Electrophilic and Nucleophilic Aromatic Substitution – Key Vocabulary

SECTION 18.1 – Distinct Reactivity of Alkenes vs. Benzene

• Comparison of addition vs. substitution
  • Alkenes react rapidly with molecular bromine (Br_2) by simple electrophilic addition, producing vicinal dibromides and destroying the \pi bond.
  • Benzene, despite also possessing \pi electrons, is inert to the same conditions because aromatic stabilization energy (≈ 36 kcal·mol^{-1}) would be lost in an addition process.
• Electrophilic Aromatic Substitution (EAS) rescue
  • In the presence of iron filings or iron(III) bromide (FeBr_3), bromination proceeds, but through substitution—hydrogen is replaced, aromaticity ultimately retained.
  • Shows that the driving force for benzene chemistry is preservation of aromaticity.
  • Practical implication: separate flammable aromatics from halogenating agents unless a Lewis acid is deliberately introduced.

SECTION 18.2 – General Mechanism of Electrophilic Aromatic Substitution

• Generation of the electrophile
  • FeBr3 (or AlBr3) accepts a lone pair from Br2, producing the strongly electrophilic bromonium species Br^+ plus FeBr4^-.
  • Analogy: like converting a blunt knife (neutral Br_2) into a razor-sharp scalpel (bare Br^+) capable of slicing into the aromatic \pi cloud.
• Two-step mechanism
  1. Formation of the σ-complex (arenium ion)
     ◦ \pi electrons attack Br^+ giving a non-aromatic, positively charged intermediate.
     ◦ Endergonic because aromaticity is lost; rate-determining step.
     ◦ Resonance: positive charge is delocalized over ortho and para positions.
  2. Deprotonation
     ◦ Base (the Lewis-acid–halide complex or another bromide) removes the proton on the sp³ carbon, restoring aromaticity.
• Chlorination parallels
  • AlCl3 + Cl2 \rightarrow Cl^+ enabling EAS.
  • Industrial production of chlorobenzene for use in herbicides and rubber.

SECTION 18.3 – Sulfonation

• Sulfur trioxide (SO_3) in fuming sulfuric acid (oleum) exists as a resonance-stabilized, but intensely electrophilic, species.
• Reaction is reversible
  • Forward: benzene + SO3/H2SO4 \rightarrow benzenesulfonic acid (PhSO3H).
  • Reverse (desulfonation): heat with dilute aqueous acid.
• Synthetic value
  • SO_3H group serves as a temporary, removable “blocking group” for regiocontrol.
  • Environmental link: SO₃ generation is also implicated in acid rain chemistry.

SECTION 18.4 – Nitration

• Mixed acid
  • HNO3 + H2SO4 \rightarrow NO2^+ + HSO4^- + H2O (equilibrium lies far right owing to strong acidity).
• NO2^+ (nitronium ion) is isoelectronic with CO2, linear, and powerful.
• Nitration is highly exothermic; temperature control (≈ <55 °C) prevents polysubstitution or oxidation.
• Reduction pathway
  • PhNO2 \xrightarrow[Fe/HCl]{Sn/HCl} PhNH2 (aniline) after basification; demonstrates two-step amino installation useful for dyes, pharmaceuticals (e.g., paracetamol synthesis).

SECTION 18.5 – Friedel–Crafts Alkylation

• Electrophile generation
  • Alkyl halide (R–X) + Lewis acid \rightarrow carbocation or bridged complex capable of EAS attack.
• Limitations
  • Carbocation rearrangements (1,2-hydride or methyl shifts) can scramble skeleton; only efficient when rearrangement either impossible (e.g., benzyl, tert-butyl) or inconsequential.
  • The carbon bearing X must be sp^3; vinyl and aryl halides fail (too unstable cations).
  • Polyalkylation: first alkyl group activates ring → further alkylation; mitigated by large excess of benzene or by using acylation–reduction workaround.
• Real-world: tert-butylbenzene production for antioxidants; difficulties scaling due to corrosive AlCl_3 waste.

SECTION 18.6 – Friedel–Crafts Acylation and the Clemmensen Strategy

• Acylium ion
  • RCOCl + AlCl_3 \rightarrow RCO^+ ([R–C \equiv O]^+ resonance with R–C^+=O) – stabilized, non-rearranging.
• Product
  • Aromatic ketone; electron-withdrawing carbonyl deactivates ring, preventing polyacylation.
• Clemmensen reduction
  • Ph–COR \xrightarrow[Zn(Hg)]{HCl, \Delta} Ph–CHR – converts carbonyl to methylene; net alkylation without rearrangement risk.
• Synthesis tip
  • Install long linear alkyl chains (e.g., C_{12}) found in detergents; direct alkylation would rearrange.

