Electrophilic and Nucleophilic Aromatic Substitution – Key Vocabulary

A comprehensive set of vocabulary flashcards covering electrophilic and nucleophilic aromatic substitution mechanisms, reagents, intermediates, directing effects, and synthetic strategies from Sections 18.1–18.15.

Electrophilic Aromatic Substitution (EAS)

A reaction in which an electrophile replaces a hydrogen atom on an aromatic ring, preserving aromaticity after deprotonation.

Bromination of Alkenes vs. Benzene

Alkenes undergo addition with Br₂, whereas benzene remains inert unless a Lewis acid (e.g., FeBr₃) is present.

Lewis Acid (in EAS)

A compound such as FeBr₃ or AlCl₃ that accepts an electron pair, activating halogens or acyl/alkyl halides toward electrophilic attack on benzene.

Iron(III) Bromide (FeBr₃)

Common Lewis acid that converts Br₂ into the strongly electrophilic bromonium (Br⁺) for bromination of benzene.

Aluminum Tribromide (AlBr₃)

Alternative Lewis acid to FeBr₃ for halogenation of aromatics.

Sigma Complex (Arenium Ion)

Carbocation intermediate formed when an electrophile adds to the aromatic ring; loses aromaticity and is endergonic to form.

Deprotonation (in EAS)

Second step of EAS where a base removes a proton from the sigma complex, restoring aromaticity.

Chlorination of Benzene

Introduction of Cl using Cl₂ and AlCl₃ (or FeCl₃) via electrophilic aromatic substitution.

Sulfonation

Reversible reaction of benzene with SO₃ (from fuming H₂SO₄) giving benzenesulfonic acid.

Sulfur Trioxide (SO₃)

Powerful electrophile present in fuming sulfuric acid, responsible for sulfonation of benzene.

Nitration

EAS process where benzene reacts with the nitronium ion (NO₂⁺) generated from HNO₃/H₂SO₄ to give nitrobenzene.

Nitronium Ion (NO₂⁺)

Strong electrophile produced from a mixture of concentrated nitric and sulfuric acids in nitration reactions.

Reduction of Nitro Group

Conversion of –NO₂ to –NH₂ (amino group) using reagents such as Fe/HCl or Sn/HCl, completing a two-step amination route.

Friedel–Crafts Alkylation

EAS that installs an alkyl group on benzene via a carbocation generated from an alkyl halide and a Lewis acid.

Carbocation Rearrangement

Migration (hydride or alkyl shift) within a carbocation; problematic in Friedel–Crafts alkylations unless rearrangement is impossible.

Polyalkylation

Multiple alkylations of a ring due to increased activation after initial substitution; minimized by controlled conditions.

Friedel–Crafts Acylation

EAS that attaches an acyl group using an acyl chloride and a Lewis acid, forming an acylium ion intermediate.

Acylium Ion

Resonance-stabilized cation (R–C≡O⁺) generated from an acyl chloride and a Lewis acid; does not rearrange.

Polyacylation

Unlikely in acylations because the acyl substituent deactivates the aromatic ring toward further reaction.

Clemmensen Reduction

Zn(Hg)/HCl reduction converting an acyl group on an aromatic ring to an alkyl group after Friedel–Crafts acylation.

Activator

Substituent that donates electron density, increasing ring reactivity and directing new groups to ortho/para positions.

Deactivator

Electron-withdrawing substituent that lowers ring reactivity; typically directs new substitution to the meta position.

Ortho-Para Director

Group that directs incoming electrophiles to the ortho and para positions relative to itself on an aromatic ring.

Meta Director

Substituent that directs electrophilic substitution to the meta position relative to itself.

Strong Activator

Group with a lone pair directly attached to the ring (e.g., –OH, –NH₂), providing intense electron donation.

Moderate Activator

Group whose lone pair is partially delocalized outside the ring (e.g., –O–C=O–R); still donates but less strongly.

Weak Activator

Alkyl group that activates by weak hyperconjugation and inductive donation.

Weak Deactivator

Halogen substituent (Cl, Br, I) that withdraws inductively yet donates by resonance, giving ortho-para direction with reduced rate.

Moderate Deactivator

Group possessing a π bond to an electronegative atom conjugated with the ring (e.g., –C=O–R, –CN), leading to meta direction.

Strong Deactivator

Highly electron-withdrawing group (e.g., –NO₂, –CF₃, –SO₃H) that powerfully deactivates and directs meta.

Blocking Group

Temporary substituent introduced to control regiochemistry, later removed after desired substitution pattern is achieved.

Retrosynthetic Analysis (for Aromatics)

Strategy of planning multi-step syntheses by disconnecting target molecules into simpler precursors, considering directing effects.

Nucleophilic Aromatic Substitution (SNAr)

Reaction where a nucleophile displaces a leaving group on an aromatic ring bearing an electron-withdrawing group (e.g., –NO₂) ortho or para.

Meisenheimer Complex

Resonance-stabilized anionic intermediate formed during nucleophilic aromatic substitution before expulsion of the leaving group.

Elimination–Addition (Benzyne Mechanism)

Substitution route involving loss of a leaving group and proton to form a benzyne intermediate, then nucleophilic addition.

Benzyne Intermediate

Highly reactive, strained aromatic species with a triple bond; evidence includes isotopic scrambling and trapping experiments.

Leaving Group (in SNAr)

Atom or group (often –Cl, –Br, –F) that departs from the aromatic ring during substitution, positioned ortho or para to –NO₂.

Electron-Withdrawing Group (EWG)

Substituent that stabilizes negative charge in SNAr and activates the ring toward nucleophilic attack; typically nitro (–NO₂).