{H_3O^+} + 2\\,\\mathrm{HSO_4^-} \\).[6](https://chem.libretexts.org/Courses/SUNY_Oneonta/Chem_322_Lecture_Content/04%3A_Arenes_Electrophilic_Aromatic_Substitution/4.05%3A_Electrophilic_Aromatic_Substitution) Stable nitronium salts such as nitronium fluoroborate (NO2BF4) and nitronium perchlorate (NO2ClO4), dissolved in nitromethane or acetic acid, nitrate aromatic compounds at room temperature in high yields.[4](https://www.chemistry-online.com/organic-chemistry/aromatic-substitution-reactions-in-benzene-and-derivatives/)\n\n**Sulfonation** uses concentrated or fuming sulfuric acid. Sources differ on the active species: one describes SO3 as the actual neutral but strongly electrophilic sulfonating agent,[6](https://chem.libretexts.org/Courses/SUNY_Oneonta/Chem_322_Lecture_Content/04%3A_Arenes_Electrophilic_Aromatic_Substitution/4.05%3A_Electrophilic_Aromatic_Substitution) while another states the electrophile is either HSO3+ or neutral SO3 depending on reaction conditions.[7](https://ecampusontario.pressbooks.pub/orgbiochemsupplement/chapter/aromatic-reactions/)\n\n**Halogenation** uses Br2 with FeBr3, chlorination with Cl2 and AlCl3, or iodination with I2 and HgO, the Lewis acid generating the X+ electrophile.[8](http://www.chem.ucla.edu/~harding/IGOC/E/electrophilic_aromatic_substitution.html) • [4](https://www.chemistry-online.com/organic-chemistry/aromatic-substitution-reactions-in-benzene-and-derivatives/)\n\n**Friedel–Crafts alkylation** reacts an alkyl halide with an arene in the presence of a Lewis acid of the AlX3 type, usually aluminum chloride, generating a carbocation electrophile; alcohols and alkenes also work with acid catalysts such as H3PO4, H2SO4, HF, or BF3.[6](https://chem.libretexts.org/Courses/SUNY_Oneonta/Chem_322_Lecture_Content/04%3A_Arenes_Electrophilic_Aromatic_Substitution/4.05%3A_Electrophilic_Aromatic_Substitution) • [4](https://www.chemistry-online.com/organic-chemistry/aromatic-substitution-reactions-in-benzene-and-derivatives/) **Friedel–Crafts acylation** introduces an acyl group RCO using acid halides, anhydrides, or the acid itself with a strong Lewis-acid catalyst that generates the acyl cation RCO+; it requires more catalyst than alkylation because the catalyst complexes with the product ketone, and it is easily controlled to monosubstitution.[6](https://chem.libretexts.org/Courses/SUNY_Oneonta/Chem_322_Lecture_Content/04%3A_Arenes_Electrophilic_Aromatic_Substitution/4.05%3A_Electrophilic_Aromatic_Substitution) The closely related Gattermann–Koch aldehyde synthesis uses the formyl cation H–C+=O as the electrophile.[6](https://chem.libretexts.org/Courses/SUNY_Oneonta/Chem_322_Lecture_Content/04%3A_Arenes_Electrophilic_Aromatic_Substitution/4.05%3A_Electrophilic_Aromatic_Substitution)\n\n## Origin\n\nThe reaction was characterized in the late 19th century as a general type of substitution distinct from substitution at C(sp3) sites, and the postulates set out at that time included a definition of the reaction intermediate, the reaction complex, and the concept of a transition state.[9](https://pubs.rsc.org/en/content/articlelanding/2021/cp/d0cp05245k) In 1929, [Christopher Kelk Ingold](https://www.edgepedia.org/christopher-kelk-ingold) published \"The principles of aromatic substitution, from the standpoint of the electronic theory of valency\" in Recueil des Travaux Chimiques des Pays-Bas, applying the electronic theory of valency to aromatic substitution, including electrophilic aromatic substitution.