Chapter 21 — Key Takeaways

What you should leave Chapter 21 with

  1. Electrophilic aromatic substitution (EAS) is the canonical reaction of aromatic compounds. A ring H is replaced by an electrophile while preserving aromaticity.

  2. Why substitution, not addition: addition would destroy the aromatic stabilization (~36 kcal/mol). Substitution preserves it.

  3. EAS mechanism (2 steps): - Step 1 (slow, rate-determining): electrophile attacks the aromatic π system; one ring C becomes sp³; positive charge delocalized over 3 ring atoms (the arenium ion or σ-complex). - Step 2 (fast): a base removes the H from the sp³ ring C; aromaticity restored.

  4. The arenium ion: cyclic positively-charged intermediate with one sp³ C; positive charge on 3 of the remaining 5 ring carbons (delocalized).

  5. Five major EAS reactions: - Halogenation (X₂ + Lewis acid): X⁺ as electrophile. - Nitration (HNO₃ + H₂SO₄): NO₂⁺ as electrophile. - Sulfonation (SO₃ in H₂SO₄): SO₃ as electrophile. - Friedel-Crafts alkylation (R-X + AlCl₃): R⁺ as electrophile. - Friedel-Crafts acylation (RCOCl + AlCl₃): R-C≡O⁺ as electrophile.

  6. Halogenation: Cl⁺ from Cl₂ + FeCl₃; Br⁺ from Br₂ + FeBr₃. Direct iodination/fluorination is harder.

  7. Nitration: nitronium ion NO₂⁺. Used industrially for nitrobenzene → aniline → dyes/drugs.

  8. Sulfonation: SO₃ as electrophile. Reversible (unlike nitration). Used for detergents (LAS), as a directing/blocking group, in dye chemistry.

  9. Friedel-Crafts alkylation: R⁺ from R-X + Lewis acid. Limitations: - Rearrangement of primary cations to 2°/3°. - Polyalkylation because alkyl groups activate the ring. - Doesn't work on rings with strong EWG (NO₂, CN).

  10. Friedel-Crafts acylation: acylium ion R-C≡O⁺. Clean mono-acylation (no rearrangement; acyl group deactivates ring → stops at one).

  11. To install linear alkyl on benzene: use FC acylation + reduction (Clemmensen Zn(Hg)/HCl, or Wolff-Kishner NH₂NH₂/KOH). The acyl C=O is reduced to CH₂; gives the linear alkyl group.

  12. Industrial applications of EAS:

    • Cumene (FC alkylation of benzene + propylene): precursor to phenol via Hock process.
    • Ibuprofen (BHC process: FC acylation + reduction + Pd carbonylation).
    • Nitrobenzene (nitration of benzene): precursor to aniline → dyes/drugs.
    • Linear alkylbenzene sulfonate (LAS): detergents.
    • TNT (3 nitrations of toluene): military and industrial explosive.

  13. Other EAS reactions:

    • Vilsmeier-Haack (DMF + POCl₃ → ArCHO): mild formylation.
    • Reimer-Tiemann (CHCl₃ + NaOH on phenol → o-OH-ArCHO): formylation of phenols.
    • Kolbe-Schmitt (phenoxide + CO₂ → salicylic acid): basis of aspirin synthesis.

  14. Selectivity: EAS is sensitive to substrate. Existing substituents on the ring direct further EAS to specific positions (Ch 22).

  15. Spectroscopy: aromatic ¹H NMR at δ 7-8; aromatic ¹³C at δ 120-150. IR substitution patterns at 700-900 cm⁻¹ (out-of-plane bends).

  16. Aromatic substitution patterns:

    • Mono-substituted: 5 H total, complex multipets at δ 7-8.
    • Para-disubstituted: 4 H, AA'BB' pattern (apparent doublets).
    • Meta-disubstituted: 4 H, more complex pattern.
    • Ortho-disubstituted: 4 H, complex pattern.

  17. Why benzene is harder to nitrate than naphthalene: extended π system of naphthalene means lower aromatic stabilization energy per ring; first nitration is faster.

  18. Kinetic vs. thermodynamic considerations in EAS: the rate-limiting step (electrophile attack on aromatic ring) determines product distribution under typical conditions (kinetic).

  19. Modern EAS alternatives: Pd-catalyzed C-H activation (Ch 37), photoredox catalysis (Ch 40), biocatalysis. These work under milder conditions and are gaining prominence.

  20. Mastery of Chapter 21 is essential for understanding: industrial drug synthesis, dye chemistry, polymer precursors, explosives, and many other applications. EAS is the workhorse of aromatic chemistry.

Cross-references

  • Chapter 15 — Electrophilic addition to alkenes (compare: aromatic vs. non-aromatic π).
  • Chapter 20 — Aromaticity (foundation for EAS).
  • Chapter 22 — Substituent effects (directing and activating/deactivating).
  • Chapter 23 — Nucleophilic aromatic substitution (alternative chemistry).
  • Chapter 25 — Carbonyl additions; reduction of acylium chemistry.
  • Chapter 26 — Acyl substitution (related: aromatic acyl chemistry).
  • Chapter 30 — Amines (aniline from nitrobenzene reduction).
  • Chapter 35 — Drug design (aspirin, ibuprofen, acetaminophen).
  • Chapter 37 — Pd cross-coupling.
  • Appendix C — Reaction summary.
  • Appendix F — Named reactions (Friedel-Crafts, Vilsmeier, etc.).

Study tip

For each EAS problem, identify: 1. What's the substrate? Is the ring activated or deactivated? 2. What's the electrophile? Generated by what reagents? 3. What's the product? Where does the substituent go (ortho/meta/para; covered in Ch 22)? 4. What's the mechanism? The two-step EAS sequence. 5. What are the limitations? Rearrangement (FC alk), polyalk, deactivated rings.

If you can answer these for any EAS problem, you've internalized Chapter 21.