SECTION 18.7 – Activating Groups & Ortho/Para Direction

• Methyl group (CH_3)
  • Hyperconjugation donates electron density, modestly lowers activation barrier; products predominantly ortho (crowding) + para.
• Methoxy group (OCH_3)
  • Lone-pair resonance donation (+M effect) greatly increases ring reactivity; strong ortho-para directing ability.
• General principle
  • All activators (lone-pair donors or alkyl) direct incoming electrophiles to positions where the intermediate cation is stabilized by resonance or hyperconjugation.

SECTION 18.8 – Deactivators & Meta Direction

• Nitro group (NO_2)
  • Strong –M and –I effects withdraw electron density; slows reaction ~ 10^6-fold vs. benzene.
  • Meta attack avoids placing positive charge adjacent to the electron-withdrawing group in the σ-complex.
• Most deactivators (carbonyl-bearing COR, SO3H, CN, CF3) behave similarly.

SECTION 18.9 – Halogens: The Exception

• Halogens are deactivating by induction (high electronegativity) yet possess lone pairs capable of resonance donation.
• Net result: slower reaction than benzene but still ortho/para orientation.
• Synthetic nuance: chlorobenzene directs further nitration to ortho/para, useful in making 4-nitrochlorobenzene, an intermediate in azo dyes.

SECTION 18.10 – Ranking Activating/Deactivating Power

• Strong activators: –NH2, –NHR, –NR2, –OH, –O^- (lone pair directly conjugated).
• Moderate activators: –O(C=O)R, –NHCOR, –OCOR (lone pair already partly delocalized), alkoxy exception –OR.
• Weak activators: alkyl groups.
• Weak deactivators: halogens.
• Moderate deactivators: –COR, –COOR, –CONH2, –SO2R, –CN (π bond to electronegative atom conjugated).
• Strong deactivators: –NO2, –NR3^+, –CF3, –CCl3 (powerful –M and/or –I).

SECTION 18.11 – Multiple Substituent Effects & Blocking Strategies

• Dominance rule: strongest activator dictates orientation unless sterics prohibit.
• Steric hindrance: bulky tert-butyl directs predominantly para vs. ortho.
• Blocking group strategy
  • Example: install SO_3H para to methyl to force subsequent nitration ortho; later remove by hydrolysis.

SECTION 18.12 – Synthesis Design & Retrosynthetic Analysis

• Work backward from target
  • Identify final substitution pattern, assign directors’ roles.
  • Plan order: install meta-directing groups before ortho/para directors if required.
• Example workflow
  1. Desired product: m-bromonitrobenzene.
  2. Start with nitrobenzene (meta director), then brominate.
• Toolset: combination of EAS, functional-group interconversion (FGI), reductions, oxidations.

SECTION 18.13 – Nucleophilic Aromatic Substitution (Addition–Elimination)

• Prerequisites (the "S┬NAr triad")
  1. Powerful –M group (e.g., NO_2).
  2. Leaving group (Cl, F, Br, OR). Fluorine often best due to stabilizing $
     σ^*$ orbital.
  3. Ortho/para relationship enabling resonance stabilization of Meisenheimer complex.
• Mechanism
  • Step 1: Nucleophile attacks ipso carbon; σ-complex (Meisenheimer) bears negative charge delocalized onto NO_2.
  • Step 2: Leaving group departs, aromaticity restored.
• Example: p-chloronitrobenzene + ^-OCH_3 \rightarrow p-methoxynitrobenzene.
• Medicinal chemistry: route to synthesize analgesic NSAIDs via S_NAr.

SECTION 18.14 – Elimination–Addition via Benzyne

• Harsh conditions (strong base like NaNH_2, >350 K) abstract ortho-proton, forming benzyne after halide loss.
• Benzyne: highly strained, formally contains a "triple bond" within benzene; reacts with nucleophiles, often giving isomer mixtures (addition can occur at either end).
• Evidence
  • Isotopic scrambling when using C6H5Cl labeled at the ipso carbon.
  • Trapping with dienes produces Diels–Alder adducts.

SECTION 18.15 – Comparing the Three Aromatic Substitution Mechanisms

• Electrophilic Aromatic Substitution (EAS)
  • Intermediate: σ-complex cation.
  • Leaving group: proton (H^+).
  • Substituent effects: activated by electron-donors; deactivated by withdrawers.
• Nucleophilic Aromatic Substitution (Addition–Elimination S_NAr)
  • Intermediate: Meisenheimer anion.
  • Leaving group: halide/OR.
  • Requires strong E-withdrawing group ortho/para.
• Benzyne (Elimination–Addition)
  • Intermediate: benzyne.
  • Leaving group: halide; no E-withdrawing substituent required.
  • Substituent effects minimal; extreme basic conditions.
• Synopsis diagram: EAS \;(σ^+) \quad\leftrightarrow\quad S_NAr \;(σ^-) \quad\leftrightarrow\quad Benzyne \;(π \text{deficient}).