[10](https://doi.org/10.1002/recl.19290480808) In the mid-20th century the Armstrong reaction complex concept was deepened and the intermediate named the σ-complex, which is why the terms σ-complex, Wheland complex, and Wheland intermediate are used synonymously today.[9](https://pubs.rsc.org/en/content/articlelanding/2021/cp/d0cp05245k) The alkylation and acylation reactions are named Friedel–Crafts reactions.[6](https://chem.libretexts.org/Courses/SUNY_Oneonta/Chem_322_Lecture_Content/04%3A_Arenes_Electrophilic_Aromatic_Substitution/4.05%3A_Electrophilic_Aromatic_Substitution)\n\n## Variants\n\nThe textbook stepwise mechanism is not universal. Computational and valence-bond studies have found a variety of concerted EAS mechanisms that do not involve σ-complex intermediates, in apparent contradiction to the generally accepted textbook picture; valence-bond modeling shows both concerted and stepwise mechanisms exist and identifies the conditions under which the intermediates appear.[11](https://pubs.acs.org/doi/abs/10.1021/jacs.9b04982) A combined computational and experimental study of anisole chlorination with Cl2 in nonpolar CCl4 found the addition–elimination pathway energetically favored, with the substitution (SEAr) mechanism proceeding concertedly via a single transition state and neither pathway involving a σ-complex in the rate-controlling stage; NMR revealed tetrachloro addition by-products, and HCl autocatalysis was confirmed by UV-visible spectroscopy.[12](https://pmc.ncbi.nlm.nih.gov/articles/PMC4104905/)\n\nNew selective variants exploit the σ-complex itself. Aryl thianthrenium salt synthesis shows unusually high para regioselectivity, superior to halogenation or borylation for various substrates; the proposed cause is reversible interconversion of Wheland-type intermediates before irreversible deprotonation, with deprotonation of the energetically accessible intermediates as the selectivity-determining step, consistent with the Evans–Polanyi principle.[13](https://pubs.acs.org/jacsat/article/143/39/16041/1498433/High-Site-Selectivity-in-Electrophilic-Aromatic) Conversely, long-lived arenium ions in superacid enable meta-selective methylation, accessing intrinsically disfavored positions that para-selective methods such as halogenation, borylation, TEDAylation, or [thianthrenation](https://www.edgepedia.org/thianthrenation) do not reach.[14](https://www.nature.com/articles/s41467-024-49421-8) For deactivated arenes, a catalytic C–H chlorination uses an elusive, highly electrophilic variant of the Willgerodt reagent, characterized by NMR and single-crystal [X-ray crystallography](https://www.edgepedia.org/x-ray-crystallography) with an unusually short I–Cl bond, formed in situ from readily available 2-iodobenzenesulfonic acid; the mild protocol has been showcased by late-stage chlorinations of highly functionalized drugs and natural products.[15](https://www.nature.com/articles/s41467-026-75774-3)\n\n## Applications\n\nSulfonation is used commercially to make sodium alkylbenzenesulfonate detergents.[6](https://chem.libretexts.org/Courses/SUNY_Oneonta/Chem_322_Lecture_Content/04%3A_Arenes_Electrophilic_Aromatic_Substitution/4.05%3A_Electrophilic_Aromatic_Substitution) [Friedel–Crafts alkylation](https://www.edgepedia.org/friedel-crafts-alkylation) underlies the industrial production of ethylbenzene from benzene and ethene using phosphoric acid catalyst; ethylbenzene is used to make styrene and then polystyrene, and isopropylbenzene is made similarly from benzene and propene.[6](https://chem.libretexts.org/Courses/SUNY_Oneonta/Chem_322_Lecture_Content/04%3A_Arenes_Electrophilic_Aromatic_Substitution/4.05%3A_Electrophilic_Aromatic_Substitution) • [7](https://ecampusontario.pressbooks.pub/orgbiochemsupplement/chapter/aromatic-reactions/) Sulfa drugs such as sulfanilamide are prepared commercially with [aromatic sulfonation](https://www.edgepedia.org/aromatic-sulfonation) as the key step.[7](https://ecampusontario.pressbooks.pub/orgbiochemsupplement/chapter/aromatic-reactions/) The nitration–reduction sequence, in which Fe, Sn, or SnCl2 reduces a nitro group to an arylamine (nitrobenzene with Fe, H3O+ and OH− yields aniline), is a key industrial route to dyes and pharmaceuticals.[7](https://ecampusontario.pressbooks.pub/orgbiochemsupplement/chapter/aromatic-reactions/)\n\n## Limitations and alternatives\n\nFriedel–Crafts alkylation has several failure modes. The alkylated product is more reactive than the starting material, so overalkylation is a problem requiring large excess benzene; overacylation is not a problem in acylation.[2](https://ocw.uci.edu/upload/files/51b_chapter18_f2014.pdf) Equilibrium-controlled isomerization can also occur, with ortho- and para-xylene converting to meta-xylene in the presence of excess catalyst.[6](https://chem.libretexts.org/Courses/SUNY_Oneonta/Chem_322_Lecture_Content/04%3A_Arenes_Electrophilic_Aromatic_Substitution/4.05%3A_Electrophilic_Aromatic_Substitution) Vinyl or aryl halides do not react under Friedel–Crafts alkylation conditions.[4](https://www.chemistry-online.com/organic-chemistry/aromatic-substitution-reactions-in-benzene-and-derivatives/) Deactivated, meta-directing benzene derivatives and aniline derivatives give poor yields of Friedel–Crafts products and should not be used.[2](https://ocw.uci.edu/upload/files/51b_chapter18_f2014.pdf) [Sulfonation](https://www.edgepedia.org/sulfonation) is reversible: heating benzenesulfonic acid in dilute sulfuric acid or water converts it back to benzene, with desulfonation favored in hot, dilute aqueous acid and sulfonation favored in strong acid.[6](https://chem.libretexts.org/Courses/SUNY_Oneonta/Chem_322_Lecture_Content/04%3A_Arenes_Electrophilic_Aromatic_Substitution/4.05%3A_Electrophilic_Aromatic_Substitution) • [7](https://ecampusontario.pressbooks.pub/orgbiochemsupplement/chapter/aromatic-reactions/)\n\nWhen EAS fails on electron-poor rings, nucleophilic aromatic substitution is the complementary method; on halobenzenes it requires a deactivating group such as −NO2 ortho or para to the halogen.[1](https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202003770) • [4](https://www.chemistry-online.com/organic-chemistry/aromatic-substitution-reactions-in-benzene-and-derivatives/)\n\n## References\n\n1. [Electrophilic and Nucleophilic Aromatic Substitutions are Mechanistically Similar with Opposite Polarity](https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.202003770)\n2. [Lecture Notes Chem 51B Chapter 18: Electrophilic Aromatic Substitution (UC Irvine OpenCourseWare)](https://ocw.uci.edu/upload/files/51b_chapter18_f2014.pdf)\n3. [Electrophilic Substitution (Imperial College organic tutorial notes)](https://www.ch.ic.ac.uk/local/organic/tutorial/EHS_1.pdf)\n4. [Aromatic Substitution Reactions in Benzene and Derivatives](https://www.chemistry-online.com/organic-chemistry/aromatic-substitution-reactions-in-benzene-and-derivatives/)\n5. [16.13: Electrophilic Aromatic Substitution of Substituted Benzenes (chem.libretexts.org)](https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map%3A_Organic_Chemistry_%28Smith%29/16%3A_Electrophilic_Aromatic_Substitution/16.13%3A_Electrophilic_Aromatic_Substitution_of_Substituted_Benzenes)\n6. [4.5: Electrophilic Aromatic Substitution - Chemistry LibreTexts](https://chem.libretexts.org/Courses/SUNY_Oneonta/Chem_322_Lecture_Content/04%3A_Arenes_Electrophilic_Aromatic_Substitution/4.05%3A_Electrophilic_Aromatic_Substitution)\n7. [22.5 Aromatic Reactions – Organic and Biochemistry Supplement](https://ecampusontario.pressbooks.pub/orgbiochemsupplement/chapter/aromatic-reactions/)\n8. [Illustrated Glossary of Organic Chemistry - Electrophilic aromatic substitution (EAS)](http://www.chem.ucla.edu/~harding/IGOC/E/electrophilic_aromatic_substitution.html)\n9. [An analysis of electrophilic aromatic substitution: a 'complex approach'](https://pubs.rsc.org/en/content/articlelanding/2021/cp/d0cp05245k)\n10. [Christopher Kelk Ingold (1929). The principles of aromatic substitution, from the standpoint of the electronic theory of valency. Recueil des Travaux Chimiques des Pays-Bas.](https://doi.org/10.1002/recl.19290480808)\n11. [Electrophilic Aromatic Substitution Reactions: Mechanistic Landscape, Electrostatic and Electric-Field Control of Reaction Rates, and Mechanistic Crossovers](https://pubs.acs.org/doi/abs/10.1021/jacs.9b04982)\n12. [Arenium ions are not obligatory intermediates in electrophilic aromatic substitution](https://pmc.ncbi.nlm.nih.gov/articles/PMC4104905/)\n13. [High Site Selectivity in Electrophilic Aromatic Substitutions: Mechanism of C–H Thianthrenation](https://pubs.acs.org/jacsat/article/143/39/16041/1498433/High-Site-Selectivity-in-Electrophilic-Aromatic)\n14. [Leveraging long-lived arenium ions in superacid for meta-selective methylation](https://www.nature.com/articles/s41467-024-49421-8)\n15. [Catalytic electrophilic arene C–H chlorination by rethinking the century-old Willgerodt reagent](https://www.nature.com/articles/s41467-026-75774-3)\n\n---\n*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Chemical principles and methods › Chemical synthesis*\n\n*Initially written Sep 29, 2026 · Edited: Oct 15, 2026*\n\n*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*\n\nLicense: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license\n", "same_as": [ "http://www.chem.ucla.edu/~harding/IGOC/E/electrophilic_aromatic_substitution.html" ], "url": "https://www.edgepedia.org/electrophilic-substitution", "markdown_url": "https://www.edgepedia.org/electrophilic-substitution.md", "license": { "name": "Edgepedia Community License 1.0", "url": "https://www.edgepedia.org/edgepedia/license", "summary": "Free with credit, commercial use included. AI training is open to everyone. For other uses, organizations over USD 100M in revenue or 100M monthly users license separately.", "spdx": "LicenseRef-Edgepedia-Community-1.0" }, "credit": "\"Electrophilic substitution\", Edgepedia (EdgeChat), https://www.edgepedia.org/electrophilic-substitution. Edgepedia Community License 1.0.", "credit_md": "\"[Electrophilic substitution](https://www.edgepedia.org/electrophilic-substitution)\", Edgepedia (EdgeChat), [https://www.edgepedia.org/electrophilic-substitution](https://www.edgepedia.org/electrophilic-substitution). [Edgepedia Community License 1.0](https://www.edgepedia.org/edgepedia/license).", "credit_html": "\"Electrophilic substitution\", Edgepedia (EdgeChat), https://www.edgepedia.org/electrophilic-substitution. Edgepedia Community License 1.0.", "speakable": "Electrophilic substitution is an organic reaction in which an electrophile replaces a hydrogen on an aromatic ring, installing halogen, nitro, sulfonic acid, alkyl, or acyl groups